Regions of the Skull

The rostrum is formed by the paired nasal, premaxilla, maxilla, ventral nasal concha, palatine, lacrimal, jugal, and mandible (ossa faciei). In Pteropus lylei CM 87972, the rostrum, as measured from the rostralmost edge of the premaxilla to the caudalmost tip that the palatine contributes to the ectopterygoid process, is slightly longer than half the length of the entire skull. The cranium is formed by the paired frontal, parietal, pterygoid, orbitosphenoid, alisphenoid, squamosal, petrosal, entotympanic, ectotympanic, and exoccipital (plus the middle ear ossicles in the auris), and the unpaired interparietal, vomer, ethmoid, presphenoid, basisphenoid, basioccipital, and supraoccipital (ossa cranii). We have examined the skull of P. lylei CM 87972 and its structures in dorsal (fig. 1), lateral (fig. 2), ventral (fig. 3), rostral (fig. 4), and oblique occipital (fig. 5) views, as well as the mandible in lateral (fig. 6), occlusal (fig. 7), and mental (fig. 8) views.

The apex (regio naris) is formed by the premaxillae ventrolaterally and the nasals dorsally, which enclose the osseous nasal opening (apertura nasi ossea) or external nasal aperture. The dorsal surface of the rostrum (regio dorsalis nasi) is formed chiefly by the nasals, which are edged by the dorsalmost extension of the maxillae laterally (fig. 1). The lateral wall of the rostrum (regio maxillaris) is formed almost in its entirety by the rostral process of the maxilla (fig. 2). The hard palate (palatum osseum) is composed of the palatine process of the maxilla rostrolaterally and the transverse process of the palatine caudally (figs. 3, 9). The premaxilla does not contribute to the hard palate because its palatine process is not ossified. The zygomatic arch (arcus zygomaticus, regio zygomatica) or zygoma is formed by the zygomatic process of the maxilla and the small jugal bone, which articulate caudally with the zygomatic process of the squamosal in the temporal region (see fig. 2 and below). The orbital region (regio orbitalis), in the middle of the skull laterally, is a large depression containing the fossa for the eyeball (bulbus oculi) or orbit (orbita). Ventral to the orbit is the pterygopalatine fossa (fossa pterygopalatina). Dorsal to the orbit is the forehead (frons) of the frontal region (sinciput, regio frontalis), which is formed exclusively by the frontal bone (fig. 1). Caudal to the orbital region is the rostral aspect of the braincase, with the fossa temporalis and fossa infratemporalis dorsally and ventrally, respectively. In the temporal region (regio temporalis), the squamosal projects rostrolaterally its zygomatic process, which comprises the posterior third of the zygomatic arch. The root of the zygomatic process (figs. 3, 5) overlies the glenoid fossa (fossa mandibularis), the cranial component of the temporomandibular joint (articulatio temporomandibularis, a synovial joint). Another component of the temporal region is the auditory region (auris), including the petrosal bone, ectotympanic, entotympanic, and the middle ear ossicles, located ventromedial to the squamosal and lateral to the basioccipital (see figs. 3, 5, and below). The dorsolateral aspect of the parietal region (regio parietalis, formed by the parietals and interparietal) is the planum parietale, the largest exposure of the braincase. The temporal and parietal regions provide both protection to the brain and origin to the m. temporalis.

In the ventral side, caudal to the hard palate (fig. 10), the choanae open anteriorly into the paired nasopharyngeal meatus (meatus nasopharyngeus). The basicranium (basis cranii externa) forms, in tandem with the ventral component of the occipital complex (see below), the floor of the braincase (fig. 3). It extends from the dorsolateral orbitosphenoid and alisphenoid bones (alae temporales et orbitales), which provide osseous basement to the caudal orbit and the infratemporal fossa, to the medial (pre- and basi-) sphenoid bones, which roof the nasopharyngeal meatus. Four ossifications—the unpaired supraoccipital and basioccipital (dorsal and ventral) and the paired exoccipitals (lateral)—as well as a small exposure of the petrosal, form the occiput, surrounding the large foramen magnum for the passage of the spinal cord and associated vessels (fig. 5).

The paired mandible consists of a tooth-bearing body connected anteriorly with its bilateral counterpart through the mandibular symphysis, and a ramus posteriorly (figs. 6, 7). The latter has a coronoid process and a mandibular condyle that articulates with the squamosal, forming the mandibular component of the temporomandibular joint.

Rostral View of the Skull

The apex of the rostrum consists of a large, almost heart-shaped external nasal aperture bordered ventrolaterally by the dorsally divergent premaxillae and dorsally by the nasals. Visible through the external nasal aperture inside the nasal cavity (cavum nasi) are the paired ventral nasal concha (concha nasalis ventralis) or maxilloturbinate, the incisive incisure of the vomer (also visible through the incisive fissure ventrally), the nasal septum (septum nasi osseum), and the very tip of the ethmoturbinals (ethmoturbinalia). Four incisors are attached to the premaxillae (see Dentition below).

Dorsal Surface of the Skull

The principal components of the rostrum are the dorsal surface of the nasals medially and the rostral process of the maxillae laterally, with a limited contribution of the premaxillae in the rostrolateral angles. The breadth of the rostrum increases abruptly at the level of the canines to accommodate the canine roots and the correspondingly prominent juga. There is only a modest widening of the rostrum caudal to the canines. The lacrimal is visible laterally in dorsal view (fig. 1). On the ventrocaudal angle of the rostrum, the large root of the zygomatic arch, in association with the alveolar process of the maxilla, forms a roughly triangular plate. The rostrolateral edge of the zygomatic root is a bridge of bone perforated by the infraorbital canal (canalis infraorbitalis). The horizontal plate of the root, widely visible in dorsal view, represents the maxillary tuberosity. The open roots of M1 and M2 protrude in that surface—which is also the floor of the pterygopalatine fossa, a space that funnels rostrally into the infraorbital canal. The caudal edge of the zygomatic root is deeply concave (fig. 3); it converges caudolaterally with the rostrolateral edge of the root, leading to the laterally compressed arches themselves. Shortly after, the maxillary part of the arch meets the small jugal, which constitutes the middle portion of the arch. The posterior zygomatic root, dorsoventrally compressed, is contributed by the zygomatic process of the squamosal, which joins the braincase laterally. The posterior zygomatic root projects rostrolaterally from the braincase and widely overlaps the jugal dorsally.

The frontal forms the central portion of the dorsal surface of the skull (fig. 1). The dorsal aspect of the frontal may be divided into a short, preorbital area, a laterally biconcave interorbital area rostrally, bounded by the supraorbital margin (margo supraorbitalis), and an approximately rectangular postorbital plate caudally. The prominent postorbital processes (processus zygomaticus) arise between the two posterior areas. The large postorbital foramen pierces the root of each postorbital process.

The braincase is large and bulbous, with a roughly piriform shape. CM 87872 shows a smooth cranial surface with no temporal lines. The rounded surface of each parietal (planum parietale) and the interparietal caudomedially are visible in dorsal view, whereas the lateral contribution from the squamosal to the braincase (the squama) is hidden in this view.

Lateral Surface of the Skull

The rostrum is formed chiefly by the maxilla in lateral view (fig. 2), and its gently convex lateral surface is marked anteriorly by the alveolar juga of the canines. Rostrally, the procumbent premaxilla projects anteriorly beyond the rostral end of the nasal; ventrally, it extends below the level of the alveolar line. The alveolar line is sinuous, with rounded valleys and peaks corresponding to the locations of individual teeth and spaces between them. The upper tooth row consists of two incisors followed by a large canine, a minute P1 and four well-developed cheek teeth (two premolars and two molars; see Maxilla and Dentition). Dorsally, the rostrum gently ascends to meet the frontal, which in turn continues rising to the coronal (= frontoparietal) suture (seen in dorsal view). Posteriorly, the thin, elongated anterior zygomatic root projects caudolaterally from the maxilla. The anterior zygomatic root is pierced by a large infraorbital canal. The anterior zygomatic root meets the jugal well away from the contact with the alveolar process of the maxilla. The jugal occupies the middle portion of the dorsoventrally thin, gently arched, mediolaterally compressed zygomatic arch, and it is widely overlaid by the zygomatic process of the squamosal.

The orbit is large, placed slightly off center in the middle portion of the skull (fig. 2). The rounded orbital outline is delimited rostroventrally by the small, triangular lacrimal and the anterior root of the zygomatic arch, and dorsally by the frontal and its postorbital process. The orbit is not bounded caudally by bone as in individuals of some large-sized species of Pteropus and allies (Acerodon, Pteralopex), in which the orbital ligament (lig. orbitale) ossifies to produce a complete osseous bar connecting the postorbital process and zygomatic arch (this bar is sometimes called the jugal spine). The orbital fossa is gently concave and is continuous caudally with the temporal fossa.

Ventral to the orbit is the pterygopalatine fossa, which is formed mainly by the oblique, mediodorsally oriented plane of the perpendicular process of the palatine. There is only a faint infratemporal crest on the alisphenoid delimiting the temporal fossa from the infratemporal fossa in CM 87972. In our sample, some specimens (e.g., AMNH 237593) show a slightly more robust infratemporal crest. In the ventrocaudal conjunction of the orbit and pterygopalatine fossa are the openings of the optic canal and sphenorbital fissure, as well as the transverse cleft that separates the palatine and the pterygoid, the pterygopalatine fissure (fig. 2). Caudal to the sphenorbital fissure, the convex alisphenoid forms the rostroventral border of the braincase. Caudal to that area is the glenoid fossa, barely visible in lateral view, underlying the posterior zygomatic root. Immediately caudal to the glenoid fossa, the external acoustic meatus (meatus acusticus externus) opens laterally, showing an arched dorsal margin that is the attachment area of the small, oval ectotympanic. The manubrium of the malleus is visible inside the ectotympanic. Where the temporal area merges with the occipital region, three successively larger protuberances appear caudally: the posttympanic process of the squamosal, the paracondylar process of the exoccipital, and the occipital condyle.

In lateral view (fig. 2), the braincase as a whole is oval in shape, with the most convex point approximately at the center of the parietal. The braincase shows two distinctive traits. First, the ventral deflection of the basicranial axis with respect to the rostral axis is extraordinarily pronounced, which is all the more noticeable if the rostral axis is held horizontally. If an imaginary line is projected caudally at the level of the alveolar line, it intersects a point on the cranium at the level of the external occipital protuberance of the supraoccipital. Second, the occipital region protrudes caudally, forming the so-called “tubular occiput” (Miller, 1907; Andersen, 1912) typical of adult pteropodine megachiropterans (especially Pteropus). In his description of the skull, Andersen (1912: 61) stated that the “occiput [is] produced backward and downward, as a short tube”. This trait is more easily observed if the skull is held with the basicranial axis aligned horizontally. In that position, the occipital condyles project caudally beyond an imaginary vertical line at the level of the incipient nuchal crest.

Ventral Surface of the Skull

The ventral surface of the skull shows the hard palate (figs. 3, 9), the basipharyngeal canal or choanal region (figs. 3, 10), the basicranium, the auditory region, and the ventral side of the zygomatic arches (fig. 3). The premaxillae project beyond the rostral edge of the canines. A wide I2–C diastema is present. The C–C distance is markedly wider than the maximum premaxillary width. The bulk of the rostrum ends abruptly in front of the canines. Rostral to the C–C line is the large incisive fissure surrounded rostrolaterally by the left and right premaxillae, which project and converge to each other rostromedially. The length of the hard palate (23.8 mm, measured from the rostralmost end of the palatine process of the maxilla to the caudalmost extension of the palatine in the midsagittal line) is considerably greater than its width (15.2 mm, measured as the distance between the lateralmost points of the left and right M1). The upper tooth row is only slightly divergent from the canine root to M2. The interdental palate is a smooth, gently concave surface perforated laterally by foramina of the palatine nerve and vessels complex (see Maxilla, Palatine, and Foramina Contents and Homology). There is a large, caudally projecting postdental palate. Laterally, the postdental palate is roughly straight, converging only slightly medially until it reaches the pterygoid, where the palate contributes to the large ectopterygoid process. Medially, the postdental palate takes the shape of a wide ∩, ending in the midpoint between the caudal border of M2 and the ectopterygoid process. This caudal edge is slightly deflected ventrally, increasing the concavity of the postdental palate. Immediately caudal is the ventrally open basipharyngeal canal, which is roofed by the presphenoid and basisphenoid and framed laterally by the pterygoids (fig. 10). In this area, the nasopharyngeal duct is wide and relatively shallow. Lateral to the pterygoid is the convex surface of the alisphenoid, which is pierced by the large foramen ovale. Lateral to the foramen ovale is the large glenoid fossa and the posterior root of the zygomatic arch.

The basioccipital presents a wide, slightly convex surface that leads caudally to the large opening of the foramen magnum and the projecting occipital condyles. Laterally, the basioccipital has a relatively narrow contact with the petrosal in the posterior basicochlear commissure. Rostral to this commissure is the basicochlear fissure, which on the left side of CM 87972 is confluent with the piriform fenestra, whereas on the right side it is separated from the latter by the small entotympanic (see details in Basioccipital). The entotympanic element is a short, peglike rostral entotympanic, preserved only on the right side in CM 87972 and variously attached to the basicranium in other specimens (see Entotympanic). Caudal to the posterior basicochlear commissure is the large jugular foramen. Caudal to this foramen is the paracondylar process of the exoccipital.

The petrosal lies lateral to the basioccipital. It has a rounded promontorium with a large, caudoventrally directed round window, a large fossa for the m. stapedius caudolaterally, and a large, rather featureless mastoid exposure caudally. The ectotympanic attaches to the border of the external acoustic meatus (of which the posttympanic process is visible ventrally). The ectotympanic is inclined ventromedially. The anterior or rostral process of the malleus is visible on the anterior surface of the ectotympanic ring, with the glenoid fossa in the background. The large postglenoid foramen appears between the glenoid fossa and the external acoustic meatus.

Caudal Surface of the Skull

The occiput (fig. 5) is dominated by the very large foramen magnum. The occipital condyles, the cranial counterparts of the atlanto-occipital joint (articulatio atlanto-occipitalis), are obliquely placed on the lateral (and slightly ventral) margin of the foramen. Flanking the condyles are the small, ventrally oriented paracondylar processes of the exoccipital and, lateral to them, the flat mastoid exposure of the petrosal. The tubular structure of the occiput is evinced, in this view, by the pronounced slope of the supraoccipital area. In adult Pteropus lylei (e.g., AMNH 237593), a strong nuchal crest (crista nuchalis) is present (see Occipital Complex and Comparisons).

Nasal

The nasal (os nasale) is a paired bone that covers the dorsum of the rostrum from its apex to the level of P4 (fig. 1). The nasals have their rostral margin free (fig. 2), and articulate with each other in the median plane, forming the straight internasal suture (sutura internasalis; fig. 1). In turn, each nasal contacts the premaxilla anteroventrally, forming the nasoincisive suture (sutura nasoincisiva), the maxilla lateroventrally, forming the nasomaxillary suture (sutura nasomaxillaris), and the frontal caudally, forming the frontonasal suture (sutura frontonasalis). The dorsal surface of the nasals is roughly flat, with the rostrolateral angle slightly deflected downward (figs. 1, 2). The rostral edge is roughly square and forms the dorsal edge of the external nasal aperture, with little nasal overhang of that aperture. The nasomaxillary suture is gently bowed inward and runs along most of the length of the nasals. The frontonasal suture is W-shaped due to the roughly triangular posterolateral frontal process (processus frontalis) of each nasal penetrating the frontal as a wedge (fig. 1). Laterally, only the anterior tip of the nasals is visible (fig. 2). There are no foramina in the nasals.

Premaxilla

The premaxilla (os incisivum) is a paired bone located in the apex of the skull, forming the ventrolateral rim of the external nasal aperture (figs. 1–Figure 2Figure 34). In the Pteropodidae, the premaxilla is composed of a lateromedially oriented body (corpus ossis incisivi), sometimes called the alveolar process of the premaxilla, and the dorsoventrally oriented nasal process (processus nasalis), whereas the palatine process (processus palatinus) is not ossified. The left and right bodies articulate by a plane suture, the short interincisive suture (sutura interincisiva). Each body bears the alveoli for two incisors (in general, alveoli dentales). The depth of the body increases from the alveolus of I1 to I2, and the alveolar line (margo alveolaris of the premaxilla) gently arches above each tooth.

The nasal process is a laterally compressed lamina that reaches the nasal dorsally, forming the nasoincisive suture, and contacts the rostral process of the maxilla caudally, forming the foliate maxilloincisive suture (sutura maxilloincisiva; equivalent to sutura incisivomaxillaris of the dog). In lateral view, the nasal process shows as a bar of bone directed dorsocaudally, maintaining a roughly constant width throughout its length. In rostral view, the two nasal processes diverge dorsally, forming a V-shaped structure, with the nasals dorsally bridging the gap between the two arms of the V, thereby closing the perimeter of the external nasal aperture. The nasoincisive suture is plane and gently convex. The maxilloincisive suture is incomplete dorsoventrally, so there is a notch separating the ventral half of the premaxilla and the maxilla. As a consequence of this notch, the premaxillary bodies do not articulate with the maxilla, creating a wide diastema between I2 and C.

In ventral view (figs. 3, 9), the left and right premaxillae surround a single large median opening between themselves and the anterior edge of the palatine process of the maxilla. This opening, the incisive fissure (the double fissura palatina of the dog), is formed by the coalescence of the paired incisive foramina (see Foramina below). There are no other foramina in the external surface of the premaxillae.

Maxilla

The maxilla (os maxillare) is paired and is the major component of the rostrum, together with the premaxilla forming the upper jaw. Each maxilla consists of a body with a large external surface or rostral process (facies facialis), which constitutes the bulk of the lateral wall of the rostrum, and a palatine process (processus palatinus), which forms the anterior portion of the hard palate (palatum osseum). At the intersection of these two processes is the alveolar surface (processus alveolaris), which bears the alveoli for six teeth and contributes to the anterior floor of the orbit caudally (figs. 2, 3, 9). The two maxillae articulate with each other along the midsagittal line in the anterior hard palate, forming the intermaxillary suture (sutura intermaxillaris, part of sutura palatina mediana in the NAV).

The rostral process of the maxilla contacts the premaxilla anteriorly, the nasal dorsally (sutures described above), the frontal posterodorsally, forming the frontomaxillary suture (sutura frontomaxillaris), the lacrimal posteriorly in the orbital margin (margo orbitalis), forming the lacrimomaxillary suture (sutura lacrimomaxillaris), and the jugal posteroventrally, forming the zygomaticomaxillary suture (sutura zygomaticomaxillaris). Inside the pterygopalatine fossa (fossa pterygopalatina) ventral to the orbit, the maxilla contacts the lacrimal dorsally and the perpendicular lamina of the palatine posteriorly, the latter forming the palatomaxillary suture (sutura palatomaxillaris in Evans, 1993; sutura palatina transversa in the NAV). Finally, the ventral limit of the rostral process in lateral view is the sinuous alveolar line (margo alveolaris of the maxilla).

The surfaces of the rostral processes are altered by the prominent juga of the canines, whose roots are located high on the side of the nasomaxillary suture. The roots of the postcanine teeth are clearly visible through the bone but do not significantly modify the rostral surface (i.e., the tooth roots do not alter the surface to form alveolar juga). Dorsal to the P4–M1 embrasure (in general, septa interalveolaria) and between the posterior root of P4 and the orbital rim, the rostral process is pierced by the infraorbital foramen (foramen infraorbitale), the anterior opening of the infraorbital canal. This foramen is elliptical in shape with its major axis oriented at ca. 45 degrees to the alveolar line. Inside the left infraorbital canal, a foramen in the floor represents the alveolar canal (canalis alveolaris) for the rostral and middle superior alveolar nerves. Apparently, the same foramen is also present on the right side but is displaced slightly posteriorly into the orbit right behind the ventral margin of the maxillary foramen (foramen maxillare), the caudal opening of the infraorbital canal (fig. 12). No other foramina are present in the rostral process, but traces of a few nutrient foramina (now obliterated) are irregularly distributed on the rostral surface.

The posterior edge of the rostral process describes an oblique line that successively contacts the frontal and the lacrimal, forms the ventral portion of the orbital rim, and continues in the zygomatic arch posteriorly (fig. 2). The frontomaxillary suture is roughly straight and is minutely serrated, running posterolaterally from the externalmost point of nasal–frontal contact to the lacrimal (this area is the frontal process of the maxilla or processus frontalis of the dog). The lacrimomaxillary suture is concave. Ventral to this, the maxilla forms the infraorbital margin (margo infraorbitalis). Posteroventrally, a flat lamina of bone bridges over the short infraorbital canal and continues backward to form the elongated zygomatic process of the maxilla (processus zygomaticus), which contacts the jugal at approximately the midpoint of the ventral orbital semicircumference. Therefore, the anterior zygomatic root lies entirely in the maxilla.

The zygomatic process is slender, with an inconspicuous ventral crest in the masseteric margin (margo massetericus of the maxilla) extending from the alveolar line (at the level of the anterior half of M1) to the contact with the jugal. This crest is for the origin of the m. masseter. The foliate zygomaticomaxillary suture is very irregular, concave laterally, and sinuous medially. Laterally, the zygomatic process rises slightly posteriorly, so the articulation with the jugal is above (ca. 1 mm) the alveolar line. The root of the zygomatic process is horizontally expanded as a flat, roughly triangular plate and joins the rear of the alveolar surface medially in the anterior orbital floor (figs. 1, 3).

The alveolar surface is in the ventrolateral portion of the maxilla. It contains the alveoli for the canine and five postcanine teeth: three premolars (P1, P3, and P4) and two molars (M1 and M2). The caudal portion of the alveolar surface of the maxilla contributes to the anterior orbital floor. In lateral view, the alveolar border (margo alveolaris) describes a low, gentle sinusoid with small rounded peaks in the P1–P3, P3–P4, and (shallower) P4–M1 embrasures, and small rounded valleys in P3 and P4 between the two roots of each tooth. Behind the P4–M1 embrasure, the alveolar border is relatively smooth and straight. The alveolus for the canine is the largest one, oval in shape, with its edge thin and sharply defined laterally and less so medially. Posteriorly, the canine alveolus is partially coalesced with the extremely small alveolus of the minute P1. There are no marked diastemata. The alveoli of P3 and P4 are similar in size and shape; the M1 alveolus is slightly longer than the P3 or P4 alveoli; the alveolus of M2 is half the length of the M1 alveolus but approximately the same width anteriorly, decreasing slightly posteriorly. Ventrally, the posterior edge of the alveolar area follows the outline of M2 and tapers to a small point. This is the pterygoid process of the maxilla (processus pterygoideus; fig. 11), which is a free projection in the dog. In Pteropus, the pterygoid process abuts medially the lateral edge of the palatine bone, and there is no free-standing portion. Within the pterygopalatine fossa, the surface of the posteriorly directed alveolar process is swollen between the shelf of the zygomatic root and the sphenoidal process of the palatine, forming the maxillary tuberosity (tuber maxillae; fig. 12). The two open roots of M1 are visible anteriorly in the orbital floor through their respective alveolar foramina (foramina alveolaria), the anterior one partially concealed by bone, as well as the two smaller (also open) roots of M2 posteriorly. On both sides, the M2 roots are displaced medially from the line of the M1 roots, the latter seemingly in line with the roots of the other cheek teeth. The rostrally directed alveolar canal, which has an intramaxillary course (see below), opens immediately dorsal and rostral to the anterior root of M1. Lateral to the M1 roots lies a large, posteriorly directed, oval opening, the maxillary foramen. The surface of the small portion of maxillary bone immediately dorsal to the anterior root of M1 and the maxillary foramen (i.e., the maxillary portion that contacts the lacrimal in the orbital surface) is porous and shows many minute foramina distributed irregularly, in addition to one slightly larger and distinct foramen present on both sides. At the juncture of the lacrimal, palatine, and maxilla, there is a small vacuity of comparable size and shape on both sides (also present bilaterally in CM 87973), the lacrimal fenestra (fig. 12).

The palatine processes form a transverse shelf of bone that occupies the anterior two-thirds of the hard palate. The free, anterior edge of the maxillary halves is biconcave, each side with a small median point that underlies the vomer's incisive incisure. The intermaxillary suture is slightly raised, forming a low crest that extends from the anterior edge of the canine to the P4–M1 embrasure. Posteriorly, the palatine processes articulate with the paired palatine bones, forming the palatal portion of the palatomaxillary suture (sutura palatomaxillaris). This suture of both maxillary halves with their respective palatine halves has the shape of a high arch or parabola whose vertex is in the midsagittal line at the level of the P4–M1 embrasure. However, the area of the vertex is very irregular and slightly convex. The palatomaxillary suture terminates immediately behind M2. The surface of the palatine process is slightly vaulted toward the midline; this feature increases gently posteriorly and continues smoothly onto the palatine bones. The major palatine foramen (foramen palatinus major) is located on the suture between the maxilla and the palatine (right side) or immediately anterior to that suture (left side). On both sides, a palatine sulcus (sulcus palatinus) extends forward from the major palatine foramen, converging near the midsagittal line and waning until it disappears at the level of P3. There are no other foramina in the palatine process of the maxilla.

Ventral Nasal Concha

The ventral nasal concha (concha nasalis ventralis), or maxilloturbinate, is a paired bone of the nasal cavity (see Internal Surfaces of the Skull). Only its rostral end is visible externally, through the external nasal aperture (figs. 3, 4, 9). In rostral view, the right ventral nasal concha of CM 87972 has a laterally compressed S-shape, with the concavity of a dorsal scroll facing medially, the concavity of a ventral scroll facing laterally, and a barely visible basal lamina that attaches laterally to the conchal crest of the internal surface (facies nasalis) of the maxilla. In CM 87972 the left side is similar to the right side, but the dorsal concavity is slightly smaller. In rostral oblique view, the rostral end is somewhat wedge-shaped. The spaces in the nasal cavity include the relatively wide common nasal meatus (meatus nasi communis) between the two ventral nasal conchae in front of the nasal septum, the middle dorsal meatus (meatus nasi medius) between the dorsal aspect of the conchae and the nasals, and the ventral nasal meatus (meatus nasi ventralis) between the ventral aspect of the conchae and the nasal surface of the maxilla. The rostral end of the ethmoturbinals (ethmoturbinalia) and the nasal septum are also visible through the external nasal aperture (fig. 4), but the surfaces available for external examination are extremely small (see Internal Surfaces of the Skull).

Palatine

The palatine bone (os palatinum) is paired and forms the caudal part of the hard palate, the lateroventral wall of the nasopharyngeal meatus, and its external counterpart, the medial wall of the pterygopalatine fossa (figs. 9–Figure 10Figure 1112). Each palatine has two thin laminae: the horizontal process (lamina horizontalis) and the perpendicular process (lamina perpendicularis). The left and right horizontal processes articulate along the midsagittal line in the posterior hard palate, forming the interpalatine suture (part of sutura palatina mediana in the NAV), which is continuous with the intermaxillary suture.

The horizontal process of the palatine rostrally contacts the palatine process of the maxilla (suture described above) and dorsally contacts the sphenoidal incisure of the vomer, forming the vomeropalatine suture (sutura vomeropalatina dorsalis). The anterior margin of the two palatines (at the transverse suture) is very irregularly concave. The lateral edge is bowed outward. The caudal edge is free and deeply concave, forming a wide ∩-shaped margin. The lateral margins of the postdental palate are slightly bowed inward and converge slightly toward the midsagittal line; in some specimens, a shallow notch is present in the midpoint alongside this margin (e.g., CM 87973). The lateral and caudal edges meet at a caudal point that contacts the pterygoid (see below). The ventral surface is arched, continuing smoothly and accentuating the trend seen in the palatine process of the maxilla. The interpalatine suture is straight and plane. The bone density of the palatine surface (facies palatina) varies in relation to the proximity of the interpalatine suture and caudal palatine margin, where the bone is noticeably thicker. In the remainder of the palatine surface, the bone is translucently thin. A series of five small lateral foramina, aligned with the lateral margin of the palatine, are found near the lateral margin of the palatine surface. The homologies of these foramina are discussed at length below on the basis of the sectioned fetus (see Foramina Contents and Homology). Based on the fetus, the rostral three of the lateral foramina are dependents of the overlying palatine canal (canalis palatinus) for the major palatine nerve and artery, which exit rostrally via the major palatine foramen; these three lateral foramina are identified here as accessory palatine foramina (fig. 9). The caudal two foramina are dependents of another, larger foramen that opens medially on the ventral surface of the palate between the level of the third and fourth lateral foramina, identified as the minor palatine foramen (foramen palatinum caudale). The bilateral discrepancy includes an extra medial foramen on the right side, the double-opening condition of the left fifth lateral foramen, and the location of the first accessory foramen (on the palatomaxillary suture on the right side, on the palatine surface on the left side). There is also a minute nutrient foramen on each side of the interpalatine suture, located in slightly different positions (left side foramen is more caudal; both foramina are placed at the level of the caudal edge of M2). The palatine surface is impressed by a wide sulcus of conical shape that expands medially and whose vertex is at the minor palatine foramen.

The dorsal surface of the palate (facies nasalis) is flat with a bilateral low ridge, the nasal crest (crista nasalis), alongside the interpalatine suture. The sphenoidal incisure of the vomer (see below) articulates with this crest in the vomeropalatine suture.

The perpendicular process of the palatine externally forms the side of the pterygopalatine fossa (fig. 11) and internally forms the lateral wall of the nasopharyngeal meatus (fig. 10). It is oriented dorsomedially at ca. 45 degrees, with a slightly arched surface. It contacts the maxilla rostrally (sutures described above), the frontal dorsally, forming the frontopalatine suture (sutura frontopalatina), the lacrimal anterodorsally, forming the palatolacrimal suture (sutura palatolacrimalis), the presphenoid dorsomedially, forming the sphenopalatine suture (sutura sphenopalatina), and the pterygoid caudally, forming the pterygopalatine suture (sutura pterygopalatina). A vacuity separates the caudomedial edge of the palatine from the rostromedial edge of the pterygoid, termed here the pterygopalatine fissure (fig. 9). The frontopalatine suture is squamous, nearly horizontal, and somewhat irregular. The palatolacrimal suture is a short (ca. 1 mm), oblique contact on the left side, and a point contact on the right side. The elliptical sphenopalatine foramen (foramen sphenopalatinum) is located anteriorly within the palatine at the level of the posterior root of M1 (fig. 12) and is a single round opening on each side (double bilaterally in CM 87973). Caudally, the pterygopalatine fissure is visible on the lateral surface. The dorsal openings of the palatine foramina complex are visible, corresponding to the fourth and fifth lateral palatine foramina described above, which are included in the sulcus that leads to the caudal opening of the palatine canal rostrally. A sphenoidal process (processus sphenoidalis) projects caudoventrally from the external surface of the perpendicular palatine lamina, contributing to the rostral portion of the tripartite ectopterygoid process (the pterygoid and the alisphenoid contribute correspondent parts). The shape of the internal surface of the perpendicular lamina is given by the roughly elliptical perimeter of the choanae in the basipharyngeal canal. This shape is distorted by the nasal crest of the palatine ventrally and by the sphenoid crest dorsally, which together split the nasopharyngeal meatus into paired openings rostrally. The two perpendicular laminae approach the presphenoid dorsally but contact it only rostrally, leaving an oblique cleft that is continuous with the pterygopalatine fissure between the caudomedial end of the palatine and the pterygoid. Another small vacuity is present in the tripartite space between the palatine, the rostral end of the presphenoid, and the caudal edge of the wing of the vomer. The vomerine wing is overlapped slightly by the palatine, but this suture is barely visible externally.

Lacrimal

The lacrimal (os lacrimale) is a paired, roughly pyramidal bone with an external triangular outline, located in the rostral margin of the orbit (figs. 2, 12). The lacrimal has two surfaces, orbital (facies orbitalis) and facial (facies facialis), separated by the orbital crest (crista orbitalis). The lacrimal contacts the palatine caudally, the maxilla rostrally and ventrally (sutures described above), and the frontal dorsally and caudally, forming the frontolacrimal suture (sutura frontolacrimalis). The last one is squamous, roughly straight on the rostral exposure, and crescentic (concave) and somewhat irregular in the orbit. The orbital rim crosses through the lacrimal, forming the orbital crest, a consequence of the two surfaces, facial and orbital, meeting at different inclinations. The facial surface is level with the surface of the rostrum, and the orbital surface lies inside the orbit.

The major feature of the large facial exposure of the lacrimal is the lacrimal foramen (foramen lacrimale), the opening of the lacrimal canal (canalis lacrimalis) that expands in the fossa for the lacrimal sac (fossa sacci lacrimalis). It is located anterior to the orbital rim, immediately dorsal to the suture with the maxilla, which participates in the ventral margin of the foramen (fig. 12) but does not contribute to the lacrimal canal, which lies entirely within the lacrimal. The dorsal margin of the lacrimal foramen is poorly defined. Inside the orbit, there is a small, unnamed foramen that is connected with the major lacrimal foramen via an oblique, anterodorsally directed canal (fig. 12). On the basis of its content (see Foramina Contents and Homology), we term it here the vascular foramen of the lacrimal. The ventral margin of this foramen is contributed by the maxilla on the left side. The orbital surface of the lacrimal is porous, with several minute foramina of irregular distribution, whereas the facial surface is smooth. In CM 87973, there is a small, oval opening in the suture between the right orbital processes of the lacrimal and frontal, slightly below the level of the lacrimal foramen. Most specimens exhibit similar openings, but a shallow pit (with no patent foramina) is present in this position on the left side in CM 87973 and on both sides in CM 87972. Based on the study of the Pteropus fetus, this opening transmits the m. obliquus ventralis and, therefore, is a lacrimal fenestra (Fig. 12; Wible and Gaudin, 2004).

Jugal

The jugal bone (os zygomaticum) is a paired, small, laterally compressed element located at the center of the zygomatic arch between the anterior root of the arch (contributed entirely by the zygomatic process of the maxilla) and the posterior root (contributed entirely by the zygomatic process of the squamosal; figs. 2, 11). The jugal has two surfaces, lateral (facies lateralis) and orbital (facies orbitalis), and two margins, the dorsal infraorbital margin (margo infraorbitalis) and the ventral masseteric margin (margo massetericus). The zygomaticomaxillary suture is described above. The jugal has a long, posteriorly directed process (processus temporalis) that articulates with the squamosal via the foliate temporozygomatic suture (sutura temporozygomatica). The lateral suture with the squamosal starts dorsally at approximately one-third the length of the jugal, continuing diagonally down until the end of the jugal ventrally (at about the midpoint of the zygomatic process of the squamosal). Medially, the jugal contributes a larger portion to the middle arch, but the shape of the suture with the squamosal is still similar to the suture on the lateral surface, only slightly bowed dorsally. The dorsal free margin of the jugal is short and extremely irregular. Immediately dorsal to the masseteric margin on the lateral surface is a noticeable longitudinal striation that runs the length of the jugal, accompanied by another shorter striation that is dorsal and anterior to the major striation. In that area, the bone shows short, irregular incisive marks and small nutrient foramina. Based on the fetus, the m. masseter, pars profunda arises from the medial side of the zygoma and the superficial masseter from the masseteric margin (and a very small part of the ventrolateral surface). The bulk of the lateral surface of the zygoma in the fetus is free of muscle (see fig. 32), suggesting that all muscle attachment probably lies ventral to the longitudinal striation in the adult. On the right side of CM 87972, there is a single nutrient foramen near the dorsal free edge of the jugal. The fetus also has one foramen that transmits a vein.

Cranial Bones (ossa cranii)

Frontal

The frontal bone (os frontale) is paired and forms the forehead of the skull (fig. 1) and the bulk of the orbital wall (pars orbitalis of the frontal; fig. 2). In CM 87972, the frontals are solidly fused with no trace of the interfrontal suture (sutura interfrontalis, by which the left and right frontals articulate along their midsagittal contact). The most prominent feature of the frontal is the large postorbital process (supraorbital or zygomatic process of the dog, processus zygomaticus).

In dorsal view (fig. 1), the frontal is a long, roughly rectangular plate (squama frontalis) with preorbital (pars nasalis), interorbital, and postorbital areas. Paired postorbital processes project from the lateral border just caudal to the midpoint of the dorsal length of the frontal. In the short preorbital area, the frontal contacts the nasal, the maxilla, and the lacrimal (sutures described above). In dorsal view, the interorbital area shows a slight but noticeable constriction. The circular orbital rim (margo orbitalis) continues caudally in the caudoventrolaterally directed postorbital process. The postorbital process has a wide base that tapers caudally to a blunt, squared-off tip (slightly bifid on the right side). The free process overhangs the orbit by 2.3 mm in CM 87972, forming a small triangular roof over the posterior half of the orbital concavity. The rostral margin of the postorbital process is almost straight in dorsal view, whereas the caudal margin is deeply concave. In the rostral margin on the left side is a minute, oblique notch, accompanied by a sulcus, which suggests the passage of a nerve and/or vessel. Based on the Pteropus fetus, this notch and sulcus may accommodate the frontal nerve and vessels (see fig. 31).

Caudal to the postorbital process, the dorsolateral exposure of the frontal is broadly overlapped by a wedge-shaped rostral process of the parietal in the coronal suture (sutura coronalis, equivalent to the sutura frontoparietalis of the dog). This suture is therefore squamous. The suture lines from each side converge slightly until they abruptly turn inward in a roughly right angle to meet each other in the midsagittal line. This point is located approximately midway between the root of the postorbital process and the interparietal.

The dorsal surface of the frontal shows two prominent features: the postorbital foramen and the frontal sinus. In the midpoint of the frontal length, on the rostral half of the root of the postorbital process, the frontal is pierced by the large postorbital foramen. Each foramen is a slightly oval opening that communicates the orbit with the skull roof. In close relationship with the postorbital foramen, there are three (right side of CM 87972) or four (left side) minute foramina associated with the frontal diploic vein. The extra foramen on the left side opens on the dorsal surface of the frontal, close to the inner edge of the postorbital foramen. The frontal sinus (sinus frontalis) can be studied because of the thinness of the frontal bone. Each of the paired sinuses is divided into a medial and a lateral part. The latter is the larger; it follows the orbital rim from the lacrimal to the postorbital foramen and shows a modestly swollen surface. As a consequence, the medial aspect of the interorbital surface is a slightly depressed passage connecting the preorbital and postorbital regions of the frontal. The medial part is not swollen, but its extension can be seen because of the transparency of the bone; it is short and placed medial to the anterior half of the lateral part. Its anterior edge is defined by the sutures with the lacrimal, maxilla, and nasal; the posterior edge of each side converges in the midsagittal line. Posterior to the postorbital processes, the surface of the frontal is smooth and slightly domed.

The frontal occupies most of the orbital wall, where it contacts the lacrimal rostrally, the palatine ventrally (sutures described above), and the orbitosphenoid caudally, forming the sphenofrontal suture (sutura sphenofrontalis). The frontal also contacts the parietal posterodorsally, continuing the frontoparietal suture into the orbit (fig. 2). The sphenofrontal suture is squamous ventrally, and it turns gradually plane dorsally. The shape of this suture resembles one side of a lyre: it is convex caudoventrally, concave caudodorsally, and gently rounded dorsally as it curves posteroventrally to meet the parietal. The thinness of the bone in the orbital surface permits observation of structures of the medial side of the orbital wall. There is a rostrally convex line running from the postorbital foramen to the point of triple articulation between the frontal, the palatine, and the orbitosphenoid. This line marks the placement of the cribriform plate of the ethmoid (lamina cribrosa; see below). Four oblique lines (the dorsalmost line is incomplete) run between the line of the cribriform plate and the rostroventral limit of the frontal. These are lines of internal attachment of the ethmoturbinates (ethmoturbinalia), whose delicate laminae scroll inside the five interlinear spaces. The surface between the line of the cribriform plate and the anterior orbital rim corresponds to a wide squamous suture by which the frontal completely overlaps the ethmoid bone laterally, the frontoethmoidal suture (sutura frontoethmoidalis). The surface between the line of the cribriform plate and the suture with the orbitosphenoid is smooth; its translucence reveals that it is devoid of bony structures medially. In the center of this area is the small, round, ventrally directed ethmoidal foramen (foramen ethmoidale). The ethmoidal foramen is very large in CM 87973 (larger than the optic foramen; see Foramina) and opens between the frontal and orbitosphenoid.

Parietal

The parietal bone (os parietale) is the major element forming the skull roof (figs. 1, 2). The braincase owes its bulbous shape (described above) chiefly to the curvature of the paired parietals. These protect the dorsolateral surface of the brain and provide most of the attachment area of the m. temporalis. In CM 87972, the surface of the bone is entirely smooth, so the limits of the temporal fossa are not evident. In adult Pteropus lylei (e.g., AMNH 237593) and most Pteropus species, however, the temporal fossa is delimited by the sagittal crest on the parietal (plus the infratemporal crest of the alisphenoid and the nuchal crest of the supraoccipital; see also Development).

The parietal has four borders: rostral or frontal (margo frontalis), dorsal or sagittal (margo sagittalis), ventral or squamous (margo squamosus), and caudal or occipital (margo occipitalis). The parietals contact each other in the corresponding sagittal border through the sagittal suture (sutura sagittalis), and in turn each parietal contacts the frontal rostrodorsally (suture described above), the orbitosphenoid rostrolaterally, forming the sphenoparietal suture (sutura sphenoparietalis), the alisphenoid anteroventrally (the continuation of the sutura sphenoparietalis), the squamosal ventrally, forming the sutura squamosa, the interparietal mediocaudally (sutura parietointerparietalis), and the supraoccipital laterocaudally, forming the occipitoparietal suture (sutura occipitoparietalis in Evans, 1993; part of lambdoid suture, sutura lambdoidea in the NAV). There is also a short contact with the mastoid exposure of the petrosal.

The sagittal suture is plane, slightly irregular, and follows the midsagittal line with little deviation. The frontoparietal suture is described above. The rostral border is an oblique suture directed caudoventrally; the suture is squamous, with the parietal slightly overlapping the frontal and the orbitosphenoid. The suture turns rostrally in its ventral quarter to meet the tripartite articulation of the orbitosphenoid, alisphenoid, and parietal. There the ventral border of the parietal begins, as a sinusoidally shaped squamous suture with the alisphenoid. The parietal is overlapped by a rounded, dorsal process of the alisphenoid. The chief suture of the ventral border is with the squamosal, whose dorsal lamina widely overlaps the parietal. The sutura squamosa is broadly wedge-shaped, with a straight anterior border directed dorsocaudally and a subtly convex posterior border directed ventrocaudally. Near the dorsal vertex of this triangle-shaped suture lies the foramen for ramus temporalis, which is dorsally directed with two small openings in the left side and a single, larger opening in the right side. The foramina are between the squamosal and the parietal, with a short sulcus present in the parietal surface leading from the foramen. The suture with the petrosal is < 2 mm in length. Next is the dorsomedially directed occipitoparietal suture. This is a squamous suture (with serrated border) where the supraoccipital is slightly overlapped by the parietal. The parietointerparietal suture shows the same structure; its caudalmost extreme is at the tripartite articulation of the parietal, supraoccipital, and interparietal. From this point, the suture runs rostrodorsally, converging with its bilateral counterpart in the midsagittal line. The curvature of the suture, which describes an arch when both sides are taken together, is somewhat interrupted by a small ingression of the parietal into the interparietal surface near the midsagittal line. This shows as a notch in the interparietal. In this small region are the first signs of fusion between the interparietal and parietal on the left side.

Our study of the Pteropus fetus revealed another part of the parietal that was not readily apparent to us in our study of the intact adult skull (but see Internal Surfaces of the Skull). In the fetus, the parietal contributes an epitympanic wing (sensu MacPhee, 1981): an exposure in the tympanic roof interposed between the sphenoid anteriorly (the basisphenoid and alisphenoids are represented by a single ossification), the squamosal laterally, and the tegmen tympani of the petrosal posteriorly (see figs. 34, 35). This parietal epitympanic wing has a foramen and a sulcus associated with the ramus superior of the stapedial artery (fig. 34). After this discovery, we subsequently identified the parietal epitympanic wing on the left side of CM 87973. It is a small, lobster claw–shaped exposure in the tympanic roof, with the open end of the claw directed anteromedially, positioned lateral to the posterolateral margin of the piriform fenestra. It contacts the alisphenoid anteriorly and medially, the petrosal posteriorly, and is underlain laterally by the squamosal. In the gap between the parietal epitympanic wing and the squamosal is the opening into a small vascular canal directed posterodorsolaterally.

Interparietal

The interparietal (os interparietale) is an unpaired bone that forms the mediocaudal portion of the skull roof between the parietals and the supraoccipital (fig. 1). The parietointerparietal suture is described above. The suture with the supraoccipital (unnamed in the dog, because the interparietal is fused with the supraoccipital prenatally; Evans, 1993) is termed here the occipitointerparietal suture (sutura occipitointerparietalis), part of the lambdoid suture (sutura lambdoidea) in the NAV, which has three sections: two bilateral portions that run from the tripartite point of articulation of the parietal, interparietal, and supraoccipital on each side, converging medially and slightly posteriorly to meet a central portion that is essentially perpendicular to the sagittal axis and slightly concave. This suture is minutely serrated, and there is no significant overlap between the two contacting bones. The surface of the interparietal is smooth and somewhat domed over the central portion of the occipitointerparietal suture, a feature accentuated in the adjacent area of the supraoccipital (see below). No foramina are found in the interparietal.

Pterygoid

The pterygoid bone (os pterygoideum) is a small, paired element located caudal to the palatine in the basipharyngeal canal, ventral to the sphenoidal complex (figs. 3, 5, 10). The pterygoid contacts the perpendicular process of the palatine rostrolaterally (suture described above), the presphenoid rostromedially, and the basi-alisphenoid complex dorsocaudally, forming the pterygosphenoid suture (sutura pterygosphenoidalis). The pterygoid has two surfaces: a nasopharyngeal surface (facies nasopharyngea, medial and facing the nasopharyngeal meatus) and a pterygopalatine surface (facies pterygopalatina, lateral and facing the pterygopalatine fossa). It has a caudoventral angle or hamulus (hamulus pterygoideus; fig. 11), and three processes seen in ventral view (fig. 10): rostromedial, lateral (ectopterygoid), and caudal (all unnamed in the NAV).

The pterygopalatine surface of the pterygoid is concave. Its rostral end projects laterally and joins the respective processes of the palatine and alisphenoid that together form the ectopterygoid process. Caudal to the ectopterygoid process, the lateral margin of the pterygoid runs beneath the sphenoid and describes a crescentic (concave) line that ends at the level of the caudal margin of the foramen ovale (see Alisphenoid and Foramina below). The pterygopalatine surface has a small, round foramen in its center on the left side and two smaller foramina in the same place on the right side in CM 87972; based on the sectioned fetus, these foramina transmit veins.

The medial surface of the pterygoid has a rectangular rostromedial projection that contacts the presphenoid and the basisphenoid. The medial surface is separated from the palatine rostrally by the pterygopalatine fissure. On both sides, there is a posteriorly directed aperture in the midlateral part of the suture with the basisphenoid. Extending anteriorly from this aperture is a faint sulcus on the dorsal surface of the pterygoid open within the cranial cavity. The faint impression of this sulcus is visible ventrally. This aperture represents the pterygoid canal (canalis pterygoideus), which connects the basipharyngeal canal to the cavum epiptericum (sensu Gaupp, 1902, 1905), the extradural space within the cranial cavity housing the trigeminal ganglion. The rostromedial process has a small foramen of uncertain function on its surface near the contact with the presphenoid, almost obliterated on the left side.

The hamulus of the pterygoid is a low and rounded process, laterally concave and slightly divergent caudally, that reaches its maximum depth at its longitudinal midpoint and ascends gently to the level of the basisphenoid as it continues posteriorly (figs. 10, 11). The ventral margin is slightly thickened, forming an inconspicuous lip.

Vomer

The vomer is an unpaired bone of the nasal cavity. It consists of a sagittal part and a horizontal part (Evans, 1993). The former is formed by two low, parallel laminae (laminae lateralis) that rest over the intermaxillary suture, with deeply forked rostral and caudal ends, the incisive and sphenoidal incisures (incisura incisiva et sphenoidalis, respectively), and a ventral crest (crista vomeris). The dorsally opened structure formed by the laminae, the sulcus septi nasi of the dog or sulcus vomeris (septalis) of the NAV, receives the septal cartilage (septum nasi, its cartilaginous rostral part removed in CM 87972), which can be fully ossified in old adults (e.g., AMNH 217045). The horizontal part is formed by bilateral wings (alae vomeris). Only the rostral and caudal ends are accessible for examination through the external nasal aperture (fig. 4) and the choanae, respectively. The incisive incisure is also visible through the incisive fenestra in ventral view (figs. 3, 9). The ventral surface was examined in a disarticulated skull of Pteropus livingstonii (see fig. 24 and Internal Surfaces of the Skull).

Posteriorly, the caudal part of the vomerine wings has a dorsal point contact with the presphenoid (sutura vomerosphenoidalis) only on the right side. The contact of the caudal edge of the vomerine wings with the facies nasopharyngea of the perpendicular process of the palatine (sutura vomeropalatina dorsalis) is barely visible inside the nasopharyngeal meatus. No foramina are present in the vomer.

Sphenoid Complex

The sphenoid complex comprises the unpaired presphenoid (os presphenoidale) and basisphenoid (os basisphenoidale), and the paired orbitosphenoids (ossa orbitosphenoidales) and alisphenoids (ossa alisphenoidales). Together, these bones constitute the rostral two-thirds of the basicranium. The presphenoid is in the midline roof of the basipharyngeal canal (fig. 10); if fused to the orbitosphenoids, as in CM 87972, it is called the corpus of the os presphenoidale. The basisphenoid is in the midline of the basicranium between the presphenoid and the basioccipital; if fused to the alisphenoid, as in CM 87972, it is called the corpus of the os basisphenoidale. The orbitosphenoids are located dorsolateral to the presphenoid, projecting into the orbital wall; if fused to the presphenoid, they are called the orbital (or lesser) wings of the (pre)sphenoid (alae orbitales of the dog). The alisphenoids are located dorsolateral to the basisphenoid, forming the anteroventral side of the braincase; if fused to the basisphenoid, they are called the temporal (or greater) wings of the (basi)sphenoid (alae temporales of the dog). The adjectives “lesser” and “greater” are of little value in megachiropterans because the orbitophenoids are unusually large bones that greatly exceed the alisphenoids in dorsal extension (figs. 2, 11). In CM 87972, conspicuous sutures and gaps distinguish the presphenoid + orbitosphenoid from the basisphenoid + alisphenoid, but there is no evidence of a division between the basisphenoid and alisphenoid, or between the presphenoid and orbitosphenoid (nor are divisions evident in the fetus examined; see fig. 32 and Skull Development). However, each of the four bones will be given a separate treatment for descriptive purposes.

Squamosal

The squamosal (os temporale, pars squamosa) is a paired bone located laterally in the braincase—the flattened squama—bearing a prominent process that arches rostrolaterally, forming the posterior half of the zygoma—the zygomatic process (processus zygomaticus of the squamosal; figs. 2, 5, 11). Ventral to the root of the zygomatic process is the glenoid fossa (figs. 3, 5). The squamosal also contacts the ectotympanic (annulus tympanicus) in the opening of the external acoustic meatus.

The squama is a lamina that widely overlaps the parietal dorsally, the alisphenoid rostroventrally (sutures described above), and the pars canalicularis of the petrosal laterally and caudally, forming the squamosomastoid suture (sutura squamosomastoidea). The shape of the squama is triangular in its dorsal portion, with a vertex pointing directly dorsally at or near the foramen for ramus temporalis in the sutura squamosa (see above). Caudally, the squama almost reaches the incipient nuchal crest, overlapping the pars canalicularis and thus concealing the lateral exposure of the petrosal.

Projecting laterally from the squama is the zygomatic process. The process has a wide, roughly triangular root placed horizontally just rostral to the external acoustic meatus. The dorsal surface of the root is a depression that provides attachment for the m. temporalis. A small, rostrally directed foramen is present on both sides at the posteromedial base of the zygomatic root (only on the right side in CM 87973), and it communicates with the postglenoid foramen (foramen retroarticulare; see below). The sectioned fetus does not have a comparable foramen. In the ventral surface of the zygomatic root is the glenoid fossa, formed entirely within the squamosal; it is elliptical in shape, wider than long, and is limited anteriorly by the concave rostral margin of the zygomatic root, and posteriorly by the gradually sloping, blunt postglenoid process (processus retroarticularis; see fig. 43). Immediately posterior to the postglenoid process is the postglenoid foramen, a large, round, ventrally directed opening (figs. 3, 5, 10). Medial to the glenoid fossa, the squama shows a triangular lamina that overlaps the alisphenoid (suture described above). From the root, the zygomatic process arises as a thin bar of bone directed first laterally, then rostrodorsally. The process describes a low arch as it reaches the orbit, where it contacts the jugal. The jugal underlies the anterior half of the zygomatic process (suture described above). As is the case with the rest of the arch, the zygomatic process of the squamosal is laterally compressed and shows two surfaces: medial (facies medialis) and lateral (facies lateralis). Both surfaces are smooth. The dorsal margin of the process is thin, whereas the ventral margin has a longitudinal concavity for the attachment of the m. massetericus.

The squamosal contributes the arched dorsal margin of the external acoustic meatus. The meatus begins rostrally immediately ventral to the postglenoid foramen, where the anterior leg or crus of the ectoympanic attaches through ligaments (see below). It ends in the blunt posttympanic process (processus retrotympanicus), located below the level of the opening of the postglenoid foramen, receiving ventrally the posterior leg of the ectotympanic. The posttympanic process is buttressed medially by the posterior continuation of the crista parotica of the petrosal.

Petrosal

The petrosal (os temporale, pars petrosa) is the paired bone in the cranial base that houses the organs of hearing and equilibration. Two divisions of the petrosal are generally recognized: the more anteroventromedial pars cochlearis, enclosing the cochlear duct and the saccule of the inner ear, and the more posterodorsolateral pars canalicularis, enclosing the utricle and the semicircular canals.

In CM 87972, the petrosal has three surfaces largely uncovered by other bones: the tympanic surface within the middle ear (facies tympanica of the dog), the cerebral and cerebellar surface within the cranial cavity (facies encephalica of the dog), and the mastoid exposure on the occiput (processus mastoideus of the dog). The pars cochlearis has about a 2-mm-long, plane contact with the basioccipital medially, point contacts with the basisphenoid anteromedially on either side of the carotid foramen, and a point contact with the alisphenoid anterolaterally at the posterolateral corner of the piriform fenestra. The pars canalicularis (via its concave, triangular facies occipitalis) contacts the exoccipital and supraoccipital posteromedially, forming the occipitomastoid suture (sutura occipitomastoidea), a foliate suture. Most of the lateral surface of the pars canalicularis, roughly trapezoidal in shape, is overlaid by the squamosal, forming the squamosomastoid suture, a squamous suture, at the junction of the occiput and the sidewall of the braincase, where the lateral aspect of the mastoid exposure abuts the posterior aspect of the squamosal. Between its contacts with the squamosal and the supraoccipital, the pars canalicularis has a short plane suture with the parietal. It also has about a 1-mm plane contact with the epitympanic wing of the parietal anterolaterally. Finally, the petrosal has contacts with the entotympanic, ectotympanic, incus, and stapes (see below). In CM 87972, four apertures are found in the anterior and lateral borders of the petrosal. From anterolateral to posteromedial, these are the piriform fenestra between the petrosal, basisphenoid, and alisphenoid; the carotid foramen (foramen caroticum internum of the dog) between the petrosal, basisphenoid, and entotympanic; the basicochlear fissure (fissura petrooccipitalis) between the petrosal and basioccipital; and the jugular foramen (foramen jugulare) between the petrosal, exoccipital, and basioccipital (right side only).

Because much of the petrosal in the intact skull is not readily accessible for study, we use as the centerpiece of our descriptions isolated petrosals of Pteropus livingstonii AMNH 274477, with differences from the CM P. lylei noted (figs. 13–Figure 1415). The therian petrosal is roughly the shape of a tetrahedron with the following four surfaces: tympanic or ventral, encephalic or dorsal, squamosal or lateral, and mastoid or lambdoid (MacIntyre, 1972; Wible, 1990). We treat each of these four sides of AMNH 274477 separately below.

In tympanic view, the most prominent feature is the dome-shaped promontorium, the main part of the pars cochlearis (fig. 13). The promontorium is not a symmetrical dome; its lateral aspect is flatter than its medial, and it is longer than wide. The shape of the promontorium is reflective of the enclosed cochlear duct, which Gray (1907) reported to be two coils in Pteropus medius (= P. giganteus), although he did not specify how the measurements were made. Two apertures are found in the posterior and posterolateral aspects of the promontorium: the round window (fenestra cochleae) and oval window (fenestra vestibuli), respectively. The fenestra cochleae is oval, wider than high, and directed posteriorly and slightly ventrally. It is closed in life by the secondary tympanic membrane (membrana tympani secundaria). The dorsal margin of the fenestra cochleae is recessed within a shallow cochlear fossula (sensu MacPhee, 1981), the posteroventral margin of which is formed by the medial section of the caudal tympanic process (see below). The cochlear fossula is broader in the CM Pteropus lylei than in AMNH 274477. The fenestra vestibuli, which accommodates the footplate of the stapes, is also oval, with a stapedial ratio of 1.81 (length/width, according to Segall, 1970). The fenestra vestibuli is directed laterally and slightly ventrally and sits within a shallow vestibular fossula (sensu MacPhee, 1981). The rim of the vestibular fossula has the same shape as the oval window except for its ventral margin, which is convex rather than concave. As a consequence of its position and orientation, the oval window is largely hidden in direct ventral view. At the anteromedial corner of the promontorium is a small, triangular process that is directed anteroventrally (apex parties petrosa). Based on CM 87973, this process is underlain by the basisphenoid, and together these form the lateral margin of the carotid foramen. The anterolateral surface of the promontorium has a very shallow depression; based on the Pteropus fetus (fig. 34), this is the fossa for the m. tensor tympani (fossa m. tensor tympani of the dog).

The ventral surface of the pars cochlearis in CM 87972 has an additional process wholly lacking from AMNH 274477. This process is a narrow, rounded shelf that is directed ventromedially from the medial aspect of the promontorium. The anterior end of this shelf is underlain by the entotympanic. There is a shallow, longitudinal concavity where this shelf meets the anteromedial promontorial surface that is directed at the carotid foramen and represents a carotid sulcus. In CM 87973, this shelf is hypertrophied, being both wider and longer, and contacts the basisphenoid to contribute to the medial margin of the carotid foramen.

The bulk of the pars canalicularis in tympanic view is represented by a broad shelf lateral to the promontorium. Near the lateral margin of this shelf is a sinuous ridge of varying height that extends the length of the shelf. The most prominent point on this ridge is posteriorly, where there is a short process that is directed toward, but falls short of, the back of the promontorium. This process is the tympanohyoid or tympanohyal (tympanohyoideum), the ossified proximal segment of Reichert's (hyoid) cartilage; the ridge to which the tympanohyoid is fused is the crista parotica. The segment of the tympanohyoid proximal to the crista parotica is rod-shaped posteriorly, with a narrow anterior shelf. Distally, the rod-shaped part bifurcates into a short anterior leg forming the medial edge of the anterior shelf and a longer posterior leg that tapers to a point. Based on CM 87972, the posterior crus of the ectotympanic abuts the ventral surface of the proximal tympanohyoid. The crista parotica extends both anteriorly and posteriorly from the tympanohyoid. The sharp posterior continuation curves dorsally onto the mastoid exposure and is covered anterolaterally by the posttympanic process of the squamosal. At the junction of the tympanohyoid and the posterior continuation of the crista parotica is the exit of the facial nerve from the middle ear, the stylomastoid notch (foramen stylomastoideum in the dog). The more rounded anterior continuation of the crista parotica decreases in height anteriorly and, opposite the oval window, leads to a crescentic depression: the fossa incudis for the crus breve of the incus. The fossa incudis is not flat, but is angled such that its lateral aspect is dorsal to its medial aspect. Based on CM 87972, the squamosal forms the stout lateral wall of the fossa incudis. The lower medial wall of the fossa incudis is formed by a sharp ridge that is continuous with the crista parotica. This ridge increases in height anteriorly, levels off to a flat area, and then decreases in height to the anterolateral margin of the petrosal. This ridge is the ventral edge of the tegmen tympani, which is unusual in bats in that it has a near vertical component rather than merely forming a horizontal roof over the middle ear (Wible and Novacek, 1988; Wible, 1992; Wible and Martin, 1993). As noted in other bats previously (Wible and Novacek, 1988), in the Pteropus fetus, the tendon of the m. tensor tympani wraps around the back of the ventral edge of the tegmen tympani (fig. 35), an arrangement that likely influences the direction of muscle action.

Medial to the crista parotica and the ventral edge of the tegmen tympani is a broad trough that curves around the lateral margin of the promontorium (fig. 13). This trough extends nearly the length of the pars canalicularis, but falls short of the anterolateral border. Just behind that border, this trough begins at a round, posteriorly directed foramen. This is the secondary facial foramen, by which the facial nerve enters the middle ear from the cavum supracochleare (Voit, 1909; geniculum canalis facialis). Forming the floor of the secondary facial foramen is a horizontal part of the tegmen tympani that connects to the promontorium. The trough from the secondary facial foramen to the stylomastoid notch is occupied by the facial nerve and, therefore, is the facial sulcus. However, in the narrow interval between the stylomastoid notch and the posterolateral margin of the petrosal, the trough is occupied by the m. stapedius and, so, is the fossa for the m. stapedius (fossa m. stapedius of the dog).

The pars canalicularis in tympanic view is also represented by a narrow shelf of bone posterior to the promontorium. The ventral margin of this shelf is the medial section of the caudal tympanic process of the petrosal, or CTPP (sensu MacPhee, 1981), which contributes to the dorsal margin of the cochlear fossula. The medial section of the CTPP is concave and, in direct posterior view, nearly completely shields the round window from view. The CM Pteropus lylei differ from AMNH 274477 in their inflation of this region. The medial section of the CTPP is more bulbous, extending the posterior margin of the petrosal, and more prominent, fully shielding the round window in direct posterior view. Whereas the CTPP is a simple, concave ridge in AMNH 274477, in the CM P. lylei the central portion of this concave ridge is raised, delimited by sharp edges, and resembles an anvil.

The encephalic view (fig. 14) is dominated by two large depressions, the smaller internal acoustic meatus (meatus acusticus internus) anteroventromedially and the larger subarcuate fossa (fossa subarcuata) posterodorsolaterally. The former lies in the pars cochlearis and the latter in the pars canalicularis. An oblong aperture (porus acusticus internus) leads into the internal acoustic meatus, which is divided into superior and inferior fossae by a low transverse crest (crista transversa). The smaller superior fossa (foramen acusticum superius) includes the facial nerve area (area nervus facialis) anterolaterally and the dorsal vestibular area (area vestibularis superior) posteromedially, for passage of the facial nerve and part of the vestibular nerve, respectively. The inferior fossa (foramen acusticum inferius) includes the large, kidney-shaped cochlear area (area cochleae) and the small, round ventral vestibular area (area vestibularis inferior), for the cochlear nerve and the remainder of the vestibular nerve, respectively. Within the cochlear area is the spiral tract of minute foramina (tractus spiralis foraminosus) for the fascicles of the cochlear nerve. The subarcuate fossa, which houses the paraflocculus of the cerebellum, is a deep, wide depression that occupies a considerable portion of the pars canalicularis. Leading into the subarcuate fossa is a kidney-shaped aperture that is narrower than the subarcuate fossa proper. Because of the thinness of the bone, the posterior semicircular canal (canalis semicircularis posterior) and the crus commune (crus osseum commune), which lie in the rim of the subarcuate fossa, are visible. Also visible is the part of the anterior semicircular canal (canalis semicircularis anterior) proximal to the crus commune.

Three additional apertures are visible on the encephalic surface (fig. 14). The largest of these is on a narrow shelf of bone anterolateral to (and sloping away from) the foramen acusticum superius and is partially hidden in direct dorsal view by the bar of bone (prefacial commissure) forming the lateral margin of the internal acoustic meatus. This anteriorly directed, oval aperture is the hiatus Fallopii, the anterior opening of the cavum supracochleare. Based on CM 87971, part of the shelf of bone posterolateral to the hiatus Fallopii is covered by the parietal (see below). The second aperture is on a narrow shelf of bone posteromedial to the foramen acusticum inferius the jugular incisure (incisura jugularis). This foramen is also partially hidden in direct dorsal view as it faces posteromedially into the jugular foramen. This is the opening by which the perilymphatic duct (ductus perilymphaticus) enters the petrosal, the cochlear canaliculus (apertura externa canaliculus cochleae). The third aperture is on a narrow shelf medial to the subarcuate fossa. This shelf has a faint depression, the fossa cerebellaris, and in the posterior part of this fossa is the oval, posteriorly directed opening by which the endolymphatic duct (ductus endolymphaticus) enters the petrosal, the vestibular aqueduct (apertura externa aqueductus vestibuli). A shallow sulcus leads into the vestibular aqueduct from behind. Lastly, posterodorsal to the subarcuate fossa (and running parallel to it) is a short, very shallow sulcus for the capsuloparietal emissary vein (sinus temporalis).

The most dominant feature in squamosal view (fig. 15) is the trapezoidal surface covered by the squamosal in the intact skull. The irregular ventral margin of this surface is formed by the crista parotica posteriorly and the tegmen tympani anteriorly. The posteroventral aspect of this surface has a noticeable bulge, which in the intact skull is covered by the posttympanic process of the squamosal. This bulge corresponds in position to the mastoid process (processus mastoideus in Schaller, 1992). As noted by MacPhee (1981), that term has been used for a variety of probably non-homologous bumps on the petrosal. Following Wible and Gaudin (2004), we prefer the term paroccipital process for this bulge. The anterodorsal aspect of the trapezoidal surface has a very shallow sulcus for the capsuloparietal emissary vein that is continuous with the sulcus reported on the encephalic surface. Anterior to the trapezoidal squamosal surface is a roughly triangular surface whose vertex is the ventral edge of the tegmen tympani. The ventral half of this shelf is covered by the parietal, based on CM 87971.

The mastoid exposure of the petrosal (figs. 5, 13, 15) is roughly rectangular, taller than wide. Dorsally, it contacts the parietal laterally and the supraoccipital medially. The contact with the parietal is shorter (ca. 2 mm) and straight; that with the supraoccipital (ca. 3 mm) is sinuous, with a degree of interdigitation. Medially, the mastoid exposure contacts the exoccipital at a slightly concave suture; this joint shows some interdigitation in the dorsal part. Laterally, it contacts the squamosal at a convex suture. The free ventral margin is concave, with a sharp prominence in the ventrolateral corner, the posterior continuation of the crista parotica. In CM 87973, a distinct ridge curves dorsomedially from the ventral end of the posterior continuation of the crista parotica and continues onto the paracondylar process of the exoccipital; this ridge is very faint in AMNH 274477. Based on the fetus, the m. sternomastoideus (Evans, 1993; musculus sternocephalicus, pars mastoidea in the NAV) attaches to this ridge, the posttympanic process of the squamosal, and the paracondylar process of the exoccipital. At the dorsomedial corner of the mastoid exposure in CM 87972 is a small mastoid foramen (foramen mastoideum), which is 0.4 mm on the left and smaller on the right. The petrosal only contributes to the border of the left mastoid foramen. Based on Pteropus capistratus AMNH 194276, the mastoid foramen is at the junction of the exoccipital and supraoccipital with the mastoid exposure.

Entotympanic

A rostral entotympanic (rostrales entotympanicum of Klaauw, 1922) is present in Pteropus, including the species described here. Novacek (1980, 1991) figured the rostral entotympanic in P. poliocephalus. This tiny, peglike element is located in the basicranium ventral to the juncture of the basisphenoid and basioccipital, at the basicochlear fissure (it is lost from the left side of the macerated skull of CM 87972). The bone presents a roughly tetragonal shape (figs. 3, 5, 10). The medial, roughly triangular surface has a concave caudal margin and a rostrodorsal vertex. The base of the entotympanic underlies the occipitosphenoid suture; the lateral and caudal sides are partially hidden. The shape is far more irregular in other specimens, and in AMNH 237596 the right entotympanic is divided in two parts. In adult specimens, when it is preserved, the entotympanic may be caudally displaced, reaching the petrosal and even fusing to it, while losing contact with the basisphenoid and exoccipital (e.g., in AMNH 237593). In the Pteropus fetus, the cartilaginous rostral entotympanic is an irregular U-shaped structure that sits beneath the anterior pole of the promontorium (see reconstruction in Wible and Martin, 1993: fig. 6). The main body of the rostral entotympanic lies anteriorly (below the carotid foramen in fig. 33) and has two posteriorly directed curved arms, one ventromedial and the other dorsolateral (the posterior end of which is seen in fig. 34).

In addition to a rostral entotympanic, the Pteropus fetus has a flat cartilaginous caudal entotympanic (caudales entotympanicum of Klaauw, 1922), longer than wide, which abuts the medial aspect of the posterior crus of the ectotympanic and fills in the space between that bone, the promontorium, and the rostral entotympanic (figs. 33–Figure 3435; see reconstruction in Wible and Martin, 1993: fig. 6). There is no trace of this element in the osteological specimens, which suggests that, unlike the rostral entotympanic, the caudal entotympanic does not ossify in the adult.

Ectotympanic

The ectotympanic (annulus tympanicus) is a small, ring-shaped bone located ventral to the concave margin of the squamosal between the postglenoid and posttympanic processes, in the dorsal border of the external acoustic meatus (joint unnamed in the NAV; figs. 2, 3, 5). The ectotympanic is elliptical in shape (fig. 16A, B) and conspicuously inclined ventromedially (figs. 3, 16C, 43). In ventral view, the annulus steadily increases in width rostrodorsally (fig. 3). Its irregular medial margin has a small lamina flaring medially (fig. 3; the styliform process of Henson, 1970). The annulus is horseshoe-shaped and has two arching branches (crura), anterior and posterior, that are continuous ventromedially (fig. 16). The crus anterior abuts the medial margin of the squamosal and extends its contact from the level of the rostral edge of the postglenoid foramen to beyond the midpoint of the arch of the external acoustic meatus (fig. 2). The lateral margin of the anterior crus is thickened. The crus posterior is noticeably thinner, and it has a restricted area of contact with the tympanohyoid (see Petrosal). The crura do not contact each other dorsolaterally, being separated by a space of ca. 1 mm. The annulus has a deep medial sulcus (sulcus tympanicus) that is roughly constant in width throughout (fig. 16C). The tympanic membrane (membrana tympani) attaches to the medial border of the sulcus. The ectotympanic and tympanic membrane form the lateral limit of the tympanic (or middle ear) cavity (cavum tympani).

In CM 87972, both ectotympanics are in place, precluding a full view of the medial aspect of the bones. Examination is possible in Pteropus livingstonii AMNH 217044, whose ectotympanics are detached from the skull base. In this specimen, the ectotympanic shows a slightly expanded medial border with the small flange described above projecting directly medially. Dorsally, the rostral process of the malleus (see below) overhangs the anterior crus. The rostral process is fused ventrally to the anterodorsal surface of the anterior crus medial to the raised lateral border of the crus. Seams are clearly visible in the contact of the rostral process and the anterior crus.

Middle Ear Ossicles

The three middle ear ossicles (ossicula auditus)—the malleus, the incus and the stapes—lie in the tympanic or middle ear cavity (cavum tympani in the auris media). As in mammals in general, the malleus is formed embryologically by two components from the fetal lower jaw (Klaauw, 1922; Jurgens, 1963). The malleus per se is derived from the articular. The second element is the rostral or anterior process, which is continuous with the malleus (hence the term outer lamella of processus gracilis in Jurgens, 1963) and homologous to the gonial bone (= prearticular). The incus (derived from the quadrate) and the stapes successively transmit sound to the inner ear through the fenestra vestibuli of the petrosal. Doran (1878) gave a rather superficial description of the malleus, incus, and stapes of Pteropus, based on P. hypomelanus and P. edulis (most likely a synonym of P. vampyrus), and Wassif (1948) provided a detailed description of the ossicles in Rousettus aegyptiacus. A description of the three bones, based on P. lylei and P. livingstonii, follows.

Occipital Complex

The occipital complex (os occipitale) is formed by the unpaired basioccipital (pars basilaris) and supraoccipital (squama occipitalis), and the paired exoccipitals (pars lateralis; fig. 5). These four embryological units, each ossified from separate centers, surround the foramen magnum, which connects the brain with the spinal cord and associated structures (fig. 5). The bones are fused seamlessly in CM 87972 except for traces of a suture between the basioccipital and exoccipitals in the medial margin of the jugular foramen (see fig. 5 and below). For that reason, precise relationships among these bones will be discussed based on Pteropus capistratus AMNH 194276, a younger specimen in which all sutures are present, in the Skull Development section.

Hyoid Apparatus

The tongue and the larynx are suspended from the skull by the hyoid apparatus (apparatus hyoideus), a derivative of the second and partially the third pharyngeal arches (arcus pharyngeus secundus et tertius; Nomina Embriologica Veterinaria, 1994 [hereafter NEV]). In general aspect (fig. 19), the hyoid apparatus is formed by a short transverse bar from which two long, hornlike structures arise bilaterally, one roughly dorsocaudolateral in position, whose proximal end attaches to skull in the ear region (the anterior cornu or hyoid arch), and one caudolateral, which articulates with the laryngeal cartilages (the posterior cornu, formed by a single element, the thyrohyoid, which is derived from the third branchial arch). Specifically, the hyoid apparatus is composed of six elements: the single basihyoid (basihyoideum) and the paired thyrohyoid (thyrohyoideum), ceratohyoid (ceratohyoideum), epihyoid (epihyoideum), stylohyoid (stylohyoideum), and tympanohyoid (tympanohyoideum). In Megachiroptera, the basihyoid is fused to the thyrohyoids, and the stylohyoid is not ossified proximally so it does not contact the tympanohyoid (tympanohyal); the remaining elements are all ossified and are connected by minute synovial joints (Sprague, 1943). These joints may fuse in adults of some forms, except the joint between the basi- and ceratohyoid, which generally remains a synovium in megachiropterans (Sprague, 1943).

Based on Rousettus (Jurgens, 1963), the basihyoid and thyrohyoids are continuous already in the embryo. The basihyoid component of this unit is an ossification of the basihyoid cartilage (cartilago basihyoidea, a composite originated in the pars ventralis of both the second and third branchial arches). It is a transverse bar that widens laterally toward the articulation with the ceratohyoid (see fig. 19 and below). The basihyoid is roughly cylindrical in shape, slightly bowed, and depressed dorsoventrally. The basihyoid contacts the ceratohyoid through an elliptical synchondrosis located in the rostrolateral angle of the basihyoid. The thyrohyoid (also termed cornu branchiale [Jurgens, 1963] and cornu majus [NAV]) is an ossification of the thyrohyoid cartilage (cartilago thyrohyoidea) of the pars ventralis of the third branchial arch (first branchial arch in Sprague's [1943] terminology). The thyrohyoids project caudolaterally from the basihyoid as slender elements twice as long as the transverse dimension of the basihyoid. In dorsal view, the thyrohyoids significantly diverge caudally; in lateral view, they form a pronounced inverted arch with almost perfect curvature. Each thyrohyoid is laterally compressed, and its caudal half forms a flange that is slightly deflected laterally. The rounded distal end of the thyrohyoid is the hyoid component of the thyrohyoid articulation (articulatio thyrohyoidea). In USNM 566608, a cap of cartilage is present in the proximal end (i.e., the end opposite to the basihyoid).

The ceratohyoid (also termed cornu minus in the NAV) is an ossification of the ceratohyoid cartilage (cartilago ceratohyoidea) of the pars ventralis of the second branchial arch (hyoid arch of Sprague [1943] and Jurgens [1963]). It is a short and flat piece that links the basihyoid with the proximal elements that suspend the hyoid apparatus from the skull (fig. 19). The ceratohyoid exhibits an apparent bending, because the major axes of its two elliptical synchondroses in the opposite ends of the bone are oriented differently: the distal synchondrosis (connecting with the basihyoid) faces medially, whereas the proximal synchondrosis (connecting with the epihyoid) faces caudodorsally. The ventrolateral angle is free, wide, and rounded. There exists a fundamental difference with respect to the ceratohyoid of the dog, as illustrated by Evans (1993: figs. 4–Figure 5Figure 6Figure 7Figure 8Figure 9Figure 10Figure 11Figure 12Figure 13Figure 14Figure 15Figure 16Figure 17Figure 18Figure 19Figure 20Figure 21Figure 22Figure 23Figure 24Figure 25Figure 26Figure 27Figure 28Figure 29Figure 30Figure 31Figure 32Figure 33Figure 34Figure 35Figure 36Figure 37Figure 38Figure 3940, 4–Figure 5Figure 6Figure 7Figure 8Figure 9Figure 10Figure 11Figure 12Figure 13Figure 14Figure 15Figure 16Figure 17Figure 18Figure 19Figure 20Figure 21Figure 22Figure 23Figure 24Figure 25Figure 26Figure 27Figure 28Figure 29Figure 30Figure 31Figure 32Figure 33Figure 34Figure 35Figure 36Figure 37Figure 38Figure 39Figure 4041). In the dog, the proximal end of the ceratohyoid is directed almost cranially and meets the epihyoid at a right angle. In Pteropus, the ceratohyoid is aligned with the epihyoid and stylohyoid in the dorsocaudolateral direction.

The epihyoid is an ossification of the epihyoid cartilage (cartilago epihyoidea) of the pars dorsalis of the second branchial arch (Reichert's cartilage). It is a simple, slender rod that contacts the ceratohyoid distally and the stylohyoid proximally (fig. 19). Its section is roughly circular, although a slight ventral ridge is present. The epihyoid exhibits an apparent torsion, because the oval-shaped synchondroses are oriented differently: the major axis of the distal synchondrosis is horizontal, whereas its counterpart in the proximal synchondrosis is vertical.

The stylohyoid is an ossification of the stylohyoid cartilage (cartilago stylohyoidea) of the pars dorsalis of the second branchial arch. It is incompletely ossified in Megachiroptera, as is also the case in our specimens. The stylohyoid is a laterally compressed, short bar that shows an unreduced synchondrosis in its distal end, and a free proximal end (fig. 19). Thus, the bony contact with the tympanohyoid is lost, as in all megachiropterans—the discreto-cornuate condition of Sprague (1963), as opposed to the integro-cornuate condition found in Microchiroptera (and many other mammals), in which all elements of the anterior cornu are ossified and the stylohyoid articulates with the tympanohyoid. In Rousettus, the tympanostyloid ligament connects the tympanohyoid and the stylohyoid (Jurgens, 1963; Sprague, 1943), but such a ligament was not found in Pteropus (Sprague, 1943). The structure and location of the tympanohyoid have been described in the tympanic view of the petrosal, because this element remains attached (and may be fused) to the crista parotica in many specimens. The tympanohyoid forms a joint with the petrosal (articulatio temporohyoidea), which serves to suspend the hyoid apparatus and larynx from the skull in forms in which the proximal anterior cornu is completely ossified (e.g., microchiropterans).

In our limited sample, there is no obvious difference between species and sexes in the structure and relative size of elements in the hyoid apparatus of Pteropus tonganus (USNM 566608, subadult female, and USNM 566610, subadult male) and Pteropus dasymallus (FMNH 14052, adult female, and FMNH 140650, adult male). The hyoid apparatus is structurally similar but larger and markedly more robust in Pteropus giganteus (FMNH 57661, adult female). An additional difference found in P. giganteus is the presence of a tubercle in the ventralmost point of the arch of the thyrohyoid, probably an attachment point of at least the m. thyrohyoideus.

Larynx

The four component elements of the larynx are the single epiglottic cartilage (cartilago epiglottica, missing in our specimens), thyroid cartilage (cartilago thyroidea), and cricoid cartilage (cartilago cricoidea) and the paired arytenoid cartilage (cartilago arytenoidea). These cartilages belong to the partes ventrales of the last three branchial arches (arcus pharyngeus quartus, quintus, et sextus; NEV). The basihyoid + thyrohyoid, and the thyroid and cricoid cartilages form a telescopic arrangement of movable pieces, each of which fits within the space of the next and therefore reduces the diameter of the laryngeal orifice caudally (see Jurgens, 1963: fig. 13). In USNM 566608, the inner distance between the caudal ends of the left and right thyrohyoids is 7.4 mm; the inner distance between the left and right caudal cornua (see below) of the thyroid cartilage is 3.7 mm; and finally the minor (horizontal) axis of the inner ring of the cricoid cartilage is 2.9 mm. The thyroid and cricoid elements, as well as the tracheal rings, are ossified in adult Pteropus (Sprague, 1943).

The thyroid is a trough-shaped structure that fits within the space between the thyrohyoids and articulates with them cranially (fig. 19). The thyroid is formed by right and left laminae (lamina dextra et sinistra) united ventrally (fig. 19). Dorsally, the lamina projects the rostral and caudal cornua (cornu rostralis et caudalis). The thin rostral cornu is the thyroid component of the thyrohyoid articulation, and it is noticeably expanded toward the joint (cranially). An ample rostral thyroid notch (incisura thyroidea rostralis) excavates ventral to the cornu at the expense of the cranial edge of the lamina. The caudal cornu is comparatively shorter and less expanded distally. A correspondingly less pronounced caudal thyroid notch (incisura thyroidea caudalis) is present in the caudal margin of the lamina. The end of the caudal cornu is the thyroid component of the cricothyroid articulation (articulatio cricothyroidea). In dorsal view, the dorsal margin of the laminae diverges cranially to meet the thyrohyoids. In lateral view, the laminae slightly reduce their width ventrally. The middle part where the laminae meet is projected cranially and approximately follows the curvature of the caudomedial margin of the basihyoid + thyrohyoid (in its postmortem position, the thyroid and the basihyoid + thyrohyoids are separated by approximately 1 mm). In the dog, the cranioventral and caudoventral margins of the thyroid exhibit a laryngeal prominence (prominentia laryngea) and a thyroid fissure (fissura thyroidea), respectively. In our Pteropus specimens, the former is absent, but a conspicuous thyroid fissure is present (fig. 19). Both the medial and lateral surfaces of the laminae are smooth.

The cricoid is a complete ring that fits within the space between the thyroid laminae; it is formed by a dorsal lamina (lamina cartilaginis cricoideae) and a ventrolateral arch (arcus cartilaginis cricoideae; fig. 20). The lamina is about 5 mm long, with caudal and lateral margins chiefly straight and parallel, and a triangular cranial margin. On the side of this triangle is the indistinct facet of the cricoid component of the cricoarytenoid articulation (articulatio cricoarytenoidea). The dorsal surface of the lamina is divided by an irregular median crest (crista mediana), and the sides are inclined ventrolaterally as a two-sided roof. The cricoid component of the cricothyroid articulation is a marked tubercle in the caudolateral angle of the lamina. Immediately cranial to it, the lateral margin of the lamina is inflected medially and then continues cranially in the lateral crest, which neatly separates the lamina from the arch. The latter is ventrally no wider than 1 mm. In lateral view, the caudal margin of the arch is roughly vertical, so the cranial edge of the arch is comparatively much longer than the caudal margin, and reaches the cranial edge of the lamina describing a gentle sigmoid line. The cranial margin of the arch is slightly enlarged as a lip. The arch is significantly thinner and becomes translucent in its lateral surface. The caudal margin of the arch connects via ligaments to the first tracheal ring.

The paired arytenoid cartilage is a small element confined to the articular area of the craniolateral facet of the cricoid lamina, in the craniodorsal region of the cricoid. The arytenoid cartilage of Pteropus is greatly simplified in comparison with the cartilage of the dog. In Pteropus, it exhibits three delicate processes that emerge from a poorly defined body and are oriented in different directions. The process located dorsally is the longest and thinnest, and is craniocaudally compressed, and its dorsal end forms the first (dorsomedial) of two points of contact of the arytenoid cartilage with the cricoid. The other two processes are similar in shape, much shorter than the dorsal, each tapering to a round point. The ventrocaudal process is the second contact with the cricoid. The cranioventral process is free. The whole arytenoid cartilage is curved such that it follows the outline of the laryngeal orifice.

Nasal Surface

The facies interna of the nasal is the roof of the nasal meatuses. It is a featureless surface, slightly concave, with subparallel margins that are slightly curved ventrally. The ventral view of the nasal generally follows the external shape of the bone; that is, it has a straight medial border and a gently bowed lateral border. The caudal portion of the bone, which is not entirely visible externally because it is overlapped dorsally by the maxilla in the nasomaxillary suture, overlaps in turn the frontal in the frontonasal suture, expanding laterally into a triangular wedge of very thin bone.

Additionally, the isolated nasal preserves the internal surfaces of two squamous sutures, the nasoincisive and nasomaxillary sutures, and one thin plane suture, the internasal suture. The first two sutures are coalesced anteriorly in the nasal. The surface of overlap forms a lateral shelf on the nasal separated from the externally visible medial part by a sharp longitudinal line, corresponding to the external outline of the two sutures. The internal surface of the internasal suture forms a low, slightly rugose ridge—the septal process (processus septalis)—of constant size throughout the anterior four-fifths of the nasal length. There is no nasoethmoidal crest or fossa, because the nasal does not contact the ethmoid.

Premaxillary Surface

The internal surface of the premaxilla is gently concave and generally smooth. The caudal edge of the premaxilla corresponds to the premaxillary part of the maxilloincisive suture. The maxilla overlaps the premaxilla in the ventral half of this suture, and the opposite is true for the dorsal part. In the ventral part, the suture exhibits a low ridge caudally and a shallow groove laterally. The surface of the groove is rugose and is penetrated by two alveolar foramina that connect directly with the large and conical alveoli of the two upper incisors (see alveolar canal below). The foramina are close together in the dorsal portion of the groove, with the dorsal foramen larger than the ventral foramen. The former supplies the alveolus of I1 and the latter the alveolus of I2. The openings of the alveoli are subequal in size, circular in outline, and separated by a thin but firm wall of bone. The depth of the alveoli can be determined from external examination because the bone is very thin. The alveolus of I1 extends half the length of the entire premaxilla, whereas the alveolus of I2 does not surpass a third of that length. A pair of very subtle juga is observable in association with the alveoli on the internal surface of the premaxilla. Ventrally on the internal surface, a small depression is present in the space between the two alveoli. The internal surface of the intermaxillary suture is roughly rectangular, higher than wide, presenting a slightly rugose surface.

The dorsal part of the maxilloincisive suture is suggested, in the premaxilla, by a change in coloration of bone in the overlap area (but see the maxillary counterpart below). In the nasoincisive suture, the nasal is laterally overlapped by the premaxilla, but the extension of this overlap is very limited; it shows as a very subtle thinning of the dorsal margin of the premaxilla.

Maxillary Surfaces

In Pteropus livingstonii AMNH 274466, the ventral nasal concha is firmly attached to the medial surface of the maxilla, precluding examination of some structures in medial view. The ventral nasal concha was missing from P. livingstonii AMNH 274515, thus exposing the medial surface of the maxilla. Both specimens were used for the description of this region (fig. 21A, B).

The facies nasalis of the maxilla is essentially a medial concave surface with one transverse shelf, the palatine process. The latter meets the medial surface of the maxillary body, a medially inclined wall to which the ventral nasal concha attaches medially through a long conchal crest (crista conchalis). The internal surfaces of the right and left maxillae contain the nasal meatuses, and the ventral nasal concha divides the nasal meatus into three distinct spaces. These divisions are the common nasal meatus (meatus nasi communis), medial to the conchal lamina, the dorsal nasal meatus (meatus nasi dorsalis), dorsal to the conchal crest (further separated by the rostral process of the ethmoturbinal from another space ventral to it, the middle nasal meatus or meatus nasi medius), and the ventral nasal meatus (meatus nasi ventralis), ventral to the conchal crest. The most conspicuous features of the facies nasalis (see details below) include the nasolacrimal canal, the alveolar canal, the internal jugum of the canine, and the wide dorsal lamina that is part of the squamous sutures with the nasal, lacrimal, and frontal.

In the midsagittal plane, two sutures are present: the plane intermaxillary suture and, dorsal to it, the vomeromaxillary suture (sutura vomeromaxillaris). The latter runs along the entire length of the intermaxillary suture, decreasing its dorsal development caudally. The dorsal surface of the palatine process is horizontal, generally smooth, and meets the maxillary body laterally in a gently concave surface that rolls to meet the conchal crest dorsally. A tiny foramen is present bilaterally in this surface at the level of the posterior root of P4, 1 mm lateral to the midsagittal plane. The concave caudal edge of the palatine surface is notched for the opening of the major palatine foramen.

The conchal crest extends from the rostral edge of the maxilla to approximately the level of the anterior root of P4. In medial view, the crest is gently convex dorsally and protrudes medially ca. 1 mm. The alveolar canal (incisivomaxillary canal of the dog, canalis maxilloincisivus) runs immediately dorsal to the crest in the substance of the maxilla. This canal supplies the alveoli of the premolars, the canine, and finally the incisors (unlike the dog, the alveolar canal also supplies the root of P4 in Pteropus). It opens caudally in a small, round foramen located medial to the maxillary foramen, just above the anterior (dorsally open) root of M1 (fig. 12). In our sample, the alveoli of the two roots of P3 and the single root of the canine are perforated by multiple alveolar foramina, whereas the alveoli of each of the two roots of P4, M1, and M2 open through a single foramen. The course of the alveolar canal is visible externally in several specimens of P. lylei, particularly AMNH 237595 (see fig. 39). In this specimen, the alveolar canal follows a curved rostrodorsal path, caudally convex, from the anterior M1 root medial to the infraorbital canal to near the canine root. This latter point is horizontally at the level of the lacrimal foramen and vertically at the level of the anterior root of P3.

One very conspicuous feature of the facies nasalis is the nasolacrimal canal (canalis lacrimalis). Preceded caudally by a wide sulcus lacrimalis, the nasolacrimal canal opens caudally at the level of the posterior root of P3, and slightly ventral to the middle of the facies nasalis dorsal to the conchal crest (fig. 21B). The opening itself is rather irregular and leads to an oblique intramaxillary course that crosses the conchal crest. The nasolacrimal canal opens in the ventral nasal meatus at the level of the P1–P3 embrasure. The course of the nasolacrimal canal is marked as a subtle relief in the facies nasalis. More conspicuous, the internal canine jugum is visible on the facies nasalis both dorsal and ventral to the conchal crest.

The dorsal margin of the maxilla is dominated by the wide nasomaxillary suture (fig. 21). The thin lamina by which the maxilla overlaps the lateral margin of the nasal increases its dorsal extension caudally to form the frontal process of the maxilla (processus frontalis), the maxillary component of the frontomaxillary suture. This process, which in the intact skulls lies between the caudolateral angle of the nasal and the lacrimal, is truncated caudally and leads ventrally to the wide, concave notch into which the lacrimal fits. Following this notch to its caudal end and immediately dorsal to the infraorbital canal is a triangular platform on which the lacrimal rests. Just medial and ventral to this platform for the lacrimal, as well as medial to the infraorbital and alveolar canals, is a thin and irregular sheet of bone that is the maxillary part of the ethmoidomaxillary suture (sutura ethmoidomaxillaris).

The rostral margin of the maxilla is entirely occupied by the maxilloincisive suture. The premaxilla fits in the deeply incised maxillary margin and is overlapped by the maxilla both medially and laterally. The rostral opening of the alveolar canal is in the middle of the suture and appears as a faint, ventrally directed sulcus. This canal divides within the substance of the maxilloincisive suture, appearing as a double canal (the division leading to each of the two incisor alveoli) on the premaxillary surface of the suture. The caudal edge of the facies nasalis is deeply concave, forming the perpendicular portion of the palatomaxillary suture.

The ventral nasal concha is an elongated vertical lamina of porous bone attached laterally to the conchal crest (fig. 21A; see also fig. 4). The shape of the concha is complex, but it can be described basically as a lamina from which four short folds arise. One fold is in the dorsal edge, as it scrolls laterally and then ventrally. The fold of the ventral edge is double, with one lamina projecting laterally and dorsally, and another lamina projecting medially and then dorsally. The last fold arises from the middle of the concha on the medial side, and scrolls ventrally, leaving a deep excavation in the ventral part of the concha. In medial view, the concha is cigar-shaped. The rostral edge is triangular, with a vertex pointing rostrally. The ventral edge is slightly convex, and it folds laterally and dorsally in the caudal two-thirds of the conchal length. The dorsal edge presents a conspicuous concavity in the rostral third. From the rostral edge of this concavity, a groove arises and crosses the medial surface obliquely in a caudoventral direction to the middle of the concha. The surface dorsal and caudal to this groove is flat and more lateral than the surface rostral and ventral to this groove. In this dorsal recess fits the rostral process of the ethmoturbinal, which lies caudal, medial, and slightly dorsal to the ventral nasal concha. After the mentioned concavity, the dorsal margin of the ventral nasal concha is convex, then it descends gently throughout the caudal extension of the concha to meet the caudal end of the bone at the level of the ventral margin. A deep, longitudinal groove excavates the medial surface of the concha between the scroll of the ventral margin and the midline, approximately at the level of the lateral attachment to the conchal crest. The dorsal edge of that groove forms the medial midline fold, whose border is altered by a continuation of the oblique groove for the dorsal concha in the form of a ventral tip. The caudal end of the ventral nasal concha exhibits a distinct caudolateral flange. The lateral view of the ventral nasal concha is not available to direct examination, but it can be inferred from the presence of the conchal crest and the development of the dorsal and ventral folds.

Palatine Surface

In the isolated palatine bone, the surfaces of the nasopharyngeal meatus (facies nasalis) and the large, squamous frontopalatine suture in the perpendicular process are the most salient internal features (fig. 22). The left and right portions of the nasopharyngeal meatus, each contained in an individual palatine bone, are each oval in outline with the medial side open and communicating with the corresponding portion in the other palatine. Medially and ventrally, the nasopharyngeal meatus is bounded by the nasal crest, which decreases in height rostrally. The articular surface of the nasal crest, or interpalatine suture, is very rugose. Medially and dorsally, the meatus is bounded by the thin sphenoethmoid lamina (lamina sphenoethmoidalis). Anteriorly, this lamina forms a medial flange partially roofing the meatus, decreasing its width rostrally. The meatus itself is a smooth tube that gradually funnels caudally into the choanae. Near the rostral end of the palatine is the opening of the sphenopalatine foramen. The crescent-shaped space between the sphenopalatine foramen and the rostral edge of the palatine is the palatine contribution to the maxillary recess (recessus maxillaris). The recess is limited ventrally by the ethmoidal crest (crista ethmoidalis), a low, rostrally concave ridge connecting the sphenopalatine foramen with the rostroventral angle of the nasopharyngeal meatus. One (left side) or two (right side) extra, smaller openings of the sphenopalatine foramen are present caudal to the main foramen in AMNH 274466 (all of them open to the lateral side of the perpendicular lamina; cf. CM 87972, 87973). Dorsal to the sphenoethmoid lamina, and oriented at a right angle to it, is the palatine contribution to the frontopalatine suture. Through this thin lamina, the palatine overlaps the pars orbitalis of the frontal bone in the frontopalatine suture laterally and also contacts the ethmoid medially in the palatoethmoidal suture (sutura palatoethmoidalis). Consequently, the frontal and ethmoid fit in the space between the sphenoethmoid lamina and the dorsal portion of the perpendicular lamina, unlike the dog, in which the contact is with the vomer and not with the ethmoid. This difference results from the reduced wings of the vomer in Pteropus (see below).

The rostral end of the palatine, not visible externally, is truncated and formed by the very thin bone of the squamous palatomaxillary suture (fig. 22). Here the palatine is overlapped widely by the maxilla laterally. The rostral external surface of the isolated palatine shows a deep wedge accommodating two connected parts of the maxilla: the caudal part of the palatine process of the maxilla medial to the maxillary tuberosity, and the pterygoid process of the maxilla. Just medial to this area, on the ventral side, is the exposed course of the palatine canal, which opens through the major palatine foramen in the palatine side of the palatomaxillary suture.

The caudal sphenoidal process (processus sphenoidalis) contributes to the ectopterygoid process (fig. 10). The sphenoidal process shows two deep concavities where the other two components of the ectopterygoid process fit: dorsolaterally is a larger, teardrop-shaped surface for the articulation of the alisphenoid contribution to the ectopterygoid process, and ventromedially is a smaller surface for the articulation of the pterygoid. The two surfaces are separated caudally by a thin and sharp lamina of bone.

Fronto-Ethmoidal Surfaces

The frontal, ethmoid, and vomer are solidly fused in the disarticulated specimens of Pteropus livingstonii available for examination (figs. 23, 24A, B). Based on AMNH 274466, two distinct internal surfaces can be distinguished. One surface is dorsocaudal, comprising the inner table of the frontal (facies interna) and the cribriform plate of the ethmoid (lamina cribrosa; fig. 23). The other surface (fig. 24) is ventrolateral and rostral, comprising the ventral surface of the vomer, the rostroventral and partially lateral surfaces of the ethmoid (pleurethmoid of Jurgens, 1963), and the rostral process of the ethmoturbinal (anterior tip of ethmoturbinal of Jurgens, 1963).

The facies interna of the frontal roofs the rostral cranial fossa (fossa cranii rostralis) and provides the dorsal and lateral (orbital) walls of the ethmoidal fossae (fossae ethmoidales), the space immediately posterodorsal to the cribriform plate (fig. 23). The facies interna of the frontal is a cup-shaped surface. A faint trace of cerebral juga corresponds to the bilateral mark left by the longitudinal fissure of the brain, which is reproduced in the facies interna as two rostrally divergent curved lines, separated caudally by 1 mm and rostrally by ca. 4 mm. The circular border of the postorbital area of the frontal bone is rather irregular. In dorsal view, the extent of the parietal overlap, through its wedge-shaped frontal process of the parietal, becomes evident in the disarticulated skull. The exposed forehead is limited to the median rectangular space marked bilaterally by a groove parallel to the sagittal plane at the level of the postorbital foramen. The caudal margin shows a frontoparietal suture that is part plane and part squamous. In the midsagittal line, a wedge-shaped medial process of the caudal margin of the frontal fits into a complementary recess of the rostral border of the parietal (the plane segment of the frontoparietal suture). Lateral to this suture, the frontal slightly overlaps the parietal via a bilateral wedge-shaped process of its caudal border (the squamous segment of the frontoparietal suture).

Rostral to the rostral cranial fossa are the bilateral ethmoidal fossae, each containing an olfactory bulb (bulbus olfactorius). In caudal view (fig. 23), the fossae form an ample space, twice as high as wide, with rounded corners and slightly narrower ventrally. This space is ill-defined as two chambers, corresponding to each ethmoidal fossa, with the division marked by a crest in the dorsal wall (contributed by the frontal) and by a low crista galli in the rostral wall, the cribriform plate of the ethmoid (lamina cribrosa). The thin and low crista galli is the dorsalmost part of the perpendicular lamina of the ethmoid (lamina perpendicularis), which forms the osseous nasal septum (septum nasi osseum). The cribriform plate is inclined so that the dorsal edge is rostral with respect to the ventral edge. The plate exhibits numerous perforations, the cribriform foramina (foramina laminae cribrosae), for the passage of bundles of the olfactory nerves (nn. olfactorii). The cribriform foramina are organized into tracts whose positions are, based on a partially damaged specimen of Pteropus livingstonii (AMNH 274515), related to the attachment of turbinates on the rostral side of the cribriform plate. In P. livingstonii AMNH 274466, the cribriform foramina are grouped into two columns, medial and lateral, within each ethmoidal fossa (fig. 23). The medial column consists of four irregular groups. The dorsal group is the largest and consists of one principal perforation with a bilaterally variable number of very small perforations, especially in the walls of the main foramen as they sink into the ethmoidal labyrinth (labyrinthus ethmoidalis). The same pattern is found in the other main foramina of the medial column, which are more closely associated among themselves. In the dorsolateral angle of the ethmoidal fossa is a group of two (right side) to four (left side) main foramina surrounded by some smaller perforations. Immediately ventral to these is the lateral series of cribriform foramina. These are grouped into five clusters of one or two larger foramina accompanied by smaller perforations, located close to the lateral edge of the cribriform plate. The left and right ventralmost groups of foramina are placed closer to the crista galli. The bone between the medial and lateral columns of foramina, as wide as one of those columns, is perforated occasionally and irregularly by small foramina, but it is essentially solid. The lateral walls of the ethmoidal fossae are smooth, presenting a wide ethmoidal notch (the ethmoidal foramen in the articulated skull, with its caudal concavity closed by the orbitosphenoid). Finally, the dorsal aspect of the ethmoidal fossae is biconcave, formed by two elongated domes separated by the low frontal crest (crista frontalis). The morphology of the cribriform plate of P. livingstonii (this study, fig. 23) is very similar in every respect to the morphology reported by Bhatnagar and Kallen (1974) for Pteropus giganteus (see their plate 1, fig. 8, p. 85).

The ventral surface of the frontoethmoidal complex includes the vomer (medially and rostrally), the ventral surface of the ethmoid (lamina basalis, caudolaterally), and the ventral surface of the rostral process of the ethmoturbinal (laterally and rostrally; fig. 24B). In the disarticulated skull, the vomer is fully exposed in ventral view. As long as half the skull length, the vomer extends from the incisive fissure to the sphenoidal sinus. Both ends (the incisive and sphenoidal incisures) are visible externally and, therefore, are described above (see Vomer). Ventrally, the vomer exhibits a paired wing (ala vomeris) in the caudal third of its length (only partially visible externally), each separated medially by a longitudinal crest (crista vomeris). The wings are very narrow, ca. 1 mm wide throughout their length. The suture with the ethmoid (sutura vomeroethmoidalis) is almost straight and barely visible. These alae vomeris are continuous caudally with the bifid ends of the sphenoidal incisure. Due to their narrow width, the alae form only the medial part of the roof of the nasopharyngeal meatus immediately rostral to the choanae. The lateral part of the roof is contributed by the ventral surface of the ethmoid (the pleurethmoid of Jurgens, 1963). The ventral surface of the ethmoid is smooth and thin, and has a roughly rhomboidal shape, with the vomer in the midline, and four borders (two caudolateral and two rostrolateral). The truncated caudal edge of the ethmoid forms the rostral edge of the sphenoidal sinus, which in ventral view appears as a paired notch lateral to the caudal edge of the vomer. Through the rostral opening of the sphenoidal sinus, the nasal septum is visible. In close examination, traces of the suture between the nasal septum (perpendicular plate, lamina perpendicularis) and the cribriform plate are visible. The caudal view of the sphenoidal sinus, on the sides of the nasal septum, shows two rostrocaudally elongated scrolls attached dorsally to the ethmoid, with its ventral border folding laterally (fig. 23). Based on Rousettus (Jurgens, 1963), these are endoturbinates (or primary ethmoturbinals; endoturbinalia), specifically endoturbinate III. The labyrinthus ethmoidalis of Rousettus exhibits one ectoturbinate and three endoturbinates, all of which are bilaminar, and a well-developed crista semicircularis that arises from the tectum nasi close to the nasal septum (Jurgens, 1963). Based on a partially damaged specimen of Pteropus livingstonii (AMNH 274515), we were able to preliminarily confirm a similar composition and configuration of ethmoidal elements in Pteropus, but a detailed anatomical description of the labyrinth requires further study.

In the caudolateral border is the frontoethmoidal suture, in which the dorsolateral component contributed by the frontal and the medioventral component contributed by the ethmoid meet at an almost right angle. The frontoethmoidal suture is incomplete in sealing the caudolateral border, leaving a cleft that is larger on the right side in AMNH 274466. The rostrolateral border of the ethmoid exhibits a ventral process from the lateral lamina of the ethmoid and the fused base of the rostral process of the ethmoturbinal medially (fig. 24).

The rostral process of the ethmoturbinal attaches to the rostroventral side of the ethmoid and projects rostrally in the form of a ventrally open tube, presenting a medial and a lateral wall joined dorsally, and defining internal and external surfaces (fig. 24A, B). The rostral process of the ethmoturbinal decreases in height and width rostrally, acquiring a roughly conical shape. The dorsal surface is domed. The medial wall is roughly vertical and is deeper than the lateral wall. The lateral wall folds its ventral margin medially. Caudally, the lateral wall exhibits a deep incisure that is separated from another, more caudal incisure by a thin, vertical, zigzag column of bone (probably part of the ectoturbinate, based on Rousettus; Jurgens, 1963). The caudalmost incisure is separated from the ethmoid by an oblique ridge that is expanded ventrally as a short flange. The space between this ridge and the frontoethmoidal suture caudally has two deep, oblique grooves. In the caudal part of this area is the articulation with the lacrimal in the lacrimoethmoidal suture (sutura lacrimoethmoidalis of the dog, probably equivalent to the sutura lacrimoconchalis of the NAV). Ventral and caudal to this part, and adjacent to the frontoethmoidal suture, is the ethmoidal component of the ethmoidomaxillary suture. All the surfaces involved are formed by noticeably porous bone. The caudal portion of the ventral nasal concha fits inside the hollowed cone of the rostral process of the ethmoturbinal, and the cleft between both conchae is the middle nasal meatus.

Presphenoid Surface

In the dorsal surface of the presphenoid-orbitosphenoid complex (fig. 25A), the most salient features are the large alae orbitales (orbitosphenoid), the long corpus (presphenoid), the yoke (jugum sphenoidale), and the optic canals (canales optici). The presphenoid body has two shallow optic grooves, each leading to their respective optic canals, separated by a medial, low, and rounded midsagittal crest, and demarcated laterally by a low ridge at the union of the alae with the corpus caudal to the optic canals. The alae are fused seamlessly to the corpus. Lateral to the corpus is the paired, spine-shaped, very small rostral clinoid process (processus clinoideus rostralis). Laterally, the alae connect smoothly to the body and, bridging over the optic canals, connect themselves again dorsal to the corpus. The outline of each optic canal is somewhat oval, with the dorsal orbitosphenoidal crest (crista orbitosphenoidalis) forming the flattened dorsal margin. The yoke, the rostral union of the left and right alae, is dorsally gently concave. The alae are very wide and emerge from almost the entire side of the long corpus. The alae first spread widely laterally, more so caudally, and then they gently curve dorsally, keeping a roughly uniform width until reaching the rounded dorsal margin. The caudal alar margin shows a distinct notch representing the rostral margin of the sphenorbital fissure, caudal and lateral to the optic canals. The elevated, roughly vertical part of the alae is oriented obliquely with respect to the midsagittal plane, converging rostrally. The anterior interalar space, where the caudal frontoethmoidal complex fits, is U-shaped in caudal view. Externally, the alae show the wide overlap of the orbitosphenoid's portion of the squamous sphenoparietal suture along the posterior third of the external surface. The corpus itself has truncated anterior and posterior ends. The anterior end shows the very small, paired cavity that the presphenoid contributes to the sphenoidal sinus (apertura sinus sphenoidalis). Ventral and slightly caudal to the sphenoidal sinuses, there is a third median cavity that probably houses a rod of cartilage (lost) connecting the presphenoid with the vomer and nasal septum rostrally. The caudal end has a mushroom-shaped section and is also in contact with a rod of cartilage (lost in most macerated specimens, but present, for instance, in Pteropus giganteus ROM 75747 and Dobsonia pannietensis AMNH 159099) linking the presphenoid and basisphenoid.

Basisphenoid Surface

The dorsal aspect of the basisphenoid-alisphenoid exhibits two large temporal alae (the alisphenoid component) united by a truncated pyramidal corpus (the basisphenoid component; fig. 25B). On each side of the truncated rostral end of the corpus is the short, spinelike pterygoid process (of the basisphenoid). The outline of the rostral end is dorsally convex, with two lateral sides converging in the ventral midline, with all angles rounded. The dorsal surface of the corpus has a large central depression, the sella turcica, with its hypophyseal fossa (fossa hypophysialis) flooring the hypophysis in life. The fossa is oval in shape, longer than wide, with its surface very smooth, bounded bilaterally by low, rounded ridges, and caudally by a thicker ridge, which represents a weak dorsum sellae. Caudally, the corpus presents a rather flat, lip-shaped articular surface for the cartilage of the spheno-occipital synchondrosis (lost). Lateral to this synchondrosis is the shallow carotid sulcus (sulcus caroticus), which transmits the contents of the carotid foramen to the sella turcica. The medial border of the carotid sulcus projects caudally as a short spine. Immediately lateral to the sulcus is a low ridge that coincides topographically with the lingula sphenoidalis of the dog and human.

The dorsal surface of the alae (facies cerebralis) is large, roughly triangular in shape, with a rostral vertex and a caudomedial base. The surface is gently concave, presenting principally two notches, medial and lateral, in the rostral edge immediately lateral to the corpus, and a large foramen ovale in the substance of the ala. The lateral notch forms the caudal edge of the sphenorbital fissure, and projects ventrally a footlike pterygoid process that rests on top of the junction of palatine and pterygoid, and is part of the ectopterygoid process. The larger medial notch presents a deep wedge shape; it is floored by the rostromedial process of the pterygoid, and thus it is not visible externally. The large foramen ovale is elliptical in shape and oriented obliquely so the major axes of the left and right foramina converge rostrally. This foramen, the result of the fusion of the foramen ovale and the caudal alar foramen of the dog, is in the center of the basal alar concavity. A fairly well marked sulcus ascends gently from the rostral edge of the foramen to the sphenorbital fissure, presumably transmitting the contents of the absent alar canal. Caudolaterally, the ala forms a lobe, the epitympanic wing of the alisphenoid, which forms the rostral margin of the piriform fenestra. The epitympanic wing projects into the ear region, where it contacts the petrosal along a serrate suture, and also the epitympanic wing of the parietal. The lateral alar border is rather irregular. The lateral side of the epytimpanic wing shows the small (ca. 1 mm wide) squamous sphenosquamosal suture, by which the squamosal overlaps the alisphenoid laterally, seen in ventral view. In dorsal view, this suture shows as a short, inverse alisphenoid-squamosal overlap, immediately rostral to the epitympanic wing of the alisphenoid, where the ventral and medialmost portions of the squamosal, in the medial angle of the glenoid fossa, contact the alisphenoid and form a partly serrated, partly squamous suture. The triangular rostral edge of the ala, in dorsal view, is divided into two portions, each representing a squamous suture. The smaller medial portion corresponds to the unnamed suture by which the alisphenoid overlaps the orbitosphenoid medially. The second suture corresponds to the portion of the sphenoparietal suture by which the alisphenoid overlaps the parietal medially.

Parietal Surface

The left and right parietals, as well as the interparietal, are articulated in AMNH 274466, forming a parietal complex that is treated as a whole here (fig. 26). Deeply concave, the ventral surface of the parietal complex (facies interna of the parietal, lamina interna of the cranium) reproduces the bulbous shape and some external features of the underlying brain. These features are the digital impressions (impressiones digitatae), separated by intermediate ridges, which correspond to the cerebral gyri and sulci, respectively.

In AMNH 274466, the plane, linear interparietal suture is clearly visible for most of its length, on both dorsal and ventral surfaces, becoming indistinguishable only ca. 1 mm rostral to the contact with the parietointerparietal suture. The latter is still visible, but it exhibits a higher degree of fusion. Against a light source, it is possible to see that the parietointerparietal suture is largely a plane suture, with small areas of overlap (by the parietal, dorsally) only in the rostral and the lateral margins.

Rostrally, the parietal widely overlaps the frontal in the coronal (= frontoparietal) suture, a large triangular area of thin bone well delimited caudally by a neat line at the level of the rostral edge of the parietal in the median plane. The externally visible area of the sinciput fits in the rectangular space between the two triangular areas of the frontoparietal suture (fig. 1). The caudal edge of that space (i.e., the rostral margin of the parietal in the median plane) has a V-shaped incisure in the sagittal suture.

The ventral margin of the exposed parietal is complex and presents several angles and borders, for which we propose the following nomenclature (fig. 26B). From rostral to caudal, the angles are the frontal (angulus frontalis), sphenoidal (angulus sphenoidalis), epitympanic (new term), vascular (new term), and mastoid (angulus mastoideus); the margins or edges are the frontal (margo frontalis, ventral part), squamous (margo squamosus), epitympanic (new term), and caudoventral (new term). The frontal angle, as described above, is the rostralmost extension of the parietal and forms the apex of the frontoparietal suture. The ventral part of the frontal margin is straight; it is inclined ventrocaudally until reaching the ventralmost point in the parietal, where it terminates at the sphenoidal angle, which marks the beginning of the sutura squamosa. The sphenoidal angle is located slightly rostral to the midpoint of the parietal complex. The squamous margin arches dorsally, descending again to the ventral tip of the triangular epitympanic wing of the parietal (i.e., the epitympanic angle), at about three-quarters of the parietal length. While the internal surface of the parietal lacks traces of sutures in this area, the external surface (facies externa) of the disarticulated parietal shows the wide overlap area of the sutura squamosa. Dorsal to the arched ventral margin, the area of the sutura squamosa extends generously over the lower region of the parietal's dorsal surface, ascending from the sphenoidal angle to a point high above the epitympanic wing. The caudal margin of the epitympanic wing, or epitympanic margin, is roughly vertical and terminates at the caudoventral margin of the parietal at an almost right angle.

In the angle where the epitympanic and caudoventral margins meet, called here the vascular angle, a complex configuration of vascular sulci is present. Two different vessel systems are involved, and they cross each other in this angle. These are the capsuloparietal emissary vein and the posterior division of the ramus superior of the stapedial artery. The capsuloparietal emissary vein drains the blood from the intracranial transverse sinus, partially encapsulated in a short sulcus in the internal surface of the parietal. The petrosal closes the canal for the capsuloparietal emissary vein medially and caudally, which becomes evident with the petrosal in place (e.g., in the broken skulls of Pteropus neohibernicus AMNH 152402 and P. vampyrus AMNH 198691). The capsuloparietal emissary vein arrives in a space immediately dorsal to the opening of the temporal canal (see below), between the squamosal and the parietal. This space appears as a depression in the external surface in the vascular angle, and is especially evident in caudolateral view in AMNH 274466. Then, the capsuloparietal emissary vein enters directly into the dorsal opening of the temporal canal in the medial side of the squamosal, exiting the skull through the postglenoid foramen (see below). In turn, the ramus superior follows an inverse, dorsally directed course (marked, in the parietal, as a shallow sulcus on the epitympanic margin), reaching the space in the angle between the caudal epitympanic edge and the caudoventral parietal margin. In that space, the posterior division of the ramus superior crosses the more external side of the space, and a conspicuous sulcus for the vessel, not visible externally, appears. Immediately dorsal, on the right side, the sulcus divides into a smaller, caudal sulcus, leading to the posterior foramen for ramus temporalis, and a larger, rostral sulcus, which continues further dorsally, leading to the anterior foramen for ramus temporalis. Both foramina open in the sutura squamosa, and their course between the squamosal and the parietal is not exposed externally. Both sulci continue 2–3 mm dorsally in the free dorsal surface of the parietal. On the left side, a single, wider sulcus reaches the sutura squamosa, and a corresponding single foramen for ramus temporalis is present.

The internal surface of the parietal reveals the relatively small size of the sulcus for the transverse sinus, as compared with the dog. The transverse sulcus has a strong, vertically directed medial wall that fuses with the skull roof some 5 mm dorsal to the vascular angle. Two nutrient foramina are present bilaterally in the sulcus. This is more clearly visible in articulated specimens without the skull roof, such as Pteropus neohibernicus AMNH 152402 and Pteropus vampyrus AMNH 198691. A low sulcus for the transverse sinus, gently concave caudally, bridges the vascular angle and the midsagittal line. The left and right sulci meet at the junction of the interparietal and parietointerparietal sutures. The rostral edge of the sulcus is more marked than the caudal edge. Rostral to the sulci, the parietal surface is lightly marked by digital impressions and intermediate ridges. There are two main intermediate ridges of thickened bone. The medial intermediate ridge runs rostrocaudally, separated from the interparietal suture by ca. 5 mm, and extends caudally up to half the distance between the medial rostral edge of the parietal and the ridge of the transverse sulcus. The lateral intermediate ridge is somewhat divergent rostrally, but is still roughly parallel to the medial intermediate ridge and separated by ca. 3 mm from it. Another, barely marked ridge extends between the caudal end of the lateral intermediate ridge and the vascular angle. A somewhat blurred vascular groove for the middle meningeal artery (sulcus arteriae meningeae mediae) and veins arises from the ventral border (right side) and may be from the epitympanic angle as well (left side). The ramifications of the meningeal vessels are not consistently marked bilaterally, being more numerous on the right side (some nine inconspicuous branches versus six on the left side).

The caudal part of the ventral surface of the parietal complex comprises most of the interparietal and the two caudolateral lobes of the parietal (fig. 26). The interparietal has a domed ventral surface, surrounded by the parietointerparietal and occipitointerparietal sutures. The left side of the parietointerparietal suture is more deeply marked than the right side. The occipital margin of the interparietal shows three concave suture segments. The two lateral segments are the areas by which the interparietal overlaps the supraoccipital (see below). In turn, the median segment shows a small squamous area on the dorsal surface by which the supraoccipital slightly overlaps the interparietal. The caudolateral lobes of the parietal exhibit two large suture areas. The parietal component of the occipitoparietal suture (which corresponds to the occipital margin of the parietal, margo occipitalis) is a flange of poorly consolidated bone presenting numerous notches and longitudinal striations. This wide lamina overlaps the supraoccipital dorsally. The caudoventral margin of the parietal is occupied by an unnamed, plane suture with the petrosal (see Petrosal) that is triangular in shape. In this area, the parietal widely overlaps the pars canalicularis of the petrosal.

Squamosal Surface

The squama of the squamosal laterally overlaps parts of the parietal, the petrosal, and the alisphenoid. The squama covers the parietal rostrodorsally, which projects its epitympanic wing ventrally almost to the level of the anterior margin of the external acoustic meatus. The squama also covers the pars canalicularis of the petrosal and the alisphenoid caudoventrally and rostroventrally, respectively. Consequently, there is a very small brain exposure on the squamosal. In medial view (fig. 27), the outline of the squama is the same as described externally (see Squamosal above).

The internal, or medial, surface of the squamosal is concave, with a very prominent feature: the dorsally directed opening of the temporal canal located in the posterior third of the squama. The temporal canal transmits the capsuloparietal emissary vein from the transverse sinus to its exit at the postglenoid foramen (see Parietal above, and Foramina Contents and Homology below). The opening of the temporal canal is surrounded anteriorly, ventrally, and posteriorly by acute ridges, and opens into a space that in turn leads dorsally to a wide sulcus for the posterior division of the ramus superior of the stapedial artery. Three minute nutrient foramina are found around this space. The posterior division of the ramus superior passes medial to the capsuloparietal emissary vein in a dorsal direction, enters the parietal, and divides shortly after, exiting the skull through two or three openings (the foramina for the rami temporales) at the sutura squamosa. Caudoventral to the opening of the temporal canal, a dorsally concave crest delimits the dorsal extension of the squamosal contribution to the fossa incudis, a cavity shared with the petrosal. The rostrodorsal semiarch of this crest is continued caudoventrally in the faint mark of the caudal part of articulation with the petrosal. Immediately caudal to this line is the posttympanic process of the squamosal.

Anteriorly, the triangular rostral lamina of the squama shows a distinctive ridge that marks the sphenosquamosal suture. About 1 mm dorsal is the less conspicuous sutura squamosa. The space between the two sutures is the very limited brain exposure on the squamosal (ca. 4 mm long × 1 mm high).

Occipital Surface

In AMNH 274466, the four occipital bones are fused into a single structure without traces of sutures internally or externally except for the ventral intraoccipital synchondrosis (the joint between basioccipital and exoccipital), which is represented bilaterally by no more than a 1-mm notch. As a consequence, the occipital complex is treated here as a whole. Centered on the foramen magnum, the cup-shaped complex of four bones forms a complete ring, with the basioccipital (pars basilaris of the occipital complex) protruding ventrally and rostrally as a long rectangular plate, the exoccipitals (partes laterales) forming the short but thick lateral sides, and the supraoccipital (pars squamosa) appearing as a relatively thin lamina partially roofing the wide space dorsal to the foramen magnum (fig. 28).

Distinctly two-sided, with each side forming a plane gently converging medially and ventrally, the dorsal surface of the basioccipital forms the sulcus medullae oblongatae. Rostrally, the concavity of the dorsal surface is accentuated (in the impressio pontina), and the surface is subsequently elevated to reach the dorsal border of the lip-shaped joint that is the basioccipital component of the spheno-occipital synchondrosis. In each of the rostrolateral angles, a short, oblique joint marks the lateral continuation of the spheno-occipital synchondrosis immediately medial to the carotid foramen. Caudally there is a gap that corresponds to the basicochlear fissure; immediately caudal to that is the inconspicuous mark of the posterior basicochlear commissure and then a notch representing the remainder of the ventral intraoccipital synchondrosis. Caudally and medially, the basioccipital is marked by the acutely concave margin of the foramen magnum, the intercondyloid notch (incisura intercondyloidea).

The exoccipital is the shortest component of the occipital complex. Rostrally, it has a dorsoventrally elongated, almost vertical articulation with a convex surface, the exoccipital component of the occipitomastoid suture, or the articulation between the exoccipital and the mastoid exposure of the petrosal (the petrosal component of the suture being deeply concave). This articulation has a notch in its dorsocaudal angle that communicates with the minute mastoid foramen and continues rostrodorsally in a short, almost horizontal suture of ca. 2 mm. This part of the suture is already in the supraoccipital, based on Pteropus capistratus AMNH 194276. Ventral to the suture, horizontally oriented, is the wide gap of the concave medial margin of the jugular foramen. This margin shows a distinct sulcus that corresponds to the sulcus for the inferior petrosal sinus (sinus petrosus ventralis) in the dog. Internally, the exoccipital has a smooth, convex surface that funnels into the foramen magnum, whose lateral margin is widely concave. Midway between the suture with the petrosal and the foramen magnum is the rounded hypoglossal foramen.

The supraoccipital is a wide, concave lamina of bone overlying the cerebellum. The lateral angle, as described above, is occupied by a notch connected to the mastoid foramen (the foramen being delimited ventrally and internally by the mastoid exposure of the petrosal). The supraoccipital has two sections of different thickness around the foramen magnum, the division roughly coincident with the external nuchal crest. The ventral portion is formed by bone that increases its thickness toward the foramen magnum. The dorsal margin of the foramen magnum is, like its ventral counterpart, acutely concave. The dorsal part of the supraoccipital has a central vermiform impression (impressio vermialis) that is delimited bilaterally by the internal occipital crest (crista occipitalis interna). Lateral to the internal occipital crests are the depressions of the impresiones digitatae, which accommodate gyri of the brain. The dorsal border of the supraoccipital is formed by the occipitointerparietal and occipitoparietal sutures. In the median part of the supraoccipital is the squamous occipitointerparietal suture by which the interparietal and the supraoccipital interlock. There are two deep notches on the sides of the vermiform impression. These notches are the areas where the supraoccipital is overlapped dorsally by the interparietal. The middle portion, immediately dorsal to the vermiform impression, is the small rounded area where the supraoccipital slightly overlaps the interparietal. This suture is better seen from the dorsal side of the isolated occipital complex. It has a very irregular dorsal edge, and an irregular area of overlap of up to ca. 2 mm across the entire dorsal margin of the supraoccipital.

Permanent Dentition

None of the individuals examined for this study exhibited anomalies in dental formulae other than those that can be attributed to age and tooth wear. Anomalies in dental formulae appear to be rare in Pteropus; Andersen (1912) examined over 600 skulls of Pteropus but found aberrations in dental formulae in only 10 individuals (less than 2%). All of the anomalies noted by Andersen (1912) were found in individuals referred to Pteropus giganteus, P. vampyrus, or P. scapulatus, including: (1) presence of a well-developed i3 on both sides in one individual; (2) presence of a supernumerary p2 in the broad diastema between p1 and p3 on one side in one individual; (3) presence of a supernumerary premolar between p3 and p4 on both sides in one individual; (4) absence of m3 (and its alveolus) on both sides in five individuals and on one side in another; (5) absence of m2 on one side in one individual (which also lacked m3); (6) presence of a m4 on one side in one individual; and (7) presence of M3 on both sides in one individual.

Deciduous Dentition

We examined the deciduous dentition in juvenile skulls of four specimens of Pteropus hypomelanus luteus (AMNH 159082–159085). The upper deciduous dentition consists of two incisors, a canine, and two deciduous premolars (Leche, 1876–1877; fig. 40). All are simple spicules. The deciduous incisors and canine are markedly recurved and are always implanted so that the tips are directed posteriorly (fig. 40). The deciduous premolars are slightly recurved at the tip, but their orientation is irregular. The anterior deciduous premolars are usually oriented vertically (e.g., AMNH 159083) but may project somewhat anteriorly (e.g., AMNH 159082). The posterior deciduous premolars are very small and may project in any direction, especially when displaced by eruption of the anterior molar (fig. 40).

The adult replacements for the deciduous dentition are typically visible in even the youngest pups (fig. 40). I1 and I2 erupt with their crowns directly lingual to dI1 and dI2. The permanent canine erupts anterior and lingual to the deciduous canine. The deciduous premolars are located more posteriorly and appear to be associated with the spaces between erupting permanent teeth. The anterior deciduous premolar lies between the erupting P3 and P4, and the posterior deciduous premolar lies between the erupting P4 and M1. There is no deciduous tooth associated with P1 in any of the specimens we examined, as is the rule in many mammals (e.g., the dog; Evans, 1993).

The lower deciduous dentition resembles the upper dentition in comprising two incisors, a canine, and two premolars (fig. 41). However, the morphology of these teeth differs somewhat from their upper counterparts. The deciduous lower incisors are both peglike and blunt, and are not recurved (figs. 41, 42). The medial incisor (di1) is markedly smaller than the lateral tooth (di2) in both length and breadth (fig. 42). The lower deciduous canine resembles the upper dC, and is a relatively large, recurved tooth. The first of the two deciduous premolars is similarly recurved, but the second deciduous premolar is more peglike and blunt in form. They also differ in their orientation: the anterior deciduous premolar projects dorsally and somewhat anteriorly (although its tip is recurved and thus points slightly posteriorly); in contrast, the posterior deciduous premolar lies almost horizontally in our specimens, with the crown pointed anteriorly.

Like their upper counterparts, the deciduous lower teeth occupy consistent positions with respect to their erupting replacements (fig. 41). The permanent incisor teeth erupt with their crowns directly lingual to di1 and di2. The permanent canine erupts anterior and lingual to the deciduous canine. The anterior deciduous premolar lies between the erupting p3 and p4, and the posterior deciduous premolar lies dorsal to the posterolabial corner of the erupting p4. As in the upper dentition, there is no deciduous tooth associated with p1 in any of the specimens we examined.

Shape Change

AMNH 194276, a pup of Pteropus capistratus, is the youngest osteological specimen of Pteropus that we examined (see fig. 43A). As compared with adults of Pteropus of the hypomelanus type, the most striking features of this specimen are the very short rostrum and the large, rounded braincase. The orbit is also very large, but this is typical of the temminckii species group and is not general for Pteropus. The large jugum of the canine occupies almost all of the space of the facies fascialis of the maxilla in AMNH 194276. In contrast, the jugum of the canine has a definite rostral position in the adult. Therefore, during growth, the maxilla is apparently displaced rostrally and gains bone between the canine and the zygomatic process, presumably as the upper cheek teeth are added to the tooth row in a rostrocaudal order. This process of maxillary elongation is accompanied by simultaneous elongation of the nasals, which, judging from the relative position of the canine juga, occurs chiefly in the caudal half of the nasals. Likewise, the nasopharyngeal meatus and the palate experience noticeable elongations during development.

At the same time that the rostrum is undergoing elongation, the frontal bones are remodeled such that a postorbital constriction (nonexistent in AMNH 194276) becomes apparent, clearly delineated by the temporal lines (linea temporalis; fig. 43C). Relative development of these features varies greatly within Pteropus; as in larger species the postorbital constriction is well marked (sometimes narrower than the interorbital constriction), and the temporal lines are so well developed that they form a ridge over the postorbital processes and join immediately caudal to the processes into a sagittal crest (crista sagittalis). In AMNH 194276, the posterior zygomatic root lies close to the braincase and is oriented rostrally (fig. 43A). In adults, the posterior zygomatic root becomes stronger and more sharply angled, arising laterally from the braincase and forming a triangular shelf that overlies the glenoid fossa (fig. 43B, C).

The occipital region elongates caudally during skull development in Pteropus. In AMNH 194276, the ventral rim of the foramen magnum lies only slightly caudal to a relatively small jugular foramen, and anterior to the paracondylar processes. In adults, the ventral rim of the foramen magnum is displaced caudally so that it lies posterior to the paracondylar processes, the jugular foramen is much larger in its rostrocaudal dimension, and the margin of the occiput is located farther caudal from the petrosal. Modifications in the occiput may account for the apparent decrease in the deflection of the rostrum seen during development in many megachiropterans. Bergmans (1989) noted that young specimens of many megachiropteran species (e.g., Nanonycteris veldkampi) show a greater degree of rostrum deflection than adults. This also holds for Pteropus: in most adult Pteropus specimens, if the alveolar line is projected caudally, it intersects the occiput at the supraoccipital (see above). In younger animals, such as AMNH 194276, the projected alveolar line intersects the occiput more dorsally, at the occipitointerparietal suture, indicating a more pronounced rostrum deflection than seen in adults. Changes in cranial deflection presumably occur during development by addition of bone in the central-stem synchondroses, which elongates the basicranium and may rotate it dorsally a few degrees.

Sequence of Bone Fusion

We investigated the sequence of bone fusion in Pteropus using a series of specimens ranging from a fetus to old adults. The two youngest specimens in our series were a fetus (Pteropus sp.; DUCEC 831) and a newborn pup (Pteropus capistratus; AMNH 194276) with milk teeth but no permanent teeth erupted. The remainder of our sample consisted of subadult and adult P. lylei, which we arranged into six composite age stages based on degree of tooth eruption, tooth wear, tooth loss, and fusion of cranial elements. Stage 1 was defined as including subadults with no tooth wear and M2 not fully erupted (examples included CM 87972, 87973, and AMNH 240005). Stage 2 was defined as including individuals with M2 fully erupted but with minimal tooth wear throughout the dentition (e.g., AMNH 237598). Stage 3 was defined on the basis of tooth wear (all cheek tooth cusps showing signs of wear but ridges and cusps still distinguishable) and degree of fusion of selected cranial elements (e.g., palatal elements partially fused; e.g., AMNH 237594, 237595, and 237599). Stage 4 was characterized by the same degree of tooth wear as in stage 3 (all cheek tooth cusps showing signs of wear) but with additional cranial fusions suggesting more advanced age (e.g., joints of the basicranium and orbit fused; e.g, AMNH 237593). Stage 5 was recognized on the basis of advanced tooth wear throughout the dentition, with no ridges or cusps distinguishable on any molar teeth (e.g., AMNH 30217). Stage 6 was defined by advanced tooth wear and tooth loss (all cheek teeth lost and canines worn to their roots; e.g., AMNH 217045). On the basis of these categories, we examined the sequence of bone fusion in our Pteropus sample. A graphic interpretation of this sequence is shown in figures 44–Figure 45Figure 46Figure 4748.

A distinct pattern of fusion of cranial elements emerges from analysis of specimens representing the eight age categories defined above. In the fetus, the alisphenoid and the basisphenoid form a single ossification, but the frontals are not completely fused to one another: the rostral and caudal ends of the interfrontal suture disappear shortly after birth (based on Pteropus hypomelanus AMNH 159082–159085). A rare example of a pteropodid specimen showing a complete interfrontal suture is Dobsonia pannietensis AMNH 108486 (fig. 49). Next, in subadult stage 1, the occipital region is consolidated in a single unit (figs. 45B, C, 48 part 1). This is followed in stage 2 by the fusion of the dorsal and caudal components of the braincase (figs. 45A, 46 part 2). Next, in stage 3, the hard palate and pterygoids start to fuse across the midline (figs. 45B, 48 part 3). Stage 4 includes a major series of fusions that affect several regions of the skull, namely the central stem, the caudal hard palate, and the lateral braincase and orbit (figs. 45B, C, 47, 48 part 4). This is followed in stage 5 by the frontal, jugal, and palate fusing with the remainder of their neighboring bones, as well as disappearance of the conspicuous nasomaxillary suture (figs. 45, 46–Figure 4748 part 5). By this stage, only the maxilla and the petrosal remain unfused to surrounding bones. Next, in stage 6, the three sutures involving each premaxilla disappear, as well as one part of the occipitomastoid suture (figs. 45, 46–Figure 4748 part 6). Specifically, the occipitomastoid joint is fused between the jugular foramen and the mastoid exposure, rostrolateral to the paracondylar process. This is the only part of a petrosal joint to be fused, at least in external view.

Although we do not have an absolute time scale to contrast with, our observations indicate that bone fusion continues to occur throughout life in an orderly fashion. Specimens exhibiting the degree of tooth wear of AMNH 30217 and 217045 (stages 5 and 6) are extremely rare in Pteropus; the most typical stage of bone fusion in adult P. lylei is represented by AMNH 237593 (stage 4), in which a number of joints or parts of joints are still clearly distinguishable (e.g., those involving the premaxilla, petrosal, lacrimal, pterygoid, jugal, and parietal, and some of their neighboring bones).

A functional interpretation can be deduced from the sequence of bone fusion, taking into account the types of joints involved (primarily plane or squamous sutures, and synchondroses) as well as the sequence of fusions. The first bone fusion visible in the pup, the interfrontal, is a plane suture. We also know from Pteropus livingstonii AMNH 274466 and 274467 that the frontals are fused to the ethmoid in the young. This frontoethmoid complex can be interpreted functionally as a solidly fused centerpiece that connects the rostrum and the cranium through widely overlapping surfaces—squamous sutures that will fuse much later. Because the overlap zone is so wide (see fig. 26), these squamous sutures may presumably be both very resistant to stress and resilient to bending forces (Evans, 1993). In addition, the zygomatic arches are rooted posteriorly to the braincase via another widely overlapping surface (the sutura squamosa), whereas the anterior root is simply a stout process of the maxilla. As a very general and simplified rule, the basic connecting parts that hold the skull together tend to fuse early, whereas wide squamous sutures tend to fuse in later stages. One large plane suture that fuses late is the interpalatine. This seemingly weak line is buttressed dorsally by the vomer and the osseous and cartilaginous nasal septum, and laterally by the maxillary tuberosity, which has a medial fixation (sensu Cobb, 1943).

Skull Shape in Adult Megachiropterans

Three major structural aspects of skull morphology vary across megachiropteran species: the relative length of the rostrum (producing a continuum from short-faced to long-faced morphologies), the relative orientation of the two fundamental cranial axes, rostral and basicranial (deflection of the rostrum), and the development of structures associated with the strength of mastication (e.g., size and orientation of the coronoid process and configuration of the temporal lines on the skull). These features were discussed by Andersen (1912) and continue to play a role in modern systematic treatments.

Pteropus is a long-faced genus in the context of Pteropodidae as a whole. In P. lylei, palatal length accounts for approximately 50% of the length of the skull (figs. 1–Figure 23). Andersen (1912) calculated the rostrum length/skull length ratios for a number of species and divided them into discrete categories for the purpose of composing a key. Springer et al. (1995) and Romagnoli and Springer (2000) used these data in their phylogenetic analyses, recognizing three character states: state 0 for rostrum “short, ¼ to ⅕ of skull”, state 1 for rostrum “medium, ⅓ to 2/7 of skull”, and state 2 for rostrum “long, ¾ to ½ of skull”. We measured a sample of 70 species from all currently recognized megachiropteran genera and found a virtually continuous distribution for the rostrum/skull length ratio, with no gaps allowing for splitting these data into discrete, nonoverlapping character states (fig. 50). As observed by Andersen (1912), epomophorines occupy the entire range of ratios, including genera with the shortest (Casinycteris) and the longest (Hypsignathus) rostra. Macroglossines sensu Andersen (1912; i.e., the six genera of nectar-feeding megachiropterans), pteropodines, and rousettines tend to exhibit longer-than-average rostra, whereas cynopterines, nyctimenines, and dobsonines have shorter-than-average rostra. Myonycterines sensu stricto (Myonycteris + Lissonycteris) occupy an intermediate position on this continuum. As clearly shown in P. lylei (see above), the length of the rostrum tends to increase ontogenetically.

Like rostral length, deflection of the rostrum also changes during ontogeny, and the state of this character may be affected by putative neoteny (e.g., in Nanonycteris; Bergmans, 1989). However, our examinations of series of individuals suggest that adults typically reach a characteristic degree of rostral deflection that is consistent across specimens of a given species. As discussed, the degree of rostral deflection can be defined with reference to the basicranial axis; if the latter is held horizontal, the alveolar line, representing the facial axis, may cross the occiput at different points: specifically, either below the basioccipital, at the foramen magnum, at the supraoccipital, or at the interparietal (character states used by Giannini and Simmons, 2005). Pteropodine megabats exhibit two of these conditions: a moderate-to-low rostrum deflection (alveolar line intersects the occiput at the foramen magnum; seen, for instance, in Pteropus tonganus and P. dasymallus, as well as in two species of Acerodon) and a moderate-to-high rostrum deflection (alveolar line intersects the occiput at the supraoccipital; seen, for instance, in Pteropus admiralitatum, P. giganteus, P. hypomelanus, and P. scapulatus, as well as in Eidolon, Neopteryx, Pteralopex, and Styloctenium). As in its close relative Pteropus giganteus, in P. lylei the alveolar line crosses the occiput at the level of the supraoccipital.

As discussed at length by Andersen (1912), there seems to exist a structural correlation among several traits related to mastication in Pteropus and related genera (probably in all Pteropodidae). Briefly, the size and vertical orientation of the coronoid process appears related to the configuration of the temporal lines on the skull. These traits, in turn, correlate to the strength of the dentition. Andersen (1912) identified two extremes of a likely continuous variation. Species characterized by an almost vertical and very wide coronoid process, and strongly marked temporal lines that join immediately caudal to the orbits (sometimes forming a sagittal crest), are also characterized by strong dentition (i.e., elements of large size, sometimes with additional cuspidation). An example is Pteropus anetianus, which also exhibits the short rostrum typical of forms with “heavy” dentition (sensu Andersen, 1912). At the other end of the spectrum, Andersen (1912) identified species with excessively weak dentition, a trait accompanied by a sloping (dorsocaudally oriented) and thin coronoid process and weak temporal lines that never join behind the orbits or form a sagittal crest. Pteropus scapulatus is an example of this type; these forms tend to be pollinators (Andersen, 1912). P. lylei belongs to an intermediate category, the hypomelanus type of Andersen (1912), which is characterized by an intermediate condition in the expression and development of the dentition, coronoid, and associated traits of the masticatory apparatus.

Interspecific Suture Variation

Several of the suturae capitis listed in the NAV are absent in the Pteropus examined. The vomeroincisive suture (sutura vomeroincisiva) is missing as a consequence of the lack of ossification of the palatine process of the premaxilla, to which the incisive incisure of the vomer articulates in most mammals, including yangochiropteran bats. The ethmoidonasal suture (sutura ethmoidonasalis) is missing because the frontal has taken over the whole caudal contact area of the nasal, precluding a nasal-ethmoid contact. The reduction of the jugal bone—a possible synapomorphy of bats (Wible and Novacek, 1988; Simmons, 1994)—accounts for the lack of a lacrimozygomatic suture (sutura lacrimozygomatica). In bats, the jugal is a small element in the middle of the zygomatic arch, so the anterior root of the zygoma is entirely within the maxilla. Therefore, the lacrimal is isolated from contact with the jugal by the zygomatic process of the maxilla. As in the dog, and most placental mammals, the nasal does not contact the lacrimal, the two bones being, in Pteropus, separated by the wide frontomaxillary contact, so the nasolacrimal suture (sutura nasolacrimalis) is absent. The frontal and the squamosal are widely separated by the parietal and orbitosphenoid, so the squamosofrontal suture (sutura squamosofrontalis) is absent. Finally, the sphenomaxillary suture (sutura sphenomaxillaris) is also absent.

The frontozygomatic suture (sutura frontozygomatica) is present in some megachiropterans, specifically those species of Acerodon, Pteralopex, and Pteropus that possess a complete orbital ring produced by ossification of the orbital ligament. The postorbital process of the frontal contacts the zygomatic arch in these taxa, but the contact occurs between the frontal and the zygomatic process of the maxilla, not the jugal, which is greatly reduced in all bats (Wible and Novacek, 1988; Simmons, 1994).

Some sutures have systematic importance (Andersen, 1912). Such is the case of the three sutures involving the megachiropteran premaxilla. First, the nature of the interincisive suture varies widely among megachiropterans. The left and right premaxillae may be separated and not contact one another (e.g., Boneia), or they may be sutured (e.g., Rousettus) or fused (e.g., Nyctimene). Similarly, the maxilloincisive suture may be sutured (the most common condition in megachiropterans) or fused (e.g., Pteralopex, Styloctenium, Neopteryx). The shape of the pathway of the suture may also vary. Some pteropodines exhibit a “squared off” maxilloincisive suture, whereas in most megabats this suture is gently curved. The nasoincisive suture varies in relative length within Megachiroptera. It is a long suture in many of the nectar-feeding bats (e.g., Macroglossus, Melonycteris, Notopteris) and exhibits relative reduction in other taxa: it is of moderate length in most megachiropterans, it is reduced to a point nasal-premaxilla contact in some others (e.g., many cynopterines and dobsonines), and the nasal-premaxilla contact is lost in a few forms (e.g., nyctimenines).

Internal Surfaces of the Skull

Descriptions of the internal surfaces of the skull are almost entirely lacking for bats and are infrequent for mammals in general. One exception is the cribriform plate, described and figured for representatives of most bat families by Bhatnagar and Kallen (1974). The cribriform plate of Pteropus livingstonii (this study) is very similar to that of Pteropus giganteus, Rousettus leschenaulti, and Cynopterus sphinx (Bhatnagar and Kallen, 1974), although the cribiform plate of the latter taxon is relatively wider. These four megachiropterans differ from all microchiropterans examined by Bhatnagar and Kallen (1974) in that the entire cribriform plate is perforated by cribriform foramina, whereas microchiropterans exhibit a cribriform plate with wide nonperforate areas.

For other structures, we compared our observations of internal surfaces in Pteropus livingstonii with the descriptions available for the dog (Evans, 1993). Pteropus shows a number of striking differences, detailed as follows.

The nasal bone of Pteropus lacks a nasoethmoidal crest and fossa. This appears to be related to the fact that the nasal does not contact the ethmoid as it does in the dog. The frontal process of the nasal overlies only the rostral surfaces of the frontal bone in Pteropus.

As discussed above when dealing with the external surfaces, the premaxilla lacks a caudal projection of the nasal process, and the palatine processes are missing altogether. These structural differences from the dog also are reflected on the internal surface of the premaxilla. Regarding the maxillary internal surface, the most significant difference with respect to the dog is the relative simplicity of the ventral nasal concha, which in Pteropus lacks the rich covering of delicate bony scrolls present in the dog. In the palatine, the chief differences from the dog are the limited dorsal extension of the perpendicular lamina in Pteropus and the noticeable caudal extension of the horizontal lamina, forming a long postdental palate inclusive of the minor palatine foramen (a notch in the dog).

The internal surfaces of the frontoethmoidal complex include the frontal, cribriform plate of the ethmoid, ethmoturbinal, and vomer. Differences between Pteropus and the dog include the following. First, the inner table of the frontal (facies interna) is not affected by the frontal sinus, as the sinus in Pteropus is located more rostrally in the interorbital area. Note, however, that in other megachiropterans, chiefly in cynopterines, nyctimenines, and dobsoniines, the frontal sinus reaches the postorbital area, as in the dog. The inner table of the frontal also is shaped differently in Pteropus; it does not show the strong, wedge-shaped internal frontal crest between the ethmoidal fossae dorsally. Second, the remarkable dorsal extension of the orbitosphenoid in Pteropus leaves a wide notch on the lateral side of the frontal, thus neatly separating the postorbital part of the squama frontalis from the pars orbitalis. Third, ventrally, the ala vomeris is extremely narrow in Pteropus, so the floor of the nasal capsule chiefly is exposed ventrally. In the dog, the ala vomeris is significantly wider.

The presphenoid and orbitosphenoids of Pteropus differ notably from those of the dog. Considering the body first, in Pteropus a differentiated rostrum (rostrum sphenoidale of the dog) is absent; the optic canals are more rostral in position, so the corpus is elongated caudally; and the sphenoidal sinuses are very small. The alae in Pteropus possess a very large vertical component that contributes significantly to the orbit wall, reaching two-thirds of the orbit height, an arrangement that stands in striking contrast to the limited orbital exposure of the orbitosphenoid in the dog. The main difference can be thought of as a combination of two modifications: an enlargement of the alae and a rostral displacement of the yoke.

The basisphenoid and alisphenoid of Pteropus also differ significantly from those of the dog, especially in dorsal view. In Pteropus, there is no trace of alar canal and hence no differentiated foramen rotundum, its contents instead being transmitted directly to the sphenorbital fissure. The sella turcica is also different in being significantly elongated and in lacking a distinct caudal clinoid process (processus clinoideus caudalis) and dorsum sellae. The sizable pterygoid processes present in the dog are minute in Pteropus. Caudally, Pteropus exhibits a very distinct epitympanic wing of the alisphenoid, which is absent or inconspicuous in the dog.

The internal surface of the parietal complex in Pteropus differs from that of the dog in several important aspects. First, Pteropus wholly lacks the tentorial process (processus tentoricus). Second, due to the fact that in the dog the vascular supply to the m. temporalis is entirely extracranial, all structures (sulci and foramina) directly associated with the intracranial course of the ramus superior and rami temporales in Pteropus are correspondingly absent in the dog. Third, the development and conspicuousness of the sulcus for the transverse sinus is much less marked in Pteropus. Fourth, the interparietal in the dog is a wedge-shaped bone fitting a narrow interparietal space and is more intimately associated with the supraoccipital than with the parietals. The interparietal in Pteropus is a subcircular, domed plate more intimately associated with the parietal than with the supraoccipital. Additionally, the interparietal of Pteropus lacks a foramen for the dorsal sagittal sinus (foramen sinus sagittalis dorsalis). Finally, the degree of parietal overlap of the frontal is much more pronounced in Pteropus than in the dog, and the parietal overlaps the supraoccipital.

Comparisons between the internal surface of the squamosal in Pteropus and the dog are difficult because in the dog the squamosal is solidly integrated in the temporal complex. One difference that should be noted, however, is the absence in the dog of the sulcus that leads the ramus superior outside the braincase through the foramina for the rami temporales. This sulcus is absent because the ramus superior and the rami temporales are entirely lacking in the dog. Also, in Pteropus, the transverse sulcus is less marked, and even less so in the parietal with respect to the squamosal (see above).

Major differences exist between Pteropus and the dog in the internal surfaces of the occipital complex. These include the absence in Pteropus of an occipital contribution to the sulcus for the venous transverse sinus (also reduced in the parietal) and the absence of the condyloid canal (canalis condylaris). Another difference occurs in the shape of the foramen magnum, which is rhomboidal in Pteropus but is dorsally constricted in the dog by the presence of the paired nuchal tubercles (tuberculum nuchale) in most adult individuals.

Foramina

Species of Pteropus including P. lylei show neither the full set of foramina present in Megachiroptera as a group, nor the same degree of development in all the foramina whose presence is shared with other forms. Here we point out the differences and generalities that arise from comparing P. lylei with other Pteropus species, as well as with other pteropodine and nonpteropodine megachiropterans.

Dentition

The most complete dental formula known in bats is I2/3, C1/1, P3/3, M3/3 × 2 = 38, which is seen in echolocating bats such as Myotis. Megachiropterans have reduced dental formulae, the most complete (I2/2, C1/1, P3/3, M2/3 × 2 = 34) being present as a rule in Pteropus but also in other genera such as Rousettus and Eidolon. P. lylei is no exception, and the morphology of its dentition is also typical of both the genus and the hypomelanus type. Significant variations in dental formulae and tooth morphology exist among megachiropterans, most of which were discussed in detail by Andersen (1912). Seven genera and many species were discovered after the publication of Andersen's monographic treatment, but no particularly different tooth morphologies occur in the genera described after 1912. We direct the reader to Andersen (1912) for a complete analysis of dental variation in Megachiroptera. Important additional discussion is found in Hill and Beckon (1978), and in the remarkable series of papers by Win Bergmans (Bergmans, 1976, 1977, 1980, 1988, 1989, 1990, 1994, 1997, 2001; Bergmans and Rozendaal, 1988).

The homology of specific dental structures (such as cusps, basins, and ridges) with respect to a typical tribosphenic molar is uncertain. Miller (1907), on the basis of comparisons with frugivorous phyllostomid microchiropterans, assumed that the megachiropteran upper premolars and molars each bear a protocone (medial cusp and ridge) and a paracone (lateral cusp and ridge), whereas the lower molars bear a metaconid (medial cusp and ridge) and a protoconid (lateral cusp and ridge). Andersen (1912) arrived at a similar conclusion based on comparisons with Talpa europaea, a mole with dilambdodont molars. Andersen (1912) postulated that the lateral ridge of the upper molars of megabats corresponded to a paracone + metacone, and the medial ridge corresponded to the protocone + hypocone; in the lower molars, the lateral ridge corresponded to a protoconid + hypoconid, and the medial ridge to paraconid + metaconid + entoconid. Both Miller (1907) and Andersen (1912) agreed that megachiropterans lack a stylar shelf; that is, the lateralmost cusps of a chiropteran tribosphenic molar (parastyle, mesostyle, and metastyle) are entirely absent.

Although most megachiropterans have simplified cheek teeth characterized by medial and lateral ridges, several taxa exhibit multicuspidate cheek teeth, most notably Harpyionycteris and Pteralopex. Miller (1907) and Andersen (1912) believed that additional cusps were the product of secondary divisions of the basic megachiropteran lateral and medial ridges. However, other authors, including Slaughter (1970; see also Thenius, 1989) homologized all individual cusps with normal tribosphenic elements, stating that an upper molar of Harpyionycteris exhibits a paracone, protocone, metacone, hypocone, and metastyle, and that the lower molar supports a protoconid, metaconid, hypoconid, entoconid, and two stylids (the proto- and metastylid). Peterson and Fenton (1970) dismissed the cusp homologies of Slaughter (1970) on the basis of inconsistent variation they observed in series of specimens of Harpyionycteris. In turn, Hill and Beckon (1978), on the basis of an extensive comparative study of the dentition in Pteralopex, favored Andersen's (1912) view that the multicuspidate condition of Pteralopex was probably derived from more typical pteropodine molars, such as those of the pselaphon species group within Pteropus. We concur with Hill and Beckon (1978) in concluding that support for detailed homology statements linking multiple cusps of molariform teeth in megachiropterans like Pteralopex with the cusps of the microchiropteran dentition is lacking or contradictory. While the loss of the stylar shelf seems a safe assumption, there is little or no support for identification of megachiropteran cusps with typical tribosphenic elements. Certainly, the most conservative approach to this unresolved problem would be to recognize that the two main ridges and accompanying cusps of the megachiropteran cheek tooth may derive from nonstylar, large tribosphenic cusps, but the specific homology of contributing cusps remains unknown.