Rostral

The anteroventral part of the rostral is eroded, as is most of the triturating margin. As in Psittacosaurus mongoliensis, P. meileyingensis, P. gobiensis, P. major, P. neimongoliensis, and P. ordosensis (Osborn, 1923; Sereno et al., 1988, 2007, 2010; Russell and Zhao, 1996; You et al., 2008), the apex of the rostral abuts the descending rostral processes of the nasals. Unlike in P. sinensis and P. lujiatunensis, the apex of the rostral does not extend between the rostral processes of the nasal (fig. 5A; Young, 1958; Zhou et al., 2006). In anterior view, the rostral is subtriangular (fig. 5A). Unlike in other Psittacosaurus species, the rostral does not uniformly taper to its apex. Instead, it narrows sharply at about two-thirds its height (fig. 5A). In lateral view, the rostral-premaxilla suture is sinuous (fig. 4). Dorsally, it is anteriorly convex and gently curved. At about half its height, the suture becomes strongly arched and anteriorly concave. The suture then trends anteroventrally to its ventral margin, in contrast to the condition seen in all other Psittacosaurus species (fig. 4). The surface of the rostral is pitted and striated, as in other Psittacosaurus species and was likely covered with a keratinous beak in life (Osborn, 1923; Sereno et al., 1988, 2010). The rostral's anterior margin is gently curved, and does not recurve posteriorly as in Psittacosaurus mongoliensis (fig. 4; Osborn, 1923; Sereno, 1987, 2010).

Premaxilla

The premaxillae are tall and broad in lateral view, as in other species of Psittacosaurus (fig. 4). Near the rostral, the lateral surfaces of the premaxillae bear a rugose texture like that of the rostral, suggesting that the keratinous beak was not confined to the rostral. The posterior expansion of the posterolateral process of the premaxilla excludes the maxilla from the external nares, which are bounded solely by the nasals and premaxillae. This is diagnostic of Ornithischia and like the condition found in all other Psittacosaurus species (Sereno, 1987; Russell and Zhao, 1996; Averianov et al., 2006; Zhou et al., 2006; Sereno et al., 2007, 2010). The external nares are oval and posterodorsally inclined, unlike the near-vertical condition found in P. meileyingensis, P. gobiensis, P. neimongoliensis, P. ordosensis, and P. lujiatunensis, and unlike the circular nares found in P. mongoliensis, P. major, P. sibiricus, and P. sinensis (fig. 4; Osborn, 1923; Young, 1958; Sereno, 1987; Sereno et al., 1988; Russell and Zhao, 1996; Averianov et al., 2006; Zhou et al., 2006; Sereno et al., 2007, 2010). Like in all other Psittacosaurus species, the lateral surfaces of the premaxillae are broadly concave (Sereno, 1987). Just posterior to the apex of the rostral, the premaxillae bear five foramina arranged in an arc (fig. 4). The two posteriormost foramina open posterodorsally and on both sides merge into a single hourglass-shaped opening. The next two foramina open dorsally and the ante-riormost foramen opens anteriorly, toward the rostral. These foramina are interpreted as neurovascular foramina, like those present in P. mongoliensis and P. gobiensis (Sereno, 1987; Sereno et al., 2010), but are more numerous than described in any known species of Psittacosaurus. Neurovascular grooves are faint but can be discerned from several of the foramina on each side of the skull. The premaxillae lack fenestrated lacrimal canals, as in P. mongoliensis, P. major, and P. meileyingensis (Osborn, 1923; Sereno, 1987; Sereno et al., 2007). The premaxillae contact the prefrontals and lacrimals posteriorly, and the maxilla ventrally; the contact with the maxilla is marked by a raised ridge, as in P. mongoliensis (see Sereno, 1987). Ventrally, the premaxillae expand medially, joining with the rostral and maxillae to form an arched secondary palate at the front of the upper jaw (fig. 3A). As in all other Psittacosaurus species, the premaxillae are edentulous.

Maxilla

The maxillae are subtriangular in lateral view (fig. 4). The premaxilla-maxilla suture is marked by a distinct ridge, which forms the anterior boundary of a triangular maxillary fossa as in most other Psittacosaurus species, with the exception of P. sinensis and P. neimongoliensis (Young, 1958; Russell and Zhao, 1996). This fossa is not considered homologous to the antorbital fossa where the antorbital fenestra occurs in other archosaurs (Sereno, 2000, 2010; You et al., 2008). The posterior sutural contacts of the maxilla are not visible due to presumed fusion, but the bone obviously contacts the jugal and seems to have a narrow contact with the lacrimal, as in most Psittacosaurus species but unlike P. sinensis and P. gobiensis (Sereno, 1987; Sereno et al., 2010). The maxilla is strongly emar-ginated, displaying the medially inset toothrow characteristic of ornithischian dinosaurs (fig. 3A). Approximately six foramina open ventrolaterally from under the buccal emargi-nation, as in P. mongoliensis (Sereno, 1987). The dentition in the left maxilla is complete, bearing 11 teeth. The teeth are heavily worn; the labial surfaces in the least-worn teeth bear a central primary ridge, flanked by two secondary ridges anterior and posterior to the primary ridge, forming five cusps in total. The lingual surfaces of the maxillary teeth are smooth, but the five cusps are discernible at the margin of the crown.

Anteriorly, the maxillae show projecting anteromedial processes that form the posterior end of the secondary palate (fig. 3A). Posteriorly, the tooth-bearing pedestal of the maxillae turns inward, forming a cupped lamina that is anteriorly concave. The posterior aspect of this cupped lamina abuts the pterygoid; this is in contrast to the condition in P. mongoliensis where the tooth-bearing pedestal contacts the pterygoid without a distinctive cupped lamina.

Palatine

The posterior portions of both palatines are preserved (fig. 3A). They contact the pterygoid and ectopterygoid posteriorly and the maxilla laterally. The contact with the maxilla is anteroposteriorly extensive, running to approximately the middle of the toothrow via a long narrow processes of the palatine. Medially, the palatines vault dorsally, creating a raised palate over the posterior maxillary toothrow, as in other Psittacosaurus specimens. Together with the maxilla and the pterygoid, the palatine forms the palatal foramen common to all ceratopsians for the passage of the maxillary branch of the trigeminal nerve (Brown and Schlaikjer, 1940; You and Dodson, 2004).

Pterygoid

The pterygoids are fused along the midline. As in other Psittacosaurus, they form a horizontal basal plate at the posterior end of the palate, which projects the palatal, mandibular, and quadrate rami of the pterygoid (fig. 3A). The palatal ramus is not exposed, but clearly articulated with the palatine to form a vaulted posterior palate. The interpterygoid space described by Sereno (1987) is not visible. The mandibular rami are thin but mediolaterally broad, passing posteroventrally to form the flanges for attachment of the M. pterygoideus jaw adductor musculature. The anteromedial part of this ramus contacts the cupped posterior lamina of the maxilla.

Posteriorly, the basal plate and mandibular rami grade into the quadrate ramus, a large sheet that in this specimen has fused with the pterygoid ramus of the quadrate to form a solid sheet of bone between the adductor chamber and the braincase (fig. 3A). The quadrate ramus also forms an anteriorly facing articular facet to receive the basipterygoid processes of the basisphenoid. This articulation is not visible in ventral view, a condition similar to that in Psittacosaurus mongoliensis, though in this case it is not due to the development of a posterior tablike process of the quadrate ramus (which is not preserved in this specimen), but rather to the overall posterior extent of the quadrate ramus and basal plate.

Ectopterygoid

The left ectopterygoid is present dorsal and lateral to the left pterygoid flange, running along the length of the flange to contact the maxilla and jugal anteriorly (fig. 3A). The ectopterygoid-pterygoid articulation takes the form of an elongate scarf joint. The ectopterygoid is excluded from the palatal foramen by the maxilla and pterygoid. The right ectopterygoid is not present.

Nasal

The majority of the nasals are eroded, however the rostral processes on both sides are preserved, the right more completely than the left (figs. 3B, 5A). The rostral processes abut the ascending process of the rostral bone, and overlay the anterior margin of the premaxilla between the rostral and the external nares. The eroded region of the specimen suggests that the external nares were framed equally by the premaxillae ventrally and the nasals dorsally. The dorsal surface of the nasals is completely eroded, and their width at the midline and contacts to the prefrontals and frontals cannot be discerned. In overall form, however, it is clear that the nasals were less strongly flexed than in other species of Psittacosaurus; combined with an anteroventrally projected rostral, this gives the snout of Psittacosaurus amitabha a more elongate appearance than even that of the notably long-snouted Psittacosaurus mongoliensis (fig. 4; Sereno, 2010).

Prefrontal

Portions of the right and left prefrontals are preserved, with the right element being more complete (fig. 3B). The prefrontal-lacrimal suture is completely obliterated, so the details of this contact cannot be discerned; however, it is clear that the anterodorsal quadrant of the orbit was framed by the prefrontal. The prefrontal portion of the orbital rim is textured, indicating liga-mentous attachment to the palpebral. There is a raised lip or “prefrontal crest” on the orbital margin of the prefrontal, as seen in Psittacosaurus mongoliensis (fig. 5A; Sereno, 1987, 2010). The posterior ramus of the prefrontal contacts the frontal in a broad scarf joint, as evidenced by broad flat articular facets on the anterodorsal frontal. The posterior terminus of these processes are squared off, and the bone appears to have expanded anteriorly, making it somewhat roughly teardrop shaped in dorsal view.

Palpebral

Part of the right palpebral is present, but disarticulated from the rest of the skull. The palpebral is markedly distinct from that of other Psittacosaurus, where the palpebral is subtriangular in dorsal view, extending through only the anterior one-third of the orbit. The palpebral of IGM 100/1132 is posteriorly expanded into a distinct tongue, extending more than half the length of the orbit in life position. The posterior tongue deflects dorsally, slightly overlapping the frontal. The ventral surface of the bone bears a distinct striated surface that likely served as insertion for the ligament attaching it to the prefrontal. The anterior region of the element is broken, but appears to have been subtriangular, as in other Psittacosaurus. In its anteroposterior extent, the palpebral is similar to the element in basal ornithopods such as Haya griva (Makovicky et al., 2011). However, in H. griva the posterior extension of the palpebral is on the lateral side, whereas the posterior tongue in IGM 100/1132 is on the medial side, articulating with the rest of the skull (Barta and Norell, in review).

Lacrimal

Both lacrimals are present; though their sutural contacts are fused, they appear to have contacted the prefrontals, premaxillae, maxillae, and jugals (fig. 4). Lacrimal-premaxilla contact varies among Psittacosaurus species; Psittacosaurus mongoliensis and Psittacosaurus major both have limited contact between these bones, while Psittacosaurus sinensis does not (Young, 1958; Sereno, 1987, 2010; Sereno et al., 2007). The lacrimal frames the anteroventral quadrant of the orbit in Psittacosaurus. The lacrimal bears a large foramen allowing neurovascular passage via the lacrimal canal from the orbit to the rostrum. The neurovascular occupants of this canal likely passed through the premaxillary foramina to supply and innervate the rostrum. The lacrimal canal does not appear to be laterally fenestrated, as in most Psittacosaurus (fig. 4). Sereno (2010) claims that this fenestra indicates incomplete ossification of the lacrimal canal at the premaxilla-lacrimal suture; however, the bone is very thin (as evidenced by bilateral damage on this specimen), suggesting that the “fenestra of the lacrimal canal” may simply be a region of thin and easily broken bone. This would explain the wide distribution of this structure and its irregular, often subrectangular shape.

Frontal

The paired frontals are both preserved completely, joined at the midline by a straight suture (fig. 3B). Anteriorly, they contacted the prefrontals and nasals via flattened scarf joints. While the anterior extent of the frontals is partially eroded, the nasal contacts appear to have formed a zigzag anterior margin that was laterally confluent with a straight, posteriorly extending contact with the prefrontal that deflects laterally at about the middle of the orbit. Medial to the prefrontal facet, the frontals have a striated texture (fig. 3B). The frontal forms a part of the dorsal orbit, between the prefrontals and postorbitals. The postorbital-frontal suture is broad, and W-shaped in dorsal view. Posteriorly, the frontals contacted the parietal in a sinuous suture. Two small but distinct fossae are present on each side of the midline, just anterior to the parietal contact, uniting to form a posteriorly angled triangular depression about the midline (fig. 3B). These fossae are absent on AMNH FARB 6253, which instead displays a prominent frontal contribution to the supratemporal fossae (Osborn, 1924). In IGM 100/1132, the frontals are nearly excluded from the supratemporal fenestrae, but do form a small portion of the rim between the parietal and postorbitals, and do not contribute to the supratemporal fossa (fig. 3B).

Postorbital

The postorbital is a triradiate bone forming approximately one-third of the dorsal margin and all the posterior margin of the orbit. The right postorbital is disassociated from the remainder of the skull, but the entire left postorbital is present in its original position (fig. 3, 4). The anterior ramus of the postorbital is dorsoventrally flat and anteroposteriorly wide, broadly contacting the frontal (see above), and narrowly contacting the parietal at the anterior rim of the supratemporal fenestra (fig. 3B). The body of the postorbital is swollen into a rugose boss that may have borne a small keratinous sheath, similar to that inferred by Sereno et al. (2010) for Psittacosaurus gobiensis. The posterior ramus of the postorbital is long and thin, forming a long scarf joint with the anterior ramus of the squamosal; the postorbital's posterior ramus is so long that it nearly contacts the head of the quadrate. This ramus forms the dorsal margin of the infratemporal fenestra, nearly excluding the squamosal from this opening (fig. 3B). The ventral ramus of the postorbital frames the posterior rim of the orbit and the anterior margin of the infratemporal fenestra. This ramus is thicker than the posterior ramus, and roughly cylindrical in cross section. The ventral ramus contacted the jugal; however, postmortem scavenging (probably by arthropods) and sutural fusion make the details of this contact difficult to discern. It does seem that the ventral ramus of the postorbital laterally overlaid the postorbital process of the jugal in a scarf joint. The region of the postorbital-jugal contact bears a distinct laterally directed eminence that represents a postorbital-jugal crest, which is known from most Psittacosaurus, and may have also borne a keratinous sheath (fig. 4A; Sereno, 2010; Sereno et al., 2010).

Jugal

The jugals have been mostly destroyed by postmortem scavenging (figs. 3–5). The jugal horns are notably missing, preventing comparison of this important structure with other species of Psittacosaurus. Sutural fusion has also made most of the contacts of the jugal difficult to discern. However, the jugal obviously contacts the lacrimal, maxilla, postorbital, and quadratojugal, and forms the ventral rim of the orbit and infratemporal fenestra. The jugal also seems to share a limited contact with the pterygoid, via a thin process intervening between the pterygoid and maxilla (fig. 3A). The quadratojugal process of the jugal does not appear to be forked (as in other species of Psittacosaurus), overlaying the quadratojugal as a simple tongue-shaped process (fig. 4).

Quadratojugal

The quadratojugal is subtriangular and mediolaterally flattened. The right quadratojugal is present, and the left is not preserved; however, the articular surface of the left quadratojugal is clearly preserved (fig. 4). The posterior margin of the bone is sinuous, posteriorly concave dorsally and posteriorly convex ventrally. It overlays the quadrate in a scarf joint along their entire contact and is similarly overlain by the jugal anteriorly. Unlike most Psittacosaurus species (but similar to CAGS-IG-VD-004, a specimen referred by You et al. [2008] to Psittacosaurus major), the quadratojugal does not extend posteriorly to cover the entire ventrolateral aspect of the quadrate in lateral view. No quadratojugal foramen is present.

Quadrate

Both quadrates are preserved. As in all Psittacosaurus species and dinosaurs generally, they can be partitioned into two divisions—a broad, sheetlike pterygoid ramus and a vertical shaft (Sereno, 1987). The pterygoid ramus forms an anteriorly convex shield that, in life, would have intervened between the temporal fenestrae and the braincase, obscuring the braincase from lateral view (fig. 4). Anteriorly, this ramus contacts the pterygoid just anterolateral to the juncture of the pterygoid and basipterygoid processes. The suture continues ventrally before deflecting sharply to project posteriorly to the posterior extent of the quadrate process of the pterygoid (fig. 3A).

The quadrate shaft is mediolaterally compressed for its dorsal half, expanding ventrally to become subrectangular in cross section (fig. 5B). The dorsal head of the quadrate shaft sits in a glenoid cup within the squamosal. A medial lamina of the squamosal glenoid prevents the quadrate head from contacting the paroccipital process (figs. 4A, 5B). The quadratojugal overlaps the quadrate laterally via a broad scarf joint (see above). A posteriorly directed crest sits on the lateral margin of the lower half of the quadrate shaft; this crest terminates just dorsal to the articular condyles (fig. 5B). The ventral margin of the condyles is roughly horizontal, and the lateral condyle larger than the medial. In lateral view, the quadrate shaft is strongly flexed anteriorly for its dorsal half, and forms most of the posterior boundary of the infratemporal fenestra.

Squamosal

The left squamosal is present in its original position; the right is dissociated from the cranium and is missing part of the postorbital ramus. The squamosal is triradiate in dorsal view, with a long barlike postorbital ramus, a short stout posterior ramus, and a flattened parietal ramus that contributes to the squamosal-parietal shelf overhanging the occiput (fig. 3B). A thin lamina descends along the anteroposterior axis of the postorbital ramus, forming a groove for the squamosal ramus of the postorbital. This lamina gives rise to an anteroventrally directed prong at its posterior extent (fig. 4A). At the origin of the prong lies a small fossa, interpreted as the attachment site for M. adductor mandibulae externus superficialis (Han et al., 2018). Posterior to this prong, the quadrate glenoid occurs within the body of the squamosal, without its own distinct process; this glenoid is framed posteriorly by another thin lamina that completely excludes the quadrate from the paroccipital processes, as in Psittacosaurus mongoliensis (fig. 3B; Sereno, 1987). In dorsal view, the squamosal expands posteriorly, with the parietal ramus contacting the parietal in a broad scarf joint to frame the posterolateral quadrant of the supratemporal fenestra (fig. 3B). Ventrally, this ramus also contacts the dorsal surface of the pterygoid ramus of the quadrate, contributing to the continuous sheet of bone separating the braincase from the temporal fenestrae. Overall, the squamosal is similar to that of AMNH FARB 6253 being distinguished by the lack of an invagination between the posterior and parietal rami of the squamosal in IGM 100/1132.

Supraoccipital

The supraoccipital is diamond shaped in posterior view and is wider than it is tall (fig. 5B). It narrowly contributes to the dorsal border of the foramen magnum. From the medial apex of the supraoccipital to its center, a mediolaterally thin posteriorly directed process arises and underlaps the parietal-squamosal shelf, forming a lamina between the right and left sides of the dorsal occiput. This platelike process was identified by Sereno (1987) as the attachment point for the nuchal ligament.

Exoccipital-Opisthotic

As in most archosaurs and all species of Psittacosaurus, the exoccipitals and opisthotics are completely fused (Sereno, 1987). These elements form the posterolaterally directed paroccipital processes, which in Psittacosaurus amitabha terminate with a distinct boot-shaped expansion (fig. 5B). In contrast, the paroccipital processes of AMNH FARB 6253 do not expand at their lateral ends (Osborn, 1924). The posterodorsal aspects of the paroccipital processes bear a marked depression that is confluent with a posterolaterally directed foramen that passes antero-medially into the braincase (fig. 5B). This is likely the entrance of the caudal middle cerebral vein (Witmer and Ridgely, 2009; Bever et al., 2013). They share an extensive dorsal contact with the supraoccipital, parietal, and squamosal, from medial to lateral; the parietal contact is comparatively brief. As noted above, a lamina of the squamosal intervenes between the quadrate and the paroccipital processes. As in Psittacosaurus mongoliensis, the exoccipital-opisthotic frames the posterior margin of the fenestra ovalis (fig. 6; Sereno, 1987).

The exoccipital-opisthotics form part of the dorsal and most of the lateral margins of the foramen magnum. Ventrally, they contact the basioccipital on a distinct peduncle. This peduncle houses the foramina of three cranial nerves, arranged so they form an inverted triangle in lateral view (fig. 6). From anterior to posterior, these foramina were occupied by: (1) the jugular canal (containing the glossopharyngeal [IX], vagus [X], and accessory [XI] nerves, as well as the jugular vein), (2) branches 1 and 2 of the hypoglossal nerve (XII), and (3) branch 3 of the hypoglossal nerve (Sereno, 1987; Averianov et al., 2006). Sereno (1987) further suggests that the anterior foramen provided passage for the glossopharyngeal nerve (IX) and the jugular vein. A lateral crest arises from the anterior extent of the exoccipital-opitsthotic peduncle, passing ventrally to merge with the basioccipital part of the basal tuber. Unlike in Psittacosaurus mongoliensis, the posterolateral edge of the peduncle does not bear a distinct ridge framing the three cranial nerve foramina.

FIGURE 6.

Detail of braincase of IGM 100/1132 in posterolateral view. Abbreviations: fo, fenestra ovalis; jc, jugular canal; V, trigeminal foramen; VII, foramen for facial nerve; XII-1, 2, foramen for first and second branches of the hypoglossal nerve; XII-3, foramen for third branch of the hypoglossal nerve.

Basioccipital

The basioccipital frames the ventrolateral and ventral margins of the foramen magnum, forming the single ball-shaped occipital condyle (fig. 5B). The occipital condyle does not feature a midline excavation in posterior view as figured by Osborn (1924) for AMNH FARB 6253. It contacts the exoccipital-opisthotics dorsally and the basisphenoid anteriorly. The latter suture is almost completely fused and is difficult to discern. The basioccipital contributes extensively to the basal tubera, forming the posterior 65% of their length. Along the midline, between the basal tubera and the occipital condyle, is a small foramen that would have entered the posterior floor of the braincase (fig. 3A). This structure is situated within a small heart-shaped fossa, and is not noted in the braincase descriptions of Sereno (1987), Averianov et al. (2006), or You et al. (2008), and may therefore be autapomorphic for this taxon. Zhou et al. (2006) note a small oval fossa between the occipital condyle and basal tubera in Psittacosaurus lujiatunensis, which may correspond to this foramen; however, it is more likely homologous to the fossa in which the foramen is situated.

Basisphenoid

The basisphenoid is well-preserved, but the basisphenoid rostrum is notably missing. This bone contacts the basioccipital posteroventrally and the exoccipital-opisthotics and prootics dorsally (figs. 3A, 6). Anteriorly, the bone projects out two stout bar-shaped basipterygoid processes, which articulate with the pterygoids. Posteriorly, it forms the anterior portions of the basal tubera. The anterior faces of the basal tubera give rise to thin anteriorly directed laminae; these merge with laminae that arise from the basisphenoid-prootic contact to form the basipterygoid processes (fig. 3A). The tuberal laminae and basal tubera themselves frame a deep invagination on the ventral aspect of the basisphenoid that does not communicate with the endocranial cavity (Sereno, 1987). The lateral space between these laminae frame the foramen for the passage of the internal carotid artery into the pituitary fossa (Sereno, 1987), which opens anterodorsally from the anterodorsal aspect of the basisphenoid and in life would have been confluent with the endocranial cavity. The posterodorsal corner of the basisphenoid contributes to the margin of the fenestra ovalis (fig. 6).

Prootic

Both prootics are preserved, though their sutures to the exoccipital-opisthotics and basisphenoid are nearly obliterated by fusion (fig. 6). They are taller than wide in lateral view, with prominent posterodorsal processes overlapping the lateral face of the paroccipital processes. They contact the basisphenoid ventrally, exoccipital-opisthotics posteriorly, the parietal dorsally, the laterosphenoids anteriorly and the broad pterygoid ramus of the quadrate laterally. The prootics form part of the lateral wall of the endocranial cavity. This bone forms most of the anterior margin of the fenestra ovalis; anterior to the fenestra ovalis, it is punctured by another foramen for the facial nerve (VII; Sereno, 1987). The prootic also frames the posterior margin of the trigeminal foramen, which is visible on both sides (fig. 6).

Laterosphenoid

The laterosphenoids are partially preserved, with only their posterior regions present (figs. 4B, 6). They contact the prootics posteriorly and the parietal, postorbitals, and frontals dorsally defining the anterolateral wall of the braincase. While the anterior region is missing on both sides, it is likely that the bone would be visible through the orbit when complete, as in other species of Psittacosaurus. The trigeminal foramen is visible on both sides, opening laterally for the passage of branches 2 and 3 of the trigeminal nerve (V). It is framed mostly by the lateros-phenoid, with less than half of the posterior opening framed by the prootic.

Mandible

The mandible of IGM 100/1132 is highly fragmentary, missing the entire predentary, surangular, splenial, coronoid, and articular (figs. 7, 8). The angular may be partially present, but the preservation is so poor that it is impossible to state this for certain; thus, the mandible is primarily represented by the dentary. The articular facet for the predentary is clearly demarcated as a shallow rugose fossa with numerous neurovascular foramina. The lateral rami of the predentary are less than half the length of the descending process and are lateral to the first dentary tooth. This represents an autapomorphy of this taxon, because in other Psittacosaurus the first dentary tooth is separated from the lateral ramus of the predentary by a short diastema. Most of the dentary toothrow is missing, so the number of dentary teeth cannot be determined. The anterior portion of the right coronoid process is present, arising as a gentle incline from the body of the dentary and laterally displaced from the toothrow (figs. 7, 8).

Postcrania

The atlantal intercentrum, as in other dinosaurs, is broad and U-shaped, with an anterior concavity to receive the occipital condyle (Sereno, 1987). These bones fit together tightly when manually articulated, confirming the identity of this fragmentary vertebral element. As in Psittacosaurus mongoliensis, there are two ventrolaterally projecting tab-shaped processes that would have articulated with the cervical ribs of the atlas (Sereno, 1987). There is an elliptical depression in the middle of the intercentrum, visible in dorsal view.

The two well-preserved dorsal ribs both come from the left side (fig. 9). The articular facets of the capitula and tubercula are elliptical in shape, dorsoventrally taller than anteroposteriorly wide. The proximal neck of the ribs is roughly cylindrical in cross section, thinning mediolaterally to the ventral extent of the rib. The anterior face of the neck has a striated strip, running ventrally from the anterior aspect of the tuberculum and eventually fading as the rib flattens ventrally. This structure is present on both ribs and may represent a scar from the intercostal musculature. The posterior face of the rib has a long costal groove that begins just ventral to the tuberculum. The medial and lateral ridges bounding this groove become more closely appressed ventrally, uniting and terminating the groove when the rib becomes maximally flattened. A nutrient foramen is present on the posterior aspect of the capitulum, just dorsal to the tuberculum, in both well-preserved dorsal ribs.

FIGURE 7.

Mandible of IGM 100/1132 in A, dorsal and B, ventral views. Abbreviations: cp, coronoid process; dt1, first dentary tooth; pdl, facet for lateral ramus of predentary; pdv, facet for ventral ramus of predentary.

The right pes is represented by 13 elements, including portions of all four metatarsals and nine phalanges (fig. 10). Digits I–III are complete on the right pes. The left pes is represented by the distal ends of metatarsals II–IV and phalanx III-1. Generally, the lateral condyles of the pedal elements extend farther distally than the medial condyles, which are larger and more bulbous in appearance. Metatarsal I is flattened laterally to receive the medial surface of metatarsal II. Phalanx I-1 is elongate compared to the other nonungual phalanges and has a poorly developed intercondylar groove at its distal end. The extensor tendon pits on each condyle are well developed. Phalanx I-2 is an ungual—like all other unguals in this specimen, it is gently bowed and rounded on its dorsal aspect, but flattened ventrally. Unlike the other unguals, it is flattened on its medial surface, suggesting that in life the animal bore weight on the ventrome-dial surface of phalanx I-1, not the ventral surface as in the other three digits. Its medial and lateral sides bear grooves, probably for the passage of the blood vessels that supplied the keratinous sheath it would have borne in life. The lateral vascular groove is flanked by a medial ridge of bone, giving the ungual a spade-shaped appearance in dorsal or ventral view.

FIGURE 8.

Mandible of IGM 100/1132 in A, anterior and B, right lateral views. Abbreviations follow those in figure 7.

Digit II is represented by the left and right metatarsal and the right phalanges. The metatarsal bears well-developed extensor pits on each condyle, and is tightly appressed to metatarsals I and III in life position (fig. 10). The lateral condyle extends further distally than the medial. The phalanges are hourglass shaped, with equally wide proximal and distal ends but appressed in the middle. They are roughly as wide as long and possesses a well-developed intercondylar groove and extensor pits (fig. 10). The dorsal intercondylar processes of the phalanges are well developed into a posterior tongue.

FIGURE 9.

Left dorsal ribs of IGM 100/1132 in posterior view. A, most complete rib and B, second most complete rib. Abbreviations: ca, capitulum; gr, costal groove; nf, nutrient foramen; tb, tuberculum.

Digit III is represented by a left and right metatarsal, all four right phalanges, and left phalanx III-1. The metatarsal shaft is straight and roughly square in cross section, with flattened edges contrasting the more sinuous and cylindrical shafts of the other three metatarsals. The distal end is medially deflected, as in Psittacosaurus mongoliensis. These phalanges resemble the phalanges of digit II in having deep intercondylar grooves, prominent extensor pits, and tonguelike dorsal intercondylar processes (fig. 10). The extensor pits are particularly prominent on phalanx III-3. The ungual bears lateral ridges on both sides, framing the edges of the vascular groove, making the bone spade shaped in ventral view (fig 10).

Digit IV is represented by the left and right metatarsals but no phalanges (fig. 10). The shaft is closely appressed to that of metatarsal III, with the head deflected slightly laterally. Psittacosaurus mongoliensis has five phalanges on digit IV (Sereno, 1987), and it seems likely that Psittacosaurus amitabha had the same number in life.

The isolated chevron is highly fragmentary, representing only a portion of the proximal region of the element. The left pedicle is intact, possessing a circular articular facet for contact with the caudal centrum. The right pedicle is broken at about one quarter its height. While it impossible to confidently assign the position of this chevron in the caudal series, its relatively large size implies that it was associated with one of the large anterior caudals.

The 21 caudal vertebrae form a continuous series. Unlike the opisthocoelous caudals of Psittacosaurus mongoliensis, these caudals are amphicoelous, with both ends of the centrum concave. Psittacosaurus generally possesses 43 caudal vertebrae, so this series likely represents about half of the total caudal number (Sereno, 1987). The presence of well-developed chevrons associated with almost every vertebra strongly suggests that the preserved caudals are caudals 2–22 (as the first chevron in Psittacosaurus occurs between caudals 2 and 3). The most proximal caudal (interpreted as caudal 2) is not associated with a chevron, and thus likely paired with the large, very fragmentary chevron described above. Caudals 3–14 and 16–22 are all associated with chevrons; caudal 15 is therefore the only element in the series that cannot be paired with a chevron. The vertebrae vary in their preservation; caudal 3 (fig. 11) is the best preserved, with an almost complete neural arch and two intact fused transverse processes (Sereno, 1987). Caudal 12 is the most poorly preserved, having split in half down the sagittal plane such that only the right side is represented. All other caudals, though generally well preserved, are missing part or all of the neural arch and transverse processes. The right transverse process is usually the better preserved; only caudal 4 preserves a left transverse process without the right. The caudal centra range in length from 23 mm (caudal 3) to 15 mm (caudal 22).

FIGURE 10.

A, left and B, right pes of IGM 100/1132. Abbreviations: lmt, left metatarsal; rmt, right metatarsal. Roman numerals denote digit number, and Arabic numerals denote phalanx number within each digit.

FIGURE 11.

Third caudal vertebra of IGM 100/1132 in A, anterior, B, posterior, and C, right lateral views. Abbreviations: ch, chevron; ct, centrum; hc, hemal canal; ns, neural spine; poz, postzygapophysis; prz, prezygapophysis; tp, transverse process.

Moving posteriorly down the series, the centra become longer anteroposteriorly than tall. All caudals have some degree of ventral keeling, which is most pronounced in caudals 7–15. The anterior neural arches project dorsally, shifting to a more posterodorsal orientation in the more posterior caudals. The chevrons shrink dramatically to half their maximum length after caudal 15. The prezygapophyses are almost vertical in the anterior caudals, with facets directed mostly medially and slightly dorsally. These facets point increasingly medially in the progressively more posterior caudals, orienting the prezygapophyses increasingly vertically. The postzygapophyses complement their articular mates. The anterior caudals have mostly laterally and slightly ventrally directed postzygapophyses. These facets reorient more dorsally in the more posterior caudals, but distinct postzygapophyses do not occur past caudal 15.

Gastroliths

Nine small gastrolith fragments representing seven gastroliths were found with the skeleton (fig. 12). They are of varying sizes and lithologies, but all are rounded with a weathered surface texture. The largest gastrolith is 24.5 mm long, 19.4 mm wide, and 10.5 mm thick. The smallest is 12.6 mm long, 9 mm wide, and 7.2 mm thick.

Endocranial Cast

The well-preserved and undistorted braincase enables the generation of an endocranial cast, visualizing the brain, several cranial nerve buds, and the inner ear (fig. 13). To accommodate the missing anterior laterosphenoids, we set a conservative boundary between remaining sections of braincase to avoid estimating the cerebrum beyond the evidence present in the specimen. Foramina were followed to their opening, giving many nerve buds the appearance of a flattened expansion at their most distal extent; this is an artifact of the endocasting process and is not interpreted as the true anatomy of the organism.

FIGURE 12.

Gastroliths of IGM 100/1132.

The overall form of the endocast is highly reminiscent of the brain of Alligator mississip-piensis (e.g., Watanabe et al., 2018) as well as that of Psittacosaurus lujiatunensis (Zhou et al., 2007). The olfactory tracts are long and hourglass shaped, extending along the ventral aspect of the frontal past the orbits, presumably to contact the nasal mucosa. The olfactory tracts grade into the cerebrum. Due to the partial laterosphenoids, the lateral extent and general form of the cerebrum are unclear; however, it was certainly small relative to the enlarged cerebrum seen in coelurosaurian theropods (Brusatte et al., 2009; Witmer and Ridgely, 2009; Bever et al., 2013). The optic lobes and cerebellum are similarly unexpanded. The posterodorsal corner of the endocast displays a dorsal peak, corresponding to the dural sinus (Witmer and Ridgely, 2009; Bever et al., 2013). The pituitary extends from the midline, under the presumed location of the optic lobe; the internal carotid arteries travel dorsomedially into the pituitary fossa from below. A series of cranial nerve buds are visible; from anterior to posterior, they are interpreted as the buds of V, VII, X/XI (the jugular canal), and branches 1 and 2 of XII (fig. 13).

FIGURE 13.

Endocranial cast of the skull of IGM 100/1132, in A, dorsal and B, right lateral view. Abbreviations: asc, anterior semicircular canal; c, cerebrum; hsc, horizontal semicircular canal; ic, internal carotid artery; ie, inner ear; jc, jugular canal; ob, olfactory bulb; ot, olfactory tract; pb, pituitary body; psc, posterior semicircular canal; V, trigeminal nerve; VII, facial nerve; XII 1, 2, first and second branches of hypoglossal nerve; XII-3, third branch of hypoglossal nerve.

The most notable feature of the endocast is the inner ear; particularly conspicuous are the robust semicircular canals. The lateral semicircular canal is oriented slightly anterodorsally, implying that at rest, the animal's head was slightly downturned (Witmer et al., 2003; Brusatte et al., 2009). The same condition is seen in Psittacosaurus lujiatunensis (Zhou et al., 2007), the endocast of which is extremely similar to that of P. amitabha.