A new deep-sea shark of the genus Scymnodalatias (Squaliformes, Somniosidae), S. kazenobon sp. nov., is described from the middle Miocene Yatsuo Group in Central Japan. This is the first fossil record of the genus Scymnodalatias from the Miocene strata and its first occurrence in the Pacific region. This discovery seems to indicate that major distributional changes occurred in the Pacific region sometime during the late Cenozoic era.
Introduction
Scymnodalatias is an extant genus of deep-sea shark belonging to the family Somniosidae in the order Squaliformes. This genus is characterized by an absence of dorsal fin spines and a long, broad, and arched mouth (Ebert et al., 2013). It contains four living species, which have rarely been recorded in deep-sea waters around the world. Somniosidae genera with relatively large teeth have frequently been recorded in the fossil record of Neogene sediments around the world, including Centroscymnus (e.g. Adnet, 2006; Marsili, 2007; Takakuwa and Suzuki, 2009; Suzuki, 2012), Centroselachus (e.g. Suzuki, 2008, 2012), Scymnodon (e.g. Phillips et al., 1976; Welton, 1981; Cappetta, 1987; Cigala Fulgosi, 1996), and Somniosus (e.g. Nishimoto and Ujihara, 1979; Cappetta, 1987; Cigala Fulgosi, 1988; Takakuwa and Suzuki, 2009; Suzuki, 2012). However, fossil records of Scymnodalatias are limited, with only two records, possibly owing to their small tooth size (Cigala Fulgosi, 1996; Adnet, 2006). Well preserved teeth of the genus Scymnodalatias have been collected from the middle Miocene Yatsuo Group in recent years. These specimens differ from all of the previously described species of the genus Scymnodalatias. The present study describes them and discusses their paleontological implications.
Geological setting
The specimens were collected from a cliff alongside a stream, 900 m south of Osedani, Fuchu-machi, Toyama City, Toyama Prefecture, Central Japan (Figure 1). The fossil horizon is situated in the upper part of the Tochiage Mudstone Member of the Higashibessho Formation of the Yatsuo Group (Hayakawa and Takemura, 1987). The strata at this locality are composed of light gray massive mudstone with small (around several centimeters in diameter) calcareous concretions (Figure 2). The Shiotani Sandstone Member comprises blue-gray, very fine-grained, massive sandstone. Fossils of deep sea sharks Centrophorus sp., Deania sp., Etmopterus sp., and Squalus sp. were collected from the same horizon.
According to Yanagisawa (1999) and Watanabe and Yanagisawa (2005), the depositional duration of the upper part of the Tochiage Mudstone Member is assigned to the interval between 15.8 and 15.6 Ma on the basis of diatom assemblages. Yanagisawa (1999) correlated the Tochiage Mudstone Member with the Doyama Formation in the Iozen area and the Asagaya Formation in the Kanazawa area. Hasegawa and Takahashi (1992) indicated that the Higashibessho Formation was deposited under the middle bathyal environments based on its benthic foraminifera assemblage.
The Scymnodalatias specimens are housed in the Ujihara Laboratory Specimens Room of the Graduate School of Environmental Studies, Nagoya University, and are designated with the specimen code NUUL (Nagoya University Ujihara Laboratory).
Systematic description
The classification and the terminology used to describe specimens largely follow those of Cappetta (1987, 2006, 2012), Yabe and Goto (1999) and Adnet and Cappetta (2001). Figure 3 shows the dental terminology.
Order Squaliformes Goodrich, 1909
Family Somniosidae Jordan, 1888
Genus Scymnodalatias Garrick, 1956
Type species.—Scymnodalatias sherwoodi (Archey, 1921).
Scymnodalatias kazenobon
sp. nov.
Figure 4
Diagnosis.—Lower tooth height much greater than width. Cusp rather upright, thin, and narrow, and triangular with straight edges. Apex of cusp sharp. Distal heel high, narrow, well differentiated, and semicircular. Distal heel forming distinct V-shaped notch where it meets main cusp. On labial face, wide flat apron extending to root basal edge. Several large foramina located at mesial and distal sides of apron. Large foramen opening above basal notch. On lingual face, straight and well-marked lingual bulge developing, and distinct central foramen situated below short uvula. One large mesio-lingual foramen opening on mesial side of central foramen. Shallow medio-lingual duct extending from central foramen to lower notch. Lower notch shallow. Base of root almost straight, except for central notch.
Material.—Holotype, NUUL127, well preserved lower symphyseal tooth (Figure 4A). Paratype, NUUL128, well preserved lower anterior or lateral tooth (Figure 4B).
Measurements.—NUUL127, maximum width 0.8 mm, maximum height 2.0 mm; NUUL128, maximum width 0.7 mm, maximum height 1.7 mm.
Type locality.—Cliff alongside stream, 900 m south of Osedani, Fuchu-machi, Toyama City, Toyama Prefecture, Japan. 36°60′37.72″N, 137°06′74.90″E.
Type horizon and age.—Tochiage Mudstone Member of Higashibessho Formation, Yatsuo Group; middle Miocene, 15.8–15.6 Ma (Yanagisawa, 1999).
Etymology.—Derived from the Japanese kazenobon, a famous festival in the Yatsuo area.
Description.—The lower teeth are much higher than they are wide and are labio-lingually compressed. The lower symphyseal tooth, NUUL127, is symmetrical and has a narrow main cusp, flanked on either side by a high, narrow, and semicircular heel. The cusp is upright, triangular, narrow, and thin. The apex of the cusp is sharp. The cutting edges are almost straight, with no serrations. The labial crown face is flat, and the lingual face is slightly convex. Each heel forms a distinct V-shaped notch where it meets the main cusp. The apron is wide and flat, and extends to the base of the root. The uvula is short and wide.
The root is rectangular, with a shallow central notch at the base. The edges of the root are almost straight and are parallel to each other. Two large margino-labial foramina open on either side of the apron. Several small foramina open around the apron. On the lingual face, a straight and well marked lingual bulge develops, and a very distinct central foramen opens below the uvula. There is a large mesio-lingual foramen on the mesial side of the central foramen. A shallow and broad medio-lingual duct extends from the central foramen to the lower notch. The base of the root is almost straight, with the exception of the central notch.
The lower anterior or lateral tooth, NUUL128, is asymmetrical. The mesial edge of the tooth is slightly concave. NUUL128 has a narrow, triangular, and thin main cusp. The cusp slightly inclines distally. The mesial and distal cutting edges of the cusp are almost straight and are not serrated. The labial crown surface is flat, and the lingual crown surface is slightly convex. The distal heel is high, narrow, and semicircular. It forms a distinct V-shaped notch where it meets the main cusp. A wide and flat apron extends to the base of the root. The uvula is short and wide.
The root is rectangular. The mesial edge of the root is concave, and the distal edge is convex. One large margino-labial foramen opens on the mesial side of the apron and three margino-labial foramina open on the distal side of the apron. On the lingual face, a straight and well marked lingual bulge develops, and a large central foramen is seen below the uvula. A large mesio-lingual foramen opens on the mesial side of the central foramen.
Figure 3.
Dental terminology of Scymnodalatias kazenobon sp. nov. (after Cappetta, 1987, 2006, 2012; Yabe and Goto, 1999; Adnet and Cappetta, 2001). A, holotype, NUUL 127, lower symphyseal tooth in labial (A1) and lingual (A2) views; B, paratype, NUUL 128, lower anterior or lateral tooth in labial (B1) and lingual (B2) views.
Discussion
Cappetta (2012) described the teeth of species of the Somniosidae family, including species of Scymnodalatias and its related genera. The Yatsuo specimens have the following diagnostic characteristics of Scymnodalatias, as mentioned by Cappetta (2012): a rather upright triangular crown, a parallelepiped root, a distinct lingual bulge with a central foramen, and a straight root base except for a single concavity. The teeth of the genus Scymnodalatias resemble those of the genus Zameus in having a rather upright triangular crown and a parallelepiped-shaped root. However, they have a less distinct medio-lingual duct. They differ markedly from those of the genus Scymnodon in having a cutting edge without serrations. The teeth of the genus Scymnodalatias are clearly distinguished from those of the genus Centroselachus by having a high and not horizontal distal heel. The symphyseal tooth of S. kazenobon is distinct from that of the genus Paraetmopterus by the higher cusp and smaller marginolabial foramina.
The genus Scymnodalatias comprises four modern species and one fossil species: S. albicauda Taniuchi and Garrick, 1986, S. garricki Kukuev and Konovalenko, 1988, S. oligodon Kukuev and Konovalenko, 1988, S. sherwoodi (Archey, 1921), and S. cigalafulgosii Adnet, 2006, from the Eocene of France. The lower teeth of S. kazenobon closely resemble those of S. cigalafulgosii in having a rather upright cusp, a rectangular root, and a high and narrow distal heel. However, S. kazenobon has a narrower cusp, a shorter root, a shallower basal notch, and a shallower medio-lingual duct. Cigala Fulgosi (1996) described S. aff. garricki from the Pliocene of Italy. He differentiated S. aff. garricki from S. garricki by the bellshaped profile of its cusp base, narrower apron, and slight difference in the style of root foramina at the side of the apron. This new species differs from S. aff. garricki in its higher and narrower distal heel and broader cusp. This new species also differs from S. albicauda, as described by Herman et al. (1989), in having a higher and narrower distal heel, shallower basal notch, shallower medio-lingual duct, indistinct broad apron, and broader width of the notch situated in the cusp-distal heel junction. In addition, the anterior or lateral tooth, NUUL128, has a convex distal edge at the root, whereas the anterior tooth of S. albicauda bears a straight edge. Scymnodalatias kazenobon is differentiated from S. garricki, as described by Cigala Fulgosi (1996), by having a higher and narrower root and weaker convexity of the mesial basal edge of the cusp. Scymnodalatias kazenobon differs from S. sherwoodi, as described by Garrick (1956), in having a higher distal heel and lower ratio between the cusp height and tooth height; the ratio of S. kazenobon is 0.4, and that of S. sherwoodi is approximately 0.5. The outline of the tooth of S. oligodon, except for the cusp, is almost square (the height is equal to the width); in contrast, this new species has a rectangular outline (the height is longer than the width). As explained above, the Yatsuo specimens are recognized as a new species of Scymnodalatias.
Fossil records of the genus Scymnodalatias are known from the Eocene of France (Adnet, 2006) and the Pliocene of Italy (Cigala Fulgosi, 1996). Figure 5 shows a map of the geographic distribution of living Scymnodalatias species in comparison with the fossil records of the genus. The Yatsuo specimens constitute the first fossil record of the genus from obvious Miocene strata and the first occurrence of it in the Pacific region. The modern species of the genus are not known in the northern Pacific Ocean, but they are distributed in the southern Pacific Ocean, southern Indian Ocean, and northern Atlantic Ocean (Ebert et al., 2013). The occurrence of the genus Scymnodalatias from the Japanese middle Miocene seems to indicate that major distributional changes of the genus occurred in the Pacific region sometime during the late Cenozoic era. Distinct discrepancies between distribution in geologic time and distribution in the modern era are also seen in other deep-sea elasmobranchs, including Scymnodon (Welton, 1979; Froese and Pauly, 2016) and Plesiobatis (Vialle et al., 2011; Froese and Pauly, 2016). This study provides important data for the Cenozoic paleobiogeography of deep-sea elasmobranchs.
Acknowledgments
We thank Yasufumi Iryu (Institute of Geology and Paleontology, Graduate School of Science, Tohoku University), Itsuki Suto, Seiji Hayashi, and Marc Humblet (Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University), and Tatsuo Oji, Sachiko Nishida, and Shin-ichi Fujiwara (Nagoya University Museum) for their useful input. This manuscript was greatly improved by the comments from four anonymous reviewers.
