Three odontaspidid fossil shark teeth previously reported from the “Lower Formation” of the Upper Cretaceous Mifune Group in Yamato Town and Mifune Town, Kumamoto, Japan as three separate taxa (“Carcharias” amonensis, “C.” cf. amonensis, and “Carcharias” sp.) are reexamined and described in detail. Restudy shows that all of these shark teeth should be assigned to the species “Carcharias” amonensis. These specimens establish that “C.” amonensis also inhabited the Pacific region and thus had a worldwide distribution in the temperate sea in the Cretaceous.
Introduction
Occurrences of the teeth of “Carcharias” amonensis (Chondrichthyes, Odontaspididae) from the “Lower Formation” of the Upper Cretaceous Mifune Group, Kumamoto Prefecture, Japan have already been reported by Kitamura (2008); these teeth are now described in detail in the present paper. Although Kitamura (2008) distinguished three “Carcharias” species (“C.” amonensis, “C.” cf. amonensis and “C.” sp.) without taxonomic particulars, they are identical to “Carcharias” amonensis in this paper. In Japan, the occurrence of odontaspidid fossil shark teeth in the Cretaceous is very rare, and few specimens have been described (Goto, 1994; Yabumoto and Uyeno, 1994). The specimens described here represent the existence of “Carcharias” amonensis in the Pacific region and show this species had a worldwide distribution in the temperate sea in the Cretaceous.
Geological setting
The Mifune Group is distributed from Mifune Town to the eastern part of Uto city in the central part of Kumamoto Prefecture, Kyushu, Japan. It was divided into three formations, “Basal", “Lower” and “Upper” by Matsumoto (1939). Teeth of “Carcharias” amonensis were collected from the “Lower Formation” of the Mifune Group in Fukura, Yamato Town and in Shimoumeki, Mifune Town, Kumamoto Prefecture (Figure 1). The “Lower Formation” consists of sandstone and shale, considered to have been deposited under brackish to shallow marine conditions (Tamura and Tashiro, 1966). The specimens of “C.” amonensis were found from a massive sandstone layer of the “Lower Formation” at approximately 20 m above the base of the formation with brackish-water mollusks (e.g. Oligoptyxis pyramidaeforme, Eomiodon matsubasensi, Pseudasaphis japonicus) showing autochthonous occurrence in Fukura (Kitamura, 2008) and from a coarse sandstone with the same brackish mollusks at Shimoumeki. The geological age of the “Lower Formation” has been considered as middle to late Cenomanian on the basis of an ammonoid (Eucalycoceras sp. cf. E. spathi) and an inoceramid (Inoceramus concentricus costatus) (Tamura and Matsumura, 1974) and as late Cenomanian to early Coniacian based on fission-track (FT) dating of tuff (Ikegami et al, 2007).
Figure 1.
Geological map and cross sections of the Mifune Group showing the location and horizon of the “Carcharias” amonensis fossils in Kumamoto, Kyushu. (Modified from Tamura, 1979; Tamura et al, 1991)
Systematic paleontology
Order Lamniformes Berg, 1958 Family Odontaspididae Müller and Henle, 1839 Genus Carcharias Rafinesque, 1810 “ Carcharias ” amonensis (Cappetta and Case, 1975) Figure 2 Carcharias sp. Kitamura, 1997, p. 35. pl. 1, fig. 10
Material and Method.—KCM (Kumamoto City Museum) 12-000293 anterior tooth from Shimoumeki, Mifune Town, “Lower Formation” of Mifune Group, Cenomanian. KCM 12-000208 and KCM 12-000209, lateral teeth from Fukura, Yamato Town, “Lower Formation” of the Mifune Group, Cenomanian. Systematics and terminology follow Cappetta (1987) and the measuring method is based on Uyeno (1975).
Description.—KCM 12-000293: The maximum height and width of the tooth are 9.3 mm and 4.6 mm, respectively. The specimen is an anterior tooth with a slender cusp that is sigmoidal in mesial view. The main cusp bears wide and low triangular lateral cusplets on each side of the crown base. The main cusp inclines distally. The cusplets are considerably shorter than the main cusp. Very well developed and continuous smooth cutting edges are present on the main cusp and on the mesial cusplet, but the cutting edges do not reach the distal foot of the crown. The lingual faces of all of the cusps are more convex than the labial faces. The lingual and labial faces of all of the cusps are completely smooth. The root is bilobed with rounded extremities, but the presence of nutrient grooves and dental bands cannot be determined because of the poor preservation. The root has a Ushaped basal margin (Figure 2, 1a–1c, 1a'–1c').
KCM 12-000208: The maximum height and width of the tooth are more than 7.3 mm and more than 7.6 mm, respectively. The specimen has a triangular crown (main cusp) bearing two wide triangular lateral cusplets on each side of the crown base. The main cusp inclines distally. All cusplets are considerably shorter than the main cusp. The most distal and mesial cusplets are smaller than the others, and the sizes of the lateral cusplets decrease distally and mesially. These cusplets incline distally. In particular, the inner distal cusplet is bent sharply to the distal side, whereas the inner pair of cusplets is slightly swollen laterally. The outermost distal cusplet is small and triangular in shape, whereas the outer mesial cusplet is missing. The lingual faces of all of the cusps are more convex than the labial faces. The lingual and labial faces of all of the cusps are completely smooth. Very well developed and continuous smooth cutting edges are present on all cusps. A narrow lingual dental band is present on the foot of the main cusp. The basal parts of the root are missing. A distinct nutrient groove bisects the weak lingual root protuberance on the teeth. The root has a weak, U-shaped basal margin (Figure 2, 2a–2d, 2a'–2d').
KCM 12-000209: The maximum height and width of the tooth are 3.0 mm and 4.9 mm, respectively. The labial side is buried in the matrix. The tooth has a wide triangular crown on the main cusp and a triangular lateral cusplet on each side of the crown base. The main cusp is strongly inclined distally. The height of the cusplets is almost half the height of the main cusp. These cusplets incline distally. The mesial cusplet is strongly swollen laterally at a point one-third of the distance from the base. The lingual faces of all of the cusps are convex and smooth. Very well developed and continuous smooth cutting edges are present on all cusps. The root is bilobed with rounded extremities. The root lobes are long, extending well beyond the crown, and the height is the same as that of the crown. A weak nutrient groove bisects the weak lingual root protuberance on the teeth (Figure 2, 3, 3').
Figure 2.
Odontaspidid fossil shark teeth “Carcharias” amonensis from the Mifune Group. 1, anterior tooth (KCM 12-000293); 1a, 1a', lingual view; 1b, 1b', labial view; 1c, 1c', distal view; 2, lateral tooth (KCM 12-000208); 2a, 2a', lingual view; 2b, 2b', labial view; 2c, 2c', distal view; 2d, 2d', mesial view; 3, 3', lateral tooth (KCM 12-000209), lingual view. Scale bars represent 5 mm unless otherwise stated. Dotted line indicates estimated shape.
Discussion
The specimens described here are assigned to the taxon “Carcharias'” amonensis because of the combination of the following seven features based on Cappetta and Case (1975) and Welton and Farish (1993): 1) A sigmoidal slender main cusp with wide and low triangular lateral cusplets on each side of the crown base (KCM 12000293, anterior tooth). 2) A triangular main cusp with two wide triangular lateral cusplets on each side of the crown base and the main cusp inclined distally (KCM 12-000208, lateral tooth). 3) A wide triangular crown on the main cusp and a triangular lateral cusplet on each side of the crown base and the distally inclined main cusp (KCM 12-000209, lateral tooth). 4) Very well developed and continuous smooth cutting edges (except for the distal foot of the crown of KCM 12-000293). 5) The root bisected by a nutrient groove. 6) The smooth and wide triangular cusp and 7) one to two pairs of lateral cusplets.
Figure 3.
Occurrence of “Carcharias”amonensis (Cappetta and Case, 1975). 1, Slaughter and Thurmond (1974a), Werner (1989a, 1990a); 2, Cappetta and Case (1975b, 1999), Welton and Fansh (1993); 3, Landemaine (1991e); 4, Case (2001); 5, Cicimurri (2001ad), Cumbaa et al. (2010); 6, Cicimurri (2001b); 7, Rage and Cappetta (2002); 8, Antunes and Cappetta (2002); 9, Shimada et al. (2006); 10, Vullo et al. (2007e); 11, Vullo et al. (2009e); 12, Cavin et al. (2010f); 13, Cumbaa et al. (2010); 14, this paper. The paleogeographic map (late Albian) is from Barron (1987). “Carcharias” amonensis was reported as aLamna planidens Willston 1900, Odontaspis amonensis Cappetta and Case, 1975, c, f Serratolamna amonensis (Cappetta and Case, 1975), Carcharias aff. amonensis (Cappetta and Case, 1975) and c“Carcharias” amonensis (Cappetta and Case, 1975). I follow the species classification scheme of Z. planidens, O. amonensis and S. amonensis proposed by Cappetta and Case (1999).
Several other odontaspidid genera have been reported in Cretaceous deposits: Odontaspis Agassiz, 1838, Pueblocarcharias Bourdon et al, 2011, Cenocarcharias Cappetta and Case, 1999, Hispidaspis Sokolov, 1978, Johnlongia Siverson, 1996, Eostriatolamia Glikman, 1980, and Roulletia Vullo et al, 2007. The specimens reported here are distinguished from the genera Cenocarcharias, Hispidaspis, Pueblocarcharias, Eostriatolamia, and Johnlongia by having a completely smooth cusp. The specimens reported here are distinguished from the genus Odontaspis by having lower and wider cusplets and almost continuous cutting edges between the cusp and the lateral cusplets (Cappetta, 1987). The specimens reported here are also distinguished from the teeth of the genus Roulletia, the morphologically closest taxon (Vullo et al, 2007), by a thinner crown and root and by the presence of one or two pairs of cusplets. In Japan, odontaspidid fossil shark teeth from the Cretaceous are very rare, and few specimens have been described (Goto, 1994; Yabumoto and Uyeno, 1994). Odontaspis cf. complanata was reported from a Cretaceous deposit (Santonian) in Hokkaido (Yabe, 1902). Odontaspis sp. was reported from the Hiraiga Formation of the Miyako Group (upper Aptian) in Tohoku and the Middle Ezo Group (Cenomanian) in Hokkaido (Research Group for Mesozoic Fossil Sharks, 1977). Odontaspis (?) sp. was reported from the Izumi Group (Campanian to Maastrichtian) (Nishimoto and Morozumi, 1979). Carcharias sp. was reported from the Lower Cretaceous Sebayashi Formation, Sanchu Group in Gunma Prefecture (Takakuwa et al, 2008). The teeth of “C.” amonensis occur in four Cretaceous oceanic regions including the Western Interior Seaway (e.g. Shimada et al, 2006), North Atlantic (e.g. Cavin et al, 2010), South Atlantic (Antunes and Cappetta, 2002), and Tethyan Ocean (e.g. Slaughter and Thurmond, 1974), from approximately 50°N to approximately 20°S (Figure 3). The specimens described here support the conclusion that “C.” amonensis also inhabited the Pacific region. The total latitude range of “C.” amonensis is more narrow than that of the contemporary large lamniform shark Cretodus semiplicatus (between about paleolatitudes 60°N and 60°S; Tomita and Kurihara, 2011) and differs from another contemporary large lamniform shark, Cardabiodon ricki, which showed an antitropical distribution (between about paleolatitudes 40° and 60° in the Northern and Southern hemispheres; Cook et al, 2010) and shows this species had a worldwide distribution in temperate seas in the Cretaceous.
Acknowledgments
The author extends grateful thanks to Masato Fujisaki, Yukio Ogata, and Kazuo and Kazutaka Furusyo for allowing access to the specimens for study. Moreover, the author gratefully thanks Toshifumi Komatsu (Kumamoto University) for his useful comments and Yoshitaka Yabumoto (Kitakyushu Museum of Natural History and Human History) for his discussion and for critically reading an early draft of the manuscript. Finally, the author would like to thank former professor Yoshitaka Hase (Kumamoto University) for his continued support and Romain Vullo (Université de Rennes 1) for reviewing the manuscript.
