新潟県小千谷市の魚沼層群(鮮新-更新統)より発見された新種の化石トドについて

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In this article, fossil sea-lion Eumetopias ojiyaensis sp. nov. is described, which was excavated by the Niigata plain Collaborative Research Group in 1971. The materials were found at the cliff along the Nobe river, Ojiya city, Niigata Prefecture, northern Central Japan (lat. 37°17′09″N, long. 138°48′34″E.). The excavated materials were composed of sixty-four bones, which were classified into canines, a incisor, a molar, manus bones, pes bones, patella, tibia, fibulla caudal vertebrae and nails. The sandstone bed which yielded those bones is included in the Lowermost Formation of the Uonuma Group (Plio-Pleistocene). According to the Research Group, the sandstone bed is assigned to the same horizon in which were found plant remains of fuglans megacineria, Metasequoia disticha, Glyptostrobus pensilis, Liquidambar formosa and Menyanthes trifoliata and is estimated to have the range between 2.5×10^6 Y.B.P. and 2.9×10^6 Y.B.P. by the fission-track method. The results of the diatom analysis indicate that the sandstone was deposited under the littoral environment, and the Contained molluscan and foraminiferal fossils suggest the presence of a cold current as supported by Clinocardium californiense, Patinopecten (Mizuhopecten) yessoensis, Buliminella elegantissima, Elphidium sp. (A≒E. echigoense), Bucella frigida (The Niigata plain Collaborative research group 1972, Shinanogawa Nojiriko Tomonokai, 1976 (MS)). The materials have common morphological characteristics of Otariidae by KING (1964) as a whole. Among them, the form of canine is quite similar to that of living female of Eumetopias jubata, and the size of root of upper incisor is as twice as that of female E. jubata. As for the molar, only one tooth was found, which may belong to any of lower second to fifth molar. Both the lingual and the buccal side of molar form imperfect ovol. A groove runs vertically along the middle of each surface, and the lingual one is deeper than the buccal one. Moreover, the grooves divide the root of tooth into two apical portions. The crown of molar consists of three cusps : medial accessory cusp, main cusp and distal accessory cusp. The medial accessory cusp is remarkably higher than that of living E. jubata (6mm in the present specimen; 4mm in the living female Eumetopias). One tibia, one fibulla and many bones of manus and pes were excavated. Most of them are nearly the same size as those of living male Callorhinus ursinus, but each has smaller size, 7/10 to 8/10, in comparison with living female of E. jubata The author made the morphological Comparison of the bones to those of living Otariidae, and he came to a conclusion that the present materials are similar to E. jubata and different from other genus of otariidae. It is well-known that the size of bones of Otariidae has a great difference between male and female. Taking this fact into consideration, it is possible to consider that the present materials belong to a female of Eumetopias. It is clear that the present materials belong not to the living species of sea-lion, Eumetopias jubata, and they must belong to another independent species, E. ojiyaensis. The materials are estimated to be more than seventeen years old. In comparison of E. ojiyaensis with Eumetopias sp. (KASENO) MITCHEL 1968 from the Pliocene of Toyama Prefecture, the molar of the present species has common characteristics in root form, grooves in the middle part of root, bifurcation of root apex, formation of accessary cusps, and development of cingulum. Eumetopias sp. (KASENO) has been considered as male animal, so it is difficult to compare E. sp with E. ojiyaensis but it is possible to assume that Eumetopias ojiyaensis is close or same to the Eumetopias sp.
地学団体研究会の論文
1981-05-25

新潟県小千谷市の魚沼層群(鮮新-更新統)より発見された新種の化石トドについて

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