樺太産のDesmostylus mirabilis NAGAOと,岐阜県産のPaleoparadoxia tabatai(TOKUNAGA)の頭蓋骨の研究

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Introduction (1) On the discovery of Desmostylus mirabilis NAGAO from south Sakhalin(p.1) In May 1933, the fossil was discovered by an unknown laborer in Miocene Honto series, at no. 4 dike of Hatsuyuki-sawa, a tributary of the Keton river, Keton, Shisukamachi, Shisuka-gun, South Sakhalin, Japanese former territory. Through excavation in October 1933 and September 1934, the whole skeleton was dug out by late Takumi NAGAOand late Saburo OISHI. The specimen is now in the custody of Department of Geology and Mineralogy, Faculty of Science, Hokkaido University. (2) On the discovery of Paleoparadoxia tabatai (TOKUNAGA) from Gifu Prefecture, Japan (p.1) The whole skeleton of this specimen was discovered and excavated from Miocene Yamanouchi formation by Shigemasa TOMATSU and Mitsuhiko AZUMA, in October 1950, at Inkyoyama, Izumi-machi, Toki city, Gifu Prefecture. This specimen is kept now at the National Science Museum of Tokyo. (3) Establishment of Desmostylus Research Committee (DEREC) (pp.1〜2) For the purpose of preservation and investigation of D.m., DEREC started in 1950. The members of this committee are Hisakatsu YABE (Chairman), Fuyuji TAKAI, Tokio SHIKAMA, Masao MINAT0 and Shoji IJIRI. (4) Progress of study (p.2) The skeletons of D.m. and P.t., were divided into three parts, viz. 1) Skulls 2) Vertebral columns and thoraxes 3) Fore-limbs and hind limbs, the investigation of which was assigned to S. IJIRI, F. TAKAI and T. SHIKAMA respectively. IJIRI finished his allotted work as early as 1954, and the present paper is his first report. I. Description of the skull bones of Din. from South Sakhalin (1) Cranium (pp.3〜4) The cranium is in an imperfect preservation : it lacks anterior parts of maxilla, nassal bone and others, and its mandibulla consists only of molor teeth and ramus of jaw. Moreover, this specimen is severely distorted and hardly any suture can be recognized. So, measurement was taken of the upper, lower, lateral (right side) and hind surfaces of the cranium (Figs. 3-7, Pl. I. 1-4). (2) Skull bones (pp.3〜4) As the sutures of the skull bones are scarcely remained, it is impossible to study each skull bone separately. We are, accordinly, obliged to give only their outlines on the covers of plates. (3) Teeth (pp.5〜6) There was given macroscopic description of P^4-M^3, ^1M-^3M ; M_2-M_3, _2M-_3M of D.m. (Pls 7-12). Dental formula of D.m. is inferred to be D=0133/1133 and its dentition to be monophyodonty. II. Description of the skull bones of P.t. from Gifu Prefectures, Japan (1) Cranium (pp.6〜10) The cranium is preserved almost perfectly except a portion of sphenoid bone, palate bone, incisors and premolars, and hyoid bone. Measurement was taken of the upper, lower, lateral (left side) and hind surfaces of the cranium ; and also of lateral and upper surfaces of the mandibulla (Figs. 9-16, P1. III, i-5, P1. IV, 1-4). (2) Skull bones (pp.10〜12) Sixteen skull bones are described macroscopically. (3) Teeth (pp.12〜15, 17) Together with the observation by X-rays, macroscopic descriptions of every tooth of P.t. are given. Hence it is concluded that P.t. is diphyodonty herbivorous and its dental formula is D=3143/ 3143. III. Taxonomy of Desmostylus and Paleoparadoxia from the view point of skull bones (1) Taxonomy of Desmostylus (pp.17〜19) Flaying no intention to go too far into the taxonomical problems, we have followed the term "Desmostylus mi rabilis" in this case, but the skull of D.m. is almost identical with that of Desmostylus hespers. As it was difficult to make comparative antomy on each individual skull bone of D.m., we depended mainly on the teeth comparative anatomy and comparative histology by means of optical- and electron-microscope. Consequently, we have brought out some problems of (i)-(viii) which will be useful for the future study of the taxonomy of Desmostylus. We have come to a new conclusion that, if it is justifiable to give a special importance to comparative anatomy of teeth, Desmostylus is similar to Perisodactyla (Tapirus) more than any other living animals (Fig. 21). (2) Taxonomy of Paleoparadoxia (pp.19〜22) Similarly to the case of D.m. we have followed the term of Reinhart, "Paleoparadoxia tabatai (TOKUNAGA)." It is true that the mandibulla of p.t. from Gifu Prefecture is quite alike to that of P.t. from California ; but the studies on the teeth of Paleoparadoxia have hitherto been so naive as in the case of Desmostylus that it is necessary to re-examine the specimens, for instance, of North America on some future occasion. On each individual skull bone and every tooth, comparative anatomy-especially with aquatic or semi-aquatic qudruped mammals such as Hippopotamus, Tapirus and Helicore-was made. On the teeth, we investigated microscopically and submicroscopically as in the case of D.m. The result of these studies are stated in (i)-(xvii), and, it can be concluded that Paleoparadoxia, like Desmostylus, is aquatic (marine) herbivorous and related most closely to Perisodactyla (Tapirus) among living animals, if it is justifiable to put a special importance on the microstructure of teeth (Fig.22). IV. Ecology of Desmostylus and Paleoparadoxia from the viewpoint of the study of skull bones (pp.22〜23) In our opinion, Desmostylus and Paleoparadoxia are unquestionably quadruped land herbivorous mammals : the former is browsing, while the latter is omnivorous grazing. It is very probable that they live mainly an aquatic life being soaked in the sea-water under far warmer climatic environment in Miocene time than the present. Considering every skull bone and tooth separately, Desmostylus and Paleoparadoxia have many resemblances to various animals. To synthesize them, as a whole, they are inferred to be much closer to Sirenia, Proboscidea and especially to Perisodactyla or Sus in Artiodactyla than any other living animals. But it can not be concluded only from the study of skull bones that they belong to Perisodactyla proper. Postscript (p.23) The correct zoological name, taxonomic position, systematic position, etc. of D.m. and P.t. ought to be decided and presented formally by DEREC, after the completion of the studies of vertebral columns, thoraxes, fore-limbs and hindlimbs. Acknowledgement (pp.23〜24) Omission.
地学団体研究会の論文
1961-03-25

樺太産のDesmostylus mirabilis NAGAOと,岐阜県産のPaleoparadoxia tabatai(TOKUNAGA)の頭蓋骨の研究

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