長野県野尻湖層より産出した齧歯類化石とその意義

概要

論文の詳細を見る
The seventh excavation of Lake Nojiri(Nojiri-ko), which was carried out from March 25th to April 1st of 1978, made a success of digging out numerous fossils and artifacts from the Latest Pleistocene Nojiri-ko Formation, distributed in the bottom of the lake. In the course of the cleaning of these fossils, several teeth of rodents were found by sieving the fine sands which were adhered to a bone fragment of fossil elephant. Since the occurrence of the Pleistocene micro-mammalian remains in Japan have been known exclusively from cave and fissure filling sediments, it can be said that these are the first rodent fossils discovered from the Pleistocene sediments outside of caves and fissures. They are described as follows: Order Rodentia Suborder Myomorpha BRANDT, 1855 Family Cricetidae ROCHEBRUNE, 1883 Subfamily Microtinae MILLER, 1896 Genus Microtus SCHRANK, 1798 Microtus sp.(Figs. 4-5; P1. 1, Figs. 1a-1d) Material: Two specimens of right M_2 (specimen numbers: 7NIIIS18-34, 7NIIIS18-35). Locality: Bottom of Lake Nojiri, Tategahana, Shi nano-machi, Kamiminochi-gun, Nagano Prefec ture (long. 138°13′E, 1at. 36°50′N) Horizon: Upper Nojiri-ko Formation I (Latest Pleis tocene, 16150±550 yr. B.P., Gak-267 in ^<14>C age). Description: The material are represented by only two fragmental right M_2. It is assumed that they originally have two lingual and two buccal triangles, and one posterior loop. The most part of the posterior loop is damaged in 7NIIIS18-34. The posterior loop, and the most part of second and fourth triangles counting from the posterior part, are lost, in 7NIIIS1835. Both specimens are grayish to dark brown in color, and rootless. The crown with prismatic shape is strongly curved buccally. Each salient angle is pointed. The lingual triangles are larger than the buccal ones. Each triangle is independent, not confluent with each other. Cementum is observed in each reentrant fold. Demensions and method of measurement are given in Table 1 and Fig. 2 respectively. Comparison and remarks: The materials are easily distinguished by their enamel pattern from Clethrionomys, Eothenomys, Aschizomys and Myopus, which are representative genera of subfamily Microtinae known from the Pleistocene and Recent of the Japanese Islands and their adjacent areas. The present materials are characterized by their rootlessness, pointed salient angles, closed triangles and cementum in reentrant folds. These characters are well coincident with those of the genus Microtus. The specific characters of the genus are present in the enamel pattern of M_1 and M^3 of the most species. As our materials are only two isolated M_2, further discussions are impossible. Myomorpha, gen. et sp. indet.(Figs. 6-7 ; P1. 1, Figs. 2a-2c) Material: One specimen of right lower incisor(7NIIIS18-33). Locality and horizon: Same as above mentioned molars of Microtus sp. Description: The material is only one fragmental lower incisor with somewhat damaged occlusal surface. Dentine of lingual side is fairly lost and dark brown to black in color. Enamel is almost preserved and yellowish gray in color, which forms a thin cover of labial side as in usual rodent incisors. In cross section, the enamel is U-shaped, which thickens in the central portion and gradually thinned towards the both ends(Fig. 7). The surface of the enamel is smooth trom front to rear. Demensions and method of measurement are shown in Table 2 and Fig. 3 respectively. Comparison and remarks: The material has shape and size quite similar to the incisor of myomorph rodents of the living and Quaternary forms of Japan. As the rodent incisor is less characteristic from taxonomic viewpoint, the present material is insufficient for more precise determination.
地学団体研究会の論文
1979-09-25

著者

河村善也
京都大学理学部地質学鉱物学教室

長野県野尻湖層より産出した齧歯類化石とその意義

スポンサーリンク

概要

著者

関連論文

スポンサーリンク