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Home , Proboscidea, Stegodon, Stegodontidae, Western Tokyo

miensis Matsumoto () from the Title Yaoroshi Formation, Akiruno City, , ( fulltext )

Author(s) AIBA, Hiroaki; BABA, Katsuyoshi; MATSUKAWA, Masaki

Citation 東京学芸大学紀要. 自然科学系, 58: 203-206

Issue Date 2006-09-00

URL http://hdl.handle.net/2309/63450

Committee of Publication of Bulletin of Tokyo Gakugei Publisher University

Rights Bulletin of Tokyo Gakugei University, Natural Sciences, 58, pp．203 ～ 206 , 2006

Stegodon miensis Matsumoto (Proboscidea) from the Pliocene Yaoroshi Formation, Akiruno City, Tokyo, Japan

Hiroaki AIBA ＊, Katsuyoshi BABA ＊, Masaki MATSUKAWA ＊＊

Environmental Sciences

（Received for Publication ; May 26, 2006）

AIBA, H., BABA, K. and MATSUKAWA, M.: Stegodon miensis Matsumoto (Proboscidea) from the Pliocene Yaoroshi Formation, Akiruno City, Tokyo, Japan. Bull. Tokyo Gakugei Univ. Natur. Sci., 58: 203 – 206 (2006) ISSN 1880–4330

Abstract

The of Stegodon from the Pliocene Yaoroshi Formation in Akiruno City, Tokyo, is described as S. miensis Matsumoto, based on morphological characteristics and dimensional characters of the specimen. The present specimen of S. miensis from the Yaoroshi Formation is shown to represent the uppermost-known horizon of the species.

Key words : Stegodon miensis, Yaoroshi Formation, Late Pliocene, molar, paleontological description

* Keio Gijyuku Yochisha, , Tokyo 150-0013, Japan. ** Department of Environmental Sciences, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan.

Corresponding author : Masaki Matsukawa ([email protected])

Introduction ulna. The materials were initially identified as Stegodon bombifrons (Itsukaichi Stegodon Research Group, 1980) and Stegodon miensis Matsumoto (Proboscidea) is the oldest- subsequently distinguished as S. shinshuensis (Taruno, known species of the Stegodon in Japan. The species 1991a). Taru and Kohno (2002) considered S. shinshuensis is interpreted to have been derived from S. zdansky, which as a junior synonym of S. miensis. may have migrated from the Asian mainland (Konishi and Recently, Baba et al. (2005) reported that these specimens Takahashi, 1999) and had a wide distribution in Japan during were found in mud beds in the upper part of the Yaoroshi the Pliocene. Consequently, the species was proposed as the Formation (they gave the formation name as the Yamada zonal index of the S. miensis Zone (ca. 4 Ma to 2.5 Ma) of Formation, but this formation is regarded as a junior synonym the Pliocene terrestrial deposits of Japan (Taruno and Kamei, of the Yaoroshi Formation by Mastukawa et al., 2006), and 1993). reported that a tuff layer in the mud beds is assigned an age of In 1978, the remains of a Stegodon were collected 2.93±0.19 Ma, based on fission track analysis. This means from mud beds of the basal part of the Kazusa Group in that the specimens identified as S. miensis from the Yaoroshi Akiruno City (formerly called Itsukaichi-machi Town), Formation represent the youngest known occurrence of this western Tokyo, Japan. This specimen consists of one molar species. The specimens are therefore very important for and more than 20 pieces of bone, including the tibia and understanding the biostratigraphy and evolution of the

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Japanese Proboscidea. Despite all this information, however, bent in an S-like form, but other ridges are almost straight. the paleontological systematics of the materials have not been The sixth ridge is partly broken but mostly preserved. The presented. This paper gives a description of the molar as a fifth ridge is preserved only in its central part. The enamel key for understanding the proboscid evolution. layer is lustrous, and its color is dark navy blue and light The specimens illustrated were deposited in the Tokyo brown in the worn points. The enamel folding is observable Metropolitan Natural History Museum at Takao, Hachioji in the sixth and seventh ridges and arranged regularly. The City, Tokyo, and were converted to keep in the Hachioji valleys between the ridges are wide and filled with the Board of Education, Tokyo. Registered numbers of materials cementum. The color of cementum is light brown. The used are inherited from the former museum. NSM-PV as thickness of the enamel layer is measured as 9.0 mm at the Institutional abbreviation refers to the National Science broken section of the lingual side at the fifth ridge, 7.4 mm at Museum, Tokyo. the sixth ridge, 7.4 mm at the eighth one. The angles of each valley form a V-shape and are 45-50 degrees. The cingulum Systematic description is developed on the lingual side. The roots of the tooth are not observable. The dentin is observed at the fractured Proboscidea Illiger, 1811 section of the fifth and sixth ridges; it is brown in color and Family Osborn, 1918 lustrous. The enamel outer and the enamel inner are Genus Stegodon Falconer and Cautley, 1846 indistinct. The shearing blade is obliterated and rounded. Its character shows Type 2 of typology of step-like structure of Stegodon miensis Matsumoto, 1941 Stegodon molar (Saegusa, 1996). The seventh ridge consists of seven (estimate ten) equal sized mammillae and the eighth Figure 1 (A-G) ridge has six (estimate seven). Ridge frequency at 100 mm is about 3.0. The fragment of the molar (Coll. no. 780304) is 1980 Stegodon bombifrons (Falconer and Cautley); 6.7+ mm wide, although it is not known which part it Itsukaichi Stegodon Research Group, p.76-91, pl. 3-9. represents. We can observe one lustrous tubercle. 1991a Stegodon shinshuensis Sawamura, Sugiyama, Tanaka, Comparison. The present specimen of upper right molar Yoshida and Suzuki; Taruno, p. 87. (M3) is similar to the left lower molar (m3) of the type 2005 Stegodon miensis Matsumoto; Baba et al., p. 185-193, specimen of S. miensis (NSM-PV2193) from the Pliocene fig. 1. Kameyama Formation of the Tokai Group of Japan (see Matsumoto, 1941, p. 385-396) in its large size and ridge Material. Upper right M3 (Coll. no. 780302) and molar frequency: the measurements of the greatest width of the fragment (Coll. no. 780304). Both specimens are kept at the crown are 112 mm and 104 mm in the present specimen and Hachioji Board of Education, Hachioji City, Tokyo (the the type specimen of S. miensis, respectively, and the ridge former Tokyo Metropolitan Natural History Museum at frequency at 100 mm of both specimens is 3.0. Fossil Takao, Hachioji City, Tokyo). Research Group (1979) reported a specimen of a Locality. Ajiro, Akiruno City, Tokyo, Japan. partial cranium with four molars, housed at the Department Horizon. The mud bed in the upper part of the Yaoroshi of Geology, Shinshu University, Matsumoto City Formation (Upper Pliocene). (Registered #SSG1001), found in the Shigarami Formation, Description of molar. Upper right M3 (Coll. no. 780302) Nagano Prefecture, Japan, and described by them as is composed of four ridges. Distal talonid is lacking or Stegodon shinshuensis sp. nov. There is a gap in this cranial covered with cement. Although mesial parts are missing, we specimen, however, such that both the left and right molars estimate the ridge formula as X8X. The length of the crown form two distinct parts, backward and forward. Fossil measures +127 mm, the widest width of crown is 112 mm at Elephant Research Group (1979) described the specimens as the sixth ridge. The distal margin of the crown is round. The M2 and M3 molars. Taruno (1991b), however, identified buccal and lingual margins are gently convex. Excepting the these parts as the mesial talon and distal talonid of M3; he eighth ridge, all the others are worn away. The eighth ridge described the M3 molar of the specimen and gave its is almost covered with cement. The seventh ridge is weakly measurements of 290± mm in length of crown, 118± mm

－ 204 － MATSUKAWA, et al：Stegodon miensis from the Pliocene Yaoroshi Formation, Tokyo, Japan in widest width of crown, 2.5 in ridge frequency and 7.4 mm its fission-track age. Bulletin of Tokyo Gakugei in enamel. These measurements are similar to those of the University, ser. 4, 57, 185-193 (in Japanese, English specimen of the upper right molar (M3) of our material. In abstract). addition, Taruno (1991b) mentioned that the width of the Fossil Elephant Research Group, 1979. New species of crown of the upper molar (M3) of S. miensis ranged from found from the Shigarami Formation, 100 mm to 130 mm. All this information suggests that the northern part of Nagano Prefecture, central Japan. Earth present specimen can be assigned to S. miensis. Science (Journal of the Association for the Geological Collaboration in Japan), 33, 11-25. Significance of this specimen for Japanese Itsukaichi Stegodon Research Group, 1980. On stegodon proboscid biostratigraphy from Itsukaichi, Nishitama-gun, Tokyo. Bunkazai no Hogo, 12, 76-91 (in Japanese). Specimens of S. miensis have been reported from more Konishi, S. and Takahashi, K., 1999. Mandibular than 10 localities in Japan. Although Taruno and Kamei morphology of from Japan, Stegodon (1993) proposed the Stegodon miensis Zone as one of the aurorae and Stegodon shinshuensis (Proboscidea, Plio- terrestrial biostratigraphic zones, and Mammalia). Earth Science (Chikyu Kagaku), 53, 3-18. estimated the zone to have an age of about 4.0 Ma to 2.5 Ma, Matsukawa, M., Kakinuma, H., Baba, K. and Ohira, H., there have been reports of S. miensis from the upper part of 2006. Stratigraphy and correlation of the Plio- the zone. S. miensis Zone should be used instead of S. Pleistocene strata along the western flank of the Kwanto shinshuensis, because S. shinshuensis is regarded as a junior Plain, Japan. Bulletin of Tokyo Gakugei University, synonym of S. miensis (Taru and Kohno, 2002). Natural Sciences, 58, 173-202. Although Taruno and Kamei (1993) defined the Stegodon Matsumoto, H., 1941. On Japanese stegodons and miensis Zone and estimated the zone to range in age from Parastegodon. Zoological Magazine, 53, 385-396 (in about 4.0 Ma to 2.5 Ma, the youngest occurrence of S. Japanese). miensis from the transition of the Ueno and Iga formations is Saegusa, H., 1996. Stegodontidae: evolutionary relationships. about 3.5 Ma (Taruno, 1999). The occurrence of S. miensis In: Shoshani J. and Tassy P. (Ed.), The Proboscidea, from the Yaoroshi Formation thus represents the uppermost Oxford University Press, Oxford, p. 178-190. part of the zone, because Baba et al. (2005) gave an age for Taru, H. and Kohno, N., 2002. Redescription and this locality of 2.93 ± 0.19 Ma, based on fission track identification of Stegodon’s tooth from Akiruno-shi, analysis for the tuff layer found just beneath the bed bearing Tokyo, with a reference to the specific name of the the fossil remains. To sum up, the taxon range of S. miensis large type Stegodon from Pliocene of Japan. Memoirs can be confirmed to reach to about 2.9 Ma. of the National Science Museum, Tokyo, 38, 33-41, pl. 1 (in Japanese, English abstract). Acknowledgments Taruno, H., 1991a. Stegodon fossils from Japan. In: Kamei, T. (Ed.), Japanese Proboscidean fossils, Tsukiji-Shokan, We thank J.W. Haggart and I. Obata for reading of the Tokyo, p. 82-99 (in Japanese). manuscript and for giving many useful suggestions. We are Taruno, H., 1991b. Systematic revision of genus grateful to Hiroshi Sato (Hachioji Kyodoshiryokan Museum) “Parastegodon” from the Japanese Islands (Mammalia: for the opportunity to study the specimens in his museum. Proboscidea). Bulletin of the Osaka Museum of Natural The Fuji Film Green Fund (2002-2003 for Matsukawa) History, 45, 5-16 (in Japanese, English abstract). supported the study. Taruno, H. and Kamei, T., 1993. Mammalian fossils of the Pliocene and Pleistocene in the Kinki District. In: References Itihara, M. (Ed), The Osaka Group, Sogen-sha, Osaka, p. 216-231 (in Japanese). Baba, K., Ohira, H. Aiba, H. and Matsukawa, M., 2005. The stratigraphic level of the specimen of Stegodon miensis (Proboscidea, Mammalia) in Akiruno City, Tokyo and

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Figure 1. Stegodon miensis from the Yaoroshi Formation, Ajiro, Akiruno City, Tokyo. A-E: Upper right M3 (Coll. no. 780302) (A: Occlusal view, B: Buccal view, C: Lingual view, D: Mesial view, E: Distal view). F-G: The fragment of the molar (Coll. no. 780304) (F: Occlusal view, G: Mesial or Distal view). Scale bar shows 10 cm.

Table 1. Measurements of molars RF: Ridge formula; U : Used ridge number; L : Length of the crown; H : Height of the crown; W : Width of the crown; F : Ridge frequency

Table 2. Measurements and dimensions of ridge of molars (mm)

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