Paleontological Research Vol. 6 No.2 June 2002 The Palaeontological Society 0 pan Co-Editors Kazushige Tanabe and Tomoki Kase Language Editor Martin Janal (New York, USA) Associate Editors Alan G. Beu (Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand), Satoshi Chiba (Tohoku University, Sendai, Japan), Yoichi Ezaki (Osaka City University, Osaka, Japan), James C. Ingle, Jr. (Stanford University, Stanford, USA), Kunio Kaiho (Tohoku University, Sendai, Japan), Susan M. Kidwell (University of Chicago, Chicago, USA), Hiroshi Kitazato (Shizuoka University, Shizuoka, Japan), Naoki Kohno (National Science Museum, Tokyo, Japan), Neil H. Landman (Amemican Museum of Natural History, New York, USA), Haruyoshi Maeda (Kyoto University, Kyoto, Japan), Atsushi Matsuoka (Niigata University, Niigata, Japan), Rihito Morita (Natural History Museum and Institute, Chiba, Japan), Harufumi Nishida (Chuo University, Tokyo, Japan), Kenshiro Ogasawara (University of Tsukuba, Tsukuba, Japan), Tatsuo Oji (University of Tokyo, Tokyo, Japan), Andrew B. Smith (Natural History Museum, London, Great Britain), Roger D. K. Thomas (Franklin and Marshall College, Lancaster, USA), Katsumi Ueno (Fukuoka University, Fukuoka, Japan), Wang Hongzhen (China University of Geosciences, Beijing, China), Yang Seong Young (Kyungpook National University, Taegu, Korea) Officers for 2001-2002 Honorary President: Tatsuro Matsumoto President: Hiromichi Hirano Councillors: Shuko Adachi, Kazutaka Amano, Yoshio Ando, Masatoshi Goto, Hiromichi Hirano, Yasuo Kondo, Noriyuki Ikeya, Tomoki Kase, Hiroshi Kitazato, Itaru Koizumi, Haruyoshi Maeda, Ryuichi Majima, Makoto Manabe, Kei Mori, Hirotsugu Nishi, Hiroshi Noda, Kenshiro Ogasawara, Tatsuo Oji, Hisatake Okada, Tomowo Ozawa, Takeshi Setoguchi, Kazushige Tanabe, Yukimitsu Tomida, Kazuhiko Uemura, Akira Yao Members of Standing Committee: Makoto Manabe (General Affairs), Tatsuo Oji (Liaison Officer), Shuko Adachi (Finance), Kazushige Tanabe (Editor in Chief, PRJ, Tomoki Kase (Co-Editor. PRJ, Kenshiro Ogasawara (Planning), Yoshio Ando (Membership), Hiroshi Kitazato (Foreign Affairs), Haruyoshi Maeda (Publicity Officer), Ryuichi Majima (Editor, "Fossils"), Yukimitsu Tomida (Editor in Chief, Special Papers), Tamiko Ohana (Representative, Friends of Fossils). Secretaries: Fumihisa Kawabe, Naoki Kohno, Shin'ichi Sato, Masanori Shimamoto (General Affairs), Isao Motoyama (Planning), Hajime Naruse (Publicity officer) Kazuyoshi Endo, Yasunari Shigeta, Takenori Sasaki (Editors of PRJ, Hajime Taru (Editor of "Fossils"), Yoshihiro Tanimura (Editor of Special Papers) Auditor: Yukio Yanagisawa Notice about photocopying: In order to photocopy any work from this publication, you or your organization must obtain permission from the following organization which has been delegated for copyright for clearance by the copyright owner of this publication. Except in the USA, Japan Academic Association for Copyright Clearance (JAACC), Nogizaka Bild., 6-41 Akasaka 9-chome, Minato-ku, Tokyo 107-0052, Japan. Phone: 81-3-3475-5618, Fax: 81-3-3475-5619, E-mail: [email protected] In the USA, Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. Phone: (978)750- 8400, Fax: (978)750-4744, www.copyright.com Cover: Typical Pleistocene fossils from the Japanese Islands. Front cover: Sinomegaceros yabei (Shikama). Back cover: Paliurus nipponicum Miki, Mizuhopecten tokyoensis (Tokunaga), Neodenticula seminae (Simonsen and Kanaya) Akiba and Yanagisawa and Emiliania huxleyi (Lohmann) Hay and Mohler. All communication relating to this journal should be addressed to the PALAEONTOLOGICAL SOCIETY OF JAPAN c/o Business Center for Academic Societies, Honkomagome 5-16-9, Bunkyo-ku, Tokyo 113-8622, Japan Visit our society website at http://ammo.kueps.kyoto-u.ac.jp/palaeontl Paleontological Research, vol. 6, no. 2, pp. 127-145, June 28, 2002 © by the Palaeontological Society of Japan Molluscan fauna of the "Miocene" Maejima Formation in Maejima Island, Okayama Prefecture, southwest Japan TAKAS HI MATSUBARA Division of Natural History, Museum of Nature and Human Activities, Hyogo, 6 Yayoigaoka, Sanda 669-1546, Japan (e-mail: [email protected]) Received August 2, 2001; Revised manuscript accepted January 15, 2002 Abstract. Molluscan fauna of the "Miocene" Maejima Formation is examined from taxonomical, biostratigraphical and paleozoogeographical points of view. It is composed of four gastropods and 14 bivalves including a new species. Two assemblages, the lsognomon and the Megangulus-Aeila assemblage, were dis­ criminated. The lsognomon assemblage is autochthonous or para-autochthonous and is composed of ele­ ments inhabited the littoral to upper sublittoral gravelly to rocky bottom in a warm sea. The Megangulus­ Acila assemblage represents a mixed composition between muddy sand and gravelly to rocky bottom elements, and was formed in an upper sublittoral muddy bottom near a rocky shore. Occurrences of Megangulus maxim us (Nagao), lsognomon (Hippoehaeta) hataii Noda and Furuichi and Chlamys (Leoehlamys) namigataensis (Ozaki) indicate the age of the Maejima Formation as the Paleogene, not the Miocene. The molluscan fauna of the Paleogene Maejima Formation contains both Tethyan Indo-Pacific elements and Northern Pacific elements. Taxonomy of selected molluscan taxa including a new arcid, Area (Area) uedai sp. nov., is described or discussed. Key words: Area (Area) uedai sp. nov., First Setouchi Series, Maejima Formation, molluscan fauna, Paleogene Introduction et aI., 1992; Yano et aI., 1995a). On the other hand, it is becoming clear that constituent The First Setouchi (or Setouti) Series (Kasama and formations of the First Setouchi Series around the eastern Huzita, 1957) is the generic name for the Miocene strata part of the Seto Inland Sea are of Eocene to Oligocene, not scattered in the median zone of southwestern Japan (or the Miocene age, as a result of studies during the last fifteen Setouchi Geologic Province; Ikebe, 1957), and all of its years (Matsuo, 1987; Ozaki and Matsuura, 1988; Suzuki constituent formations had been regarded as uppermost et ai., 1995; Ozaki et al., 1996; Yamamoto et at., 2000b; lower to lower middle Miocene mainly on the basis of the Kurita et ai., 2000, 2001). Yamamoto et ai. (2000b) found lithology, sedimentary cycle and molluscs (e.g. Huzita, latest middle to late Eocene calcareous nannofossils and 1962; Itoigawa and Shibata, 1973, 1986, 1992; Ishida, dinoflagellate cysts from the "Miocene" Iwaya Formation 1979). It has been known that the "Miocene" molluscan of the Kobe Group in Awajishima Island, and considered assemblages in the coastal area of the eastern Seto Inland that the difference between the "Miocene" molluscan as­ Sea (= Setouchi-Engan Belt; Yano et ai., 1995a) are differ­ semblages in the coastal area of the eastern Seto Inland Sea ent from those in the neighboring backbone area (= Bihoku and those from the backbone area in the western Setouchi Belt; Yano et ai., 1995a), both in the western Setouchi Geologic Province is due to chronologic difference. Geologic Province (e.g. Huzita, 1962; Itoigawa, 1969, However, their opinion conflicts with the previous 1971, 1983; Veda, 1991; Yano et ai., 1995a). There are molluscan data because the Miocene species have often two current interpretations explaining this difference; some been listed from the First Setouchi Series in the eastern part paleontologists have postulated the existences of different of the Seto Inland Sea (Huzita, 1962; Saito, 1962; Saito water masses in the two areas in the late early to early mid­ et ai., 1970; Bando and Furuichi, 1978; Itoigawa, 1983; dle Miocene (e.g. Itoigawa, 1983), while others have as­ Huzita and Maeda, 1984; Mizuno et at., 1990; Okumura sumed a paleogeographic barrier between the two areas in and Sato, 1999). However, most of these studies are unac­ the late early to early middle Miocene except during times companied by either figures or descriptions of molluscan of maximum transgression (e.g. Veda, 1991; Takayasu taxa. Thus, it is necessary to reinvestigate the molluscan 128 Takashi Matsubara Sea Seto t]n(and Sea 500m DAlluvium Figure 1. Location and geologic maps of the study area. A. Location of study area (arrow) and distribution of the "Miocene" sediments (hatched area; modified from Shibata and Itoigawa, (980). The geologic province division follows Yano et al. (l995a) partly modified after Seto et af. (2000). AW: Awajishima Island, MJ: Maejima Island, SH: Shodoshima Island, TS: Teshima Island. B. Geologic map of the western part of Maejima Island, Okay am a Prefecture. fauna of the area, especially from the taxonomical point of treated herein as the lower and the upper part with a revi­ view. sion of the boundary (Figure 2A). The Maejima Formation is the Tertiary in the western The lower part of the Maejima Formation is less than part of Maejima Island, Ushimado Town, Oku County, 10m in thickness and is composed mainly of granule to southeastern Okayama Prefecture (Figure lA, B), and has pebble conglomerate with numerous fragments of balanids, been regarded as one of the constituents of the Miocene brachiopods, calcareous algae and molluscs. This part is First Setouchi Series (e.g. Itoigawa, 1969, 1983; Itoigawa well exposed on the southwestern coast of Maejima Island, and Shibata, 1986). The distribution of the Tertiary in this which is designated to be the type locality of the formation. island was for the first time reported by Sato (1938), and The upper part (30 m+) consists mainly