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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Paleobiogeographical Changes of Late Early Jurassic Ammonite Assemblage from East Asia 178 K. Nakada 1 , A. Matsuoka 2 , J.R. Yin 3 1. Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan (E-mail: f08j002d@mail.cc.niigata-u.ac.jp) 2. Department of Geology, Faculty of Science, Niigata University, Niigata 950-2181, Japan (E-mail: matsuoka@geo.sc.niigata-u.ac.jp) 3. China University of Geosciences, Beijing 100083, China (E-mail: yjr@cugb.edu.cn) The Early Jurassic period is especially attracted by the analyses of the Early Toarcian Oceanic Anoxic Event (OAE), and transitions of marine biota affected by this paleoecological event have also been discussed in many taxa. On the other hand, worldwide paleobiogeographical change of the Jurassic ammonites was analyzed by Page (2008). However, detailed transitional pattern of late Early Jurassic ammonite assemblages from Japan have not been analyzed, and the lack of these data have disturbed the discussions of Asian ammonite paleobiogeography of this period. The aim of this study are to analyze the chronological changes of the late Early Jurassic ammonite faunal compositions from Japan by using the assemblages from the Toyora Group and the Kuruma Group and to discuss the paleobiogeographical positions of the each time assemblages by comparing with fauna from outside Japan, for ex- ample, NW European, Mediterranean, northern Tibetan (Yin et al., 2006), Northeast Russian, and Fareast Russian provinces. These discussions are carried out on the basis of the ammonite zonal scheme reexamined in the Toyora Group (Nakada and Matsuoka, 2007). The Toyora Group exposed in southwest Japan is the Lower to Middle Jurassic continental shelf sedi- ments and is divided into the Higashinagano Formation, the Nishinakayama Formation, and the Utano Formation in ascending order. The Nishinakayama Formation, mainly composed of black mudstone, is known as the most major source of the latest Pliens- bachian-Early Toarcian ammonoids (e.g. Hirano, 1973; Nakada and Matsuoka, 2010). The Kuruma Group is the Lower Jurassic epicontinental deposits distributed in central Japan. This group is divided into six for- mations, the Jogodani Formation, the Kitamatadani Formation, the Negoya Formation, the Teradani For- mation, the Shinadani Formation, and the Otakidani Formation in ascending order, and some Late Pliens- bachian ammonites were recognized in the Teradani Formation (Sato, 1955). This formation is mainly consisted of sandy-mudstone. The Late Pliensbachian ammonoid assemblage from the Stokesi Subzone (Margaritatus Zone) to the Apyrenum Subzone (Spinatum Zone) recognized in the Teradani Formation is mainly consisted of the genus Amaltheus, typical in Boreal Realm faunas in this period. This assemblage contains the Russian endemic species of this genus, and the faunal composition is similar to that of the Northeast Russian assemblage. However, the genus Amaltheus is absent in latest Pliensbachian (Hawskerense Subzone, Spinatum Zone), and the assemblage of this period is dominated by a variety of Arieticeratinae, typical in the Tethyan Realm faunas. A same transition is also detectable in almost coeval (Apyrenum Subzone) assemblage from the Nishinakayama Formation, whereas this turnover recognized in Japanese assemblages is earlier than the similar faunal changes common in NW European assemblages, chronologically corresponded with Pliensbachian/Toarcian boundary. The main composition of the assemblage from Toyora Group is replaced by Harpoceratinae, dominant in Tethyan Realm faunas, in Howskerense Subzone. These latest Pliensbachian fauna show strong affinities with those of the Mediterranean province. The earliest Toarcian ammonite assemblage from the Toyora Group is dominated by Dactylioceratidae and high-diversity Harpoceratinae. This Tethyan- derived fauna continues during the Tenuicostatum Zone (earliest Toarcian) and has the generic similarities with that of the Mediterranean province. However, the fauna of the Serpentinum Zone (middle Early Toarcian) is characterized by the occurrence of Cleviceras exaratum (YOUNG and BIRD). During the middle Early Toarcian, Boreal Realm faunas are dominated by the genus Cleviceras. Therefore the association of this assemblage suggests the mixing of Tethyan and Boreal faunas in East Asia. This mixed fauna is similar to that of the Fareast Russian province rather than the coeval northern Tibetan fauna, mainly consisting by the genus Cleviceras and Dactylioceratidae. The late Early Jurassic ammonite assemblages from Japan is characterized by the transition from Boreal fauna to Tethyan fauna at latest Pliensbachian (Hawskerense Subzone) and the mixing of Boreal genus with Tethyan-dominated fauna at middle Early Toarcian (Serpentinum Zone). These transitional pat- terns are broadly similar to those of the Fareast Russian faunas. The elucidation of the late Early Jurassic ammonite transition in East Asia is important for analyzing the worldwide ammonite paleobiogeography
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 of this period. Key words: Ammonite paleobiogeography; Late Early Jurassic; Faunal transition; Southwest Japan References: Hirano H. Biostratigaraphic study of the Jurassic Toyo- ra Group, Pt. 3. Transactions and Proceedings of the Palaeontological Society of Japan, New Series, 1973, 90: 45-71. Nakada K., Matsuoka A. Ammonite biostratigraphy of the Nishinakayama Formation in the Lower Jurassic Toyora Group, Southwest Japan. The 114th Annual Meeting of the Geological Society of Japan, Abstract, 2007: 65 (in Japanese). Nakada K., Matsuoka A. On the Pliensbachian/Toar- cian boundary in the Lower Jurassic Toyora Group in southwest Japan. Volumina Jurassica, 2010, 7: 51-58. Page K.N. The evolution and geography of Jurassic ammonoids. Proceedings of the Geologists’ As- sociation, 2008,119: 35-57. Sato T. Les ammonites recueillies dans le Groupe de Kuruma, Nord du Japon Central. Transactions and Proceedings of the Palaeontological Society of Japan, New Series, 1955, 20: 111-118. Yin J., Gao J., Wang Y., et al. Jurassic ammonites in anoxic black shales from Sewa and Amdo, northern Tibet. Acta Palaeontologica Sinica, 2006, 45: 311-331 (in Chinese). 179
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