Molluscan Faunal Changes from Brackish to Freshwater Deposits in the Lower Cretaceous Itsuki Formation of the Tetori Group, Japan

Shinji Isaji

doi:10.2517/PR220022

Abstract

Abstract.

This paper focuses on the molluscan faunal changes in the nonmarine deposits of the Lower Cretaceous (Barremian) Itsuki Formation of the Tetori Group, central Japan. In the lower part of the Itsuki Formation, the transition from brackish to freshwater environments is recognized by lithofacies, sedimentary structures, and fossil indices of molluscs and plant remains. The molluscan assemblages include eleven gastropod species and six bivalve species, and one new gastropod species, Probaicalia okurai sp. nov., is described. The brackish habitats in the delta-front were dominated by Myrene tetoriensis, which usually formed shell beds. The newly founded freshwater habitats on the fluvial environments were populated by Campeloma onogoense and Sphaerium coreanicum as pioneer immigrants. With the development of the fluvial floodplain, a variety of aquatic and semiaquatic microenvironments formed, thereby providing various habitats suitable for small gastropods.

DOI: http://dx.doi.org/10.2517/PR220022

Received 01 1月 2023 Accepted 01 1月 2023

Acknowledgments:

I would like to express my sincere thanks to the late Masatoshi Okura (Konan, Aichi), who initiated the fossil research in Shimoyama area studied in this paper, for his support during the fieldwork. I also wish to thank the Ono City Board of Education for permission for geological survey and collecting fossils. Thanks are also due to anonymous reviewers for their critically reading the manuscript and providing valuable comments. This study was partly supported by a Grant-in-Aid for Scientific Research (KAKENHI) from the Japan Society for the Promotion of Science to S.I. (no. 19654077, 2007–2009).

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