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

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.