Mode of occurrence and composition of bivalves of the Middle Jurassic Mitarai Formation, Tetori Group, Japan

Abstract

The Middle Jurassic Mitarai Formation distributed in Shokawa Village, Gifu Prefecture, central Japan, is interpreted as deposits of wave-dominated shelf environments. The muddy shelf deposits yield abundant molluscs showing various modes of occurrence that can be divided into three fossil assemblages : (1) the autochthonous Modiolus maedae-Tetorimya carinata assemblage with commonly in-situ preserved semi-infauna and deep burrowers; (2) the Entolium inequivalve assemblage characterized by mixed autochthonous infauna and parautochthonous free-living elements; and (3) the allochthonous Inoceramus maedae assemblage. The Entolium inequivalve assemblage contains the chemosymbiotic bivalve Solemya and is associated with a low-diversity ichnofauna suggestive of low oxygen conditions.