New bivalve species of Tucetona, Pycnodonte and Ezocallista from the Lower Miocene Kurosedani Formation at Fukushima, Yatsuo-machi in Toyama City, central Japan

Abstract

Sixty-nine molluscan species have been recorded from the Lower Miocene Kurosedani Formation at the hitherto unknown locality, Fukushima, Yatsuo-machi in Toyama City. Most species are shallow-water dwellers living in tropical and subtropical areas. Based on their occurrence, they were transported from shallow water to the deep sea. The assemblage includes three new bivalve species: Tucetona tsudai n. sp., Pycnodonte (Phygraea) matsubarai n. sp. and Ezocallista toyamaensis n. sp. Four species of Tucetona including T. tsudai n. sp. have been described from the Lower Miocene to lowest Middle Miocene in Honshu. The last occurrence of Pycnodonte was also in the Early to earliest Middle Miocene in Honshu. They disappeared from Honshu by the Miocene Climatic Transition (MCT). Ezocallista might have first appeared in the tropical area during the Miocene Climatic Optimum (MCO) and adapted to colder water since MCT or the Late Miocene Cooling (LMC).