Chemosymbiotic Bivalves from the Lower Miocene Kurosedani Formation in Toyama Prefecture, Central Honshu, Japan

Abstract

Many molluscan fossils have been found from the turbidite deposits of the lower Miocene Kurosedani Formation at Kakuma in Toyama City. They include six chemosymbiotic bivalves, namely Solemyidae gen. et sp. indet., Nucinella sp., Luinoma cf. acutilineatum (Conrad), Conchocele yatsuoensis sp. nov., Pliocardia kawadai (Aoki) and Adulomya chitanii Kanehara. This is the oldest record of chemosymbiotic species in the Japan Sea region. Nucinella has been unknown from the Cenozoic deposits in the Japan Sea borderland and from the Recent Japan Sea. Moreover, the vesicomyids P. kawadai and A. chitanii are shared with those from the lower Miocene Honya Formation in Fukushima Prefecture and from the middle Miocene Nupinai Formation in eastern Hokkaido. This supports our hypothesis that the invasion of chemosymbiotic species from the Pacific side to the Japan Sea took place soon after the formation of a deep-sea basin in the Japan Sea.