Abstract
Ten species in the Nine genera of decapod crustacean fossils have been obtained from the Korematsu Formation of the early Middle Miocene Bihoku Group in the Shobara area, Hiroshima Prefecture, southwest Japan. They are grouped into three fossil assemblages on the basis of the association of several dominant species. The paleoecologic characters assigned from the occurrence of these assemblages as well as the ecological data of recent decapods is as follows: (1) Eucalliax yatsuoensis-Callianassa nishikawai assemblage: intertidal zone, sandy mud bottom. (2) Miosesarma japonicum assemblage: mud bottom of the upper part of lower sublittoral zone. (3) Carcinoplax antiqua assemblage: sand or sandy mud bottom of the upper to upper part of lower sublittoral zone.
The latter two assemblages composed of sublittoral dwellers are the first report from the western part of the Setouchi Geologic Province. The crustacean fossil assemblages suggest that, the depositional environment of the Korematsu Formation has been intertidal zone of inner bay to upper part of lower sublittoral zone under warm current.
