Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 6

Paleoenvironmental and paleoecological characteristics of the Miocene molluscan assemblages reported from the central San-in district and its vicinity, southwest Japan, are discussed on the basis of data of living molluscan genera. Results are illustrated as diagrams shown in Figs.1-4 which demonstrate effectively the characteristics of the assemblages. The early Middle Miocene assemblages from Masuda, Hamada, Nima and Shobara approximately represent an environmental transition from intertidal zone of tropical water, through eulittoral zone of warm water, to sublittoral zone of mixed warm and cold water. The middle to late Middle Miocene assemblages are represented by the Macoma-Cultellus assemblage in the Izumo area. The assemblage indicates neritic mixed water environment and is composed of suspension feeders of endobenthos living in sandy mud bottom condition. The earlier assemblages ecologically allied with the Macoma-Cultellus assemblage appear as the Phacosoma-“Vasticardium” assemblage in Shobara area, the Turritella-Glycymeris assemblage in Hamada area and the Nima fossil molluscan assemblage in ascending order. The cold water elements become remarkable since the Nima assemblage in this district.