Abstract
We examined Plio-Pleistocene mollusks from the Taihei and Minehama areas in Akita Prefecture with the aim of elucidating the change in faunal composition at Datum A (2.75 Ma), when glaciers expanded in the Northern Hemisphere. In both areas, the deposits below the Datum A consist of finer sediments than those above. In the Taihei area, 105 species of mollusks were recovered from 10 localities. On the other hand, 139 species were found from seven localities in the Minehama area. In terms of paleobathymetry, the sediments below the Datum A in both areas were deposited in the lower sublittoral zone, while those above the datum were deposited in the upper sublittoral zone. In both areas, the assemblages above the Datum A contain a higher proportion of individuals of cold-water species, despite the fact that they lived in shallower water depths. Moreover, some species now living in Hokkaido and northwards are confined to the upper horizons than the Datum A. In both areas, two Miocene relict species were collected from the deposits below the Datum A. These species might suffer from extinction by the cooling event at the Datum A. The results show that the cooling event at 2.75 Ma affected the molluscan fauna in Akita Prefecture.
