2012 Volume 118 Issue 12 Pages 810-822
Molluscan fossils from the Zukawa Formation in Toyama Prefecture, central Honshu, were examined to investigate faunal changes across Datum A (2.75 Ma), the timing of the expansion of ice sheets in the Northern Hemisphere. Calcareous nannofossils indicate that the lowest part of the formation can be assigned to the Lower Pliocene NN15 Zone. Datum A was recognized in the upper part of the Zukawa Formation. A total of 125 species of mollusks were recovered from 11 localities in the Zukawa Formation. Among them, Tridonta-Limatula and Mizuhopecten-Chlamys associations are recognized. Tridonta-Limatula association might live in a lower sublittoral zone and occurring in horizons below Datum A. On the other hand, Mizuhopecten-Chlamys association inhabiting upper sublittoral zones was found from horizons above Datum A. In terms of paleobathymetry, the sediments below Datum A were deposited in a lower sublittoral zone, whereas the sediments above Datum A were deposited in an upper sublittoral zone. The assemblages immediately above Datum A contain higher ratio of number of individuals of cold-water species than the assemblages immediately beneath Datum A, despite the former might live in shallower water depths than the latter. In addition, and barring Megacrenalla columbiana, two species now living in Hokkaido and northwards are confined to horizons above Datum A. This suggests that the 2.75 Ma cooling event affected the molluscan fauna in the western part of Toyama Prefecture.
