Geographic distribution of the late Cenozoic bivalve Fortipecten in the northwestern Pacific

摘要

The distribution and migration history of the genus Fortipecten, a characteristic late Cenozoic bivalve, is evaluated on the basis of specimens from Hokkaido, northern Japan. Fossil occurrences indicate that Fortipecten species lived in Hokkaido from about 7.0 to 1.2 Ma. The geological occurrences and age of Fortipecten in the northwestern Pacific imply that the biogeographic history of the species was strongly influenced by climatic fluctuations. The latest Miocene migration of its geographic range from central Hokkaido southward to northern Honshu was caused by global cooling. Its early Pliocene expansion northward to Kamchatka resulted from warm, high-stand conditions, and its subsequent range contraction resulted from stepwise cooling in the late Pliocene to early Pleistocene. The species became extinct at about 1.2 Ma as the result of extreme climatic cooling. Four species of Astartidae co-occurred with Fortipecten takahashii from three horizons of Hokkaido. The lowermost horizon (about 6-5 Ma) in the Atsuga Formation is correlated with the Astartidae-bearing horizon (5.5-4.8 Ma) in the Bear Lake Formation in southwestern Alaska, and indicates the timing of the initial opening of Bering Strait.