Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 6

Yabetecten tokunagai and Mizuhopecten tokyoensis are important taxa of the Omma-Manganzi fauna in the Japan Sea borderland. The former has hitherto been considered to be frigophilic and the latter the thermophilic extralimital taxon.

Yabepecten recently found from the Atsuga Formation, together with Fortipecten in Hokkaido indicates that this taxon is of provincial late Miocene-earliest Pliocene age (about 6.0-5.0 Ma), and is very similar to Y. condoni from the Montesano Formation in the western North America. Consequently Yabepecten extended its distribution to Japan Sea borderland from the northern Japan or the western North America, and Y. tokunagai branched off from Y. condoni stock as a result of its southward migration followed by localization, and became extinct in a short period

Mizuhopecten tokyoensis hokurikuensis was restricted to tropic waters in the southern Japan in earliest Pleistocene, but M. tokyoensis (s. s.) branched off from M. t. hokurikuensis extended its distribution to Hokkaido and was found in a large number of individuals together with M. yessoensis. This taxon certainly inhabited cool-cold waters as a result of its northward migration to Hokkaido.

As mentioned above, Hokkaido is considered to have been in the special paleobiogeographic position, invaded alternately by the southern and the northern extralimital taxa during the Neogene time. Such paleobiogeographic conditions are seemed to have been favorable to the appearances of characteristic molluscs in the adjacent waters of Hokkaido: some taxa seem to have the western North American affinities and some suggest the primary southern Asiatic relationships.