New ostracod (Crustacea) species of the genus Robertsonites from the Upper Pliocene Kuwae and Sasaoka formations, central and northeast Japan

摘要

The ostracod genus Robertsonites is widely distributed in high-latitude seas of the Northern Hemisphere. It also occurs abundantly in Plio-Pleistocene strata along the coast of the Sea of Japan, which were deposited under the influence of the Japan Sea Proper Water mass, below 150 in. The assemblages of the genus Robertsonites in the seas around Japan are quite different from those in the high-latitude seas of the Northern Hemisphere. A difference in the valve outlines of the genus in the two areas is also apparent. Nine species of the genus Robertsonites are restricted to strata and marine deposits around Japan, although only 12 species have been described world-wide, including the three new taxa described here. The Upper Pliocene Sasaoka and Kuwae formations in northeast and central Japan, contain seven species, including three new taxa, Robertsonites irizukii sp. nov., Robertsonites leptoreticulata sp. nov., and Robertsonites tabukii sp. nov., and one open-nomenclature taxon. [References: 52]