Eocene ostracode assemblages with Robertsonites from Hokkaido and their implications for the paleobiogeography of Northwestern Pacific

Abstract

We report on Eocene ostracode species from Hokkaido, northern Japan for the first time and discuss their paleobiogeographic implications. Five species were found in the lower part of the Sankebetsu Formation in the Haboro area, and the Akabira and Ashibetsu Formations in the Yubari area, central Hokkaido. The ostracode assemblages are characterized by Robertsonites, a circumpolar Arctic genus of the modern fauna. This finding marks the southernmost occurrence of the Eocene record of the genus, as well as the oldest occurrence. As the characteristics of the Eocene Hokkaido fauna are similar to those of the modern-day fauna in the Seto Inland Sea, a contrast in species composition is observed between the Seto Inland Sea and Kyushu. This contrast is attributable to differences in sea temperature. Five species are described, including Robertsonites ashibetsuensis sp. nov.