Paleobiological study of Ostracoda (Crustacea) in relation to paleogeographical changes of Kyushu to the Ryukyu Islands

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The aim of this study is to clarify the relationships between migration and speciation of embayment ostracodes and paleogeographical changes related to relative sea-level changes in Kyushu and the Ryukyu Islands on the basis of biological and paleontological methods. As no studies on recent ostracodes from small bays around the Tokara Strait, which is situated between Amami-ohshima and Tanega-shima Islands, surface sediment samples were collected from Kasari, Isu and Sumiyou Bays in Amami-ohshima Island, central Ryukyu in 2003, and those from Urauchi Bay and Namako Pond in Koshiki-jima Island, southern Kyushu in 2004, to make Recent ostracode faunal and biological analyses. Fossil ostracodes were also obtained especially from Pleistocene deposits in several areas in central Honshu to southern Kyushu. As a result, distribution patterns of embayment ostracodes such as Spinileberis, Bicornucythere, Neomonoceratina and Sinocytheridea were clarified in time and space as follows :1:Sinocytheridea, which is living in high-turbidity and low-salinity bays along the Chinese coast was dominant in Honshu to Kyushu during marine oxygen isotope stage(MIS) 15 to 11. It was extinct in most bays after MIS 10.2:Subtropical taxa (e.g., Neomonoceratina) migrated from the Ryukyu Islands to Kyushu at least until marine oxygen isotope stage(MIS) 12 and were dominant in Honshu to Kyushu during MIS 11 to 5.3.Subtropical taxa became extinct in inner bays along the Pacific side of Honshu and Kyushu because of the decreasing of water temperature during MIS 2.4.The Tokara Strait is the barrier for the subtropical species to migrate from Amami-ohshima Island to southern Kyushu.Thus, enclosed muddy bay ostracodes have been influenced by temporal changes of glacio-eustacy, location of the warm Kuroshio Current, depth and width of the Tokara Strait and the opening of local straits.