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

本研究的目的是通过生物学和古生物学的方法，阐明九州和琉球群岛海湾介形类的迁徙和物种形成之间的关系，以及与相对海平面变化相关的古地理变化。由于没有对位于大岛和谷儿岛群岛之间的托卡拉海峡周围小海湾的新近介形类动物进行研究，2003年从琉球中部大岛友美大岛的Kasari、ISU和Sumiyou海湾采集了表层沉积物样品，2004年从九州南部Koshiki-Jima岛的Urauchi湾和Namako池塘采集了表层沉积物样品，以进行最新的介形虫动物群和生物学分析。还发现了介形类化石，特别是在本州中部至九州南部几个地区的更新世沉积中。结果表明：(1)本州至九州海洋氧同位素阶段(MIS15-11)，生活在高浊度、低盐度海湾的中华龙目(Sincytheridae)为优势种；在海洋氧同位素阶段(MIS11-5.3)期间，本州至九州的亚热带类群处于优势地位。在MIS2.4期间，由于水温下降，本州和九州太平洋一侧的内湾亚热带类群灭绝。托卡拉海峡是亚热带物种从大岛岛迁徙到九州南部的屏障。因此，封闭的泥质海湾介形虫受到冰期变化、温暖的黑潮位置、托卡拉海峡的深度和宽度以及当地海峡开放的影响。

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.