Sea Level Change in Mid-Pleistocene Time and Effects on Japanese Ostracode Faunas

The ostracode Sinocytheridea impressa existed in Osaka Bay, southwestern Japan, for an interval of only 0.12 My or more from pre-0.44 Ma to 0.32 Ma. This is in striking contrast to that the modem representative of this form still dominates shallow waters along mainland China. Also almost all of the ostracode species associated with Sin. impressa in Osaka Bay are still widely found in shallow waters around the Japanese Islands. The disappearance of Sin. impressa from Osaka Bay can be attributed to the extensive Spirillina transgression (0.30 to 0.35 Ma) and the preceding long period of regression. Noteworthy in relation to the above is the development of Japanese shallow-water-inhabiting ostracode assemblages of late Cenozoic age: Bicornucythere bisanensis apparently first occurred in late Pliocene time, representing a pioneering taxon, and was followed by Cytheromorpha acupunctata which appeared at least by the earlier half of the Pleistocene. Afterward, Spinileberis quadriaculeata, Pistocythereis bradyi and P. bradyformis appeared to form a more diversified shallow-water inhabiting assemblage in association with Sin. impressa which disappeared shortly thereafter. After this disappearance, Sp. quadriaculeata dominated most of the habitat previously co-occupied with Sin. impressa, and became the predominant element of shallow-water ostracode assemblages.