抄録
Thirty-seven ostracode species are identified from 15 samples from lithological Units II and III at the type locality of the middle Pleistocene (Marine Isotope Stage 11: ca. 0.41 Ma) Takatsukayama Member of the Meimi Formation, Hyogo Prefecture, western Japan. Three ostracode biofacies are distinguished on the basis of Q-mode cluster analysis. The depositional environments of each biofacies can be inferred as bay sandy coast (BC), enclosed inner bay (IB) and middle bay influenced by freshwater inflows from rivers on the Harima Plain (CB). A rapid increase in water depth from bay coast to middle bay occurred at the deposition of the middle part of Unit II of the Takatsukayama Member. Sinocytheridea impressa (Brady), which probably disappeared or rapidly decreased from paleo-Osaka Bay at about 0.3-0.35 Ma, is abundant at the study site. One of the reasons for the disappearance or rapid decrease of this species is likely that paleo-Osaka Bay changed from a low-salinity, enclosed nutrient-rich and highly turbid bay to a moderate salinity, ventilated bay with good circulation, together with the vicariance event suggested by previous studies. This change was probably linked with both widespread inundation in the neighboring paleo-Harima-nada Bay and the development of large-scale tidal currents from the Akashi Strait connecting both bays. Thereafter, Neomonoceratina deticata Ishizaki and Kato replaced S. impressa and became the dominant taxon in paleo-Osaka Bay. Sinocytheridea impressa which lived in paleo-Osaka and Harimanada Bays has larger valves than any specimens ofthis species from Recent seas. The cause of this large size in the past might have been an abundant food supply or dissolved chemicals from freshwater inflows into paleo-Osaka and Harima-nada Bays. Alternatively, whether large-valve populations occurred as a result of genetic changes in relation to the geographical isolation or not, natural selection operated in favor of increased valve size in paleo-Osaka and Harima Bays for reasons that are as yet uncertain.
