抄録
One hundred and forty-nine ostracode species are identified among 42 samples from the lower to middle part of the Middle Miocene Kobana Formation, Arakawa Group, distributed in Tochigi Prefecture, Central Japan. Dominant throughout the formation are such species living around Japan as Schizocythere kishinouyei (Kajiyama), Paracytheridea neolongicaudata Ishizaki, Cornucoquimba moniwensis (Ishizaki), Cornucoquimba saitoi (Ishizaki) and Cythere omotenipponica Hanai. Tropical to warm-temperate species living off Southwest Japan are abundant, but circumpolar and cryophilic species are also present (less than 10%) in most samples. Q-mode cluster and principal components analyses disclose the following sequence of ostracode faunas : 1) The lowest part is dominated by nearshore and subtropical species and contains about 5-13% of circumpolar and cryophilic species; 2) The lower middle part is dominated by lower sublittoral to upper bathyal muddy dwellers, suggesting a rapid increase of water depth. The horizon characterized by the increase of deep cool-water species is recognized; 3) The middle to upper part, characterized by lower to middle sublittoral subtropical to warm-temperate water species mixed with intertidal to upper sublittoral ones; 4) The upper part containing many Laperousecythere species, suggesting a cooler climate. Analyses of ostracode faunas indicate that after the mid-Neogene climatic optimum, benthic environments underwent fluctuations of water temperatures, with no rapid decrease, in the Karasuyama area.
