Appearance process of the Cenozoic cold-water molluscan assemblages in the Japanese Islands

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The purpose of this research is to make clear the appearance process of Cenozoic cold-water fauna in the Japanese Islands on the basis of shallow-marine molluscan records. Firstly, the definition of the cold-water realm from zoogeographic point of view has been studied based on modern marine zoogeographic studies in relation to various data of surface temperature. As the result, the relationship between the zoogeographic divisions and annual mean temperature of Northern Pacific can be divided into the Subarctic (less than 8ﾟC), Cool-temperate (14-9ﾟC), Mild-temperate (16-15ﾟC), Warm-temperate (20-18ﾟC) and so on, respectively. Secondly, comparative analysis and interpretations from paleomarine-climatical viewpoint on the Japanese Cenozoic molluscan assemblages have been done based on modern analogous method as referring the characteristic molluscan genera/species of the arctic/subarctic realms. Consequently, it can be concluded that l) the Oyashio-type cold-water mass has not been appeared in Northern Pacific during the Miocene, 2) shallow marine cold-water molluscs have first appeared at latest Plioecne to early Pleistocene time at southern Hokkaido, that are of the cool-temperate origin, and 3) Sea of Japan and Sea of Okhotsk might be addorded the important biotope (biotic places) for such adaptation as from the cool-temperate to subarctic marine conditions corresponding world-wide cooling event towards the Glacial age (Quaternary).