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

本研究的目的是根据浅海软体动物记录，弄清日本列岛新生代冷水动物群的出现过程。首先，根据现代海洋动物地理学的研究，结合各种表面温度数据，从动物地理学的角度研究了冷水域的定义。结果表明，北方太平洋动物地理区划与年平均温度的关系可分为亚北极（<8 ℃）、冷温（14-9 ℃）、温和温（16-15 ℃）、暖温（20-18 ℃）等4个区系。其次，以现代类比方法为基础，参照北极/亚北极地区特有的软体动物属/种，对日本新生代软体动物组合进行了古海洋气候学的对比分析和解释。2）浅海冷水软体动物最晚出现在上新世至早更新世，起源于北海道南部，属冷温性; 3）日本海和鄂霍次克海可能是重要的生境（生物的地方），为这样的适应，从冷温带到亚北极海洋条件相应的全球范围内的冷却事件到冰期（第四纪）。

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).