Permalink : http://hdl.handle.net/2115/36764

The Pleistocene marine deposits bearing many molluscan fossils are widely distributed in the Ishikari Lowland and its surrounding hills, the Nopporo, Ishikari and Umaoi Hills. The abundant molluscan fossils in this area are divided into three major faunas in ascending order; the Early Pleistocene (ca. 1.8 to 0.8 Ma), the Middle Pleistocene (ca. 0.4 Ma) and the Late Pleistocene (ca. 0.13 to 0.026 Ma) faunas. Judging from the faunal characteristics and the ratio of the extinct species, the Early Pleistocene fauna is mainly characterized by extralimital cold water species. This fauna is correlative with that of the Setana Formation in the Kuromatsunai Lowland, southwestern Hokkaido. It is noteworthy that the ratio of extinct species to the total number of species decreases from about 12.7% to about 5.6% during Early Pleistocene time. A significant feature of the Middle Pleistocene fauna is the large representation of extralimital warm water species of molluscs. About 20% of total species has modern distributional patterns that are entirely south of the fossil lacalities. Also extinct species has not yet usually been found. This fauna may be indicated a second climatic optimum event throughout the late Cenozoic time in Hokkaido. Such differences of faunal elements existing between the above-mentioned Early and Middle Pleistocene faunas are in close relation with changes of environmental conditions or resulted from changes through the lapse of time. The Late Pleistocene fauna is, as a whole, characterized by recent species which are now living in shallow water along the Japan Sea or Pacific Ocean in the vicinity of this Lowland. Such prosperity and decay in distribution of northern and southern molluscs are the characteristics of Pleistocene molluscan faunas in Hokkaido.