The Omma-Manganji ostracod fauna (Plio-Pleistocene) of Japan and the zoogeography of circumpolar species

Abstract. The Omma-Manganji fauna of Japan signifies a time during the late Pliocene and Pleistocene when arctic-subarctic species migrated far south of their present geographic range in response to oceanographic changes. Omma-Manganji deposits exposed on Hokkaido, northern Honshu, and Sado Islands yielded about 224 species of marine Ostracoda. At least 26 are circumpolar species known previously from Arctic seas off the British Isles, eastern North America, Scandinavia and Europe, comprising between 14 and 47% of the ostracod assemblage in eight of ten formations studied. The 26 circumpolar species and 21 other western Pacific cryophilic species are illustrated and their distribution in Japanese deposits is discussed.