Vicissitudes of ostracode assemblages and mechanisms involved

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Fossil ostracodes from the Cretaceous of Hokkaido were published in two papers proving the position of the Japanese Islands in relative to Tethys at the time and hererochroneity of ostracode faunas between Japan and Europe. Analysis of ostracode fauna from the Pleistocene Omma Formation distributed in and around Kanazawa City disclosed a prominent spike of Kuroshio flowing into the Japan Sea at 1.2 Ma, and distinct fluctuations of water conditions in a cycle of 0.12 to 0.09 My (Alcheringa, in press).Afterward, I concentrated in collecting fossils ostracodes from the Moniwa Formation, one of the Neogene distributed in and around Sendai City, to enable work supplementary to the report in 1966. Ostracode faunas from the Tsunaki Formation distributed near the southern border of Sendai City,were found containing the form which has been reported to date as Cornucoquimba moniwensis from the Pleistocene in Northeast Japan, together with Cornucoquimba moniwensis (s.s.), both being morphologically separated clearly by their carapace ornamentation. The dimorphism invoked at this time is shown subject to an increasing warming toward the end of Miocene time, leading their habitats extended to shallower waters, where a new form was established to linger on, at least, in Pleistocene shallow sea. With respect to the Cretaceous, the genus Krithe was found first appearing from the lowest part of the Hakobuchi Group (Campanian). This occurrence, the oldest record in Japan, may represent some changes in ostracode faunas to allow the introduction of a new element of ostracode faunas, offering us one of the very attractive subjects of our study.