This study investigated an early Pleistocene ostracod fauna from the Kazusa Group in the Tama Hills (ca. 1.7-1.4 Ma) along the western side of Tokyo Bay, central Japan. We report for the first time an early Pleistocene ostracod fauna from the Kanto District. This fauna consists of 56 species, with three assemblages defined by Q-mode cluster analysis. The two most abundant species are Bicornucythere bisanensis and Spinileberis quadriaculeata in the assemblage BS, Pontocythere subjaponica and Buntonia hanaii in the assemblage PB, and Loxoconcha ikeyai and Pontocythere subjaponica in the assemblage LP. These assemblages indicate the following depositional environments: (1) the innermost to central area of an inner bay having relatively low salinity (assemblage BS); (2) the outer area of the inner bay and upper-shelf area influenced by open sea water with relatively high salinity (assemblage LP); and (3) the central to outer area of the inner bay having salinity intermediate between that of (1) and (2) (assemblage PB). We also report first occurrences on the Pacific side of Japan near Tokyo Bay for two cryophilic taxa, Laperousecythere robusta and Pectocythere sp. Laperousecythere robusta moved southward from the Japan Sea coast, probably through the Tsugaru Strait before reaching central Japan near Tokyo Bay by 1.6 Ma. Pectocythere sp. might have first appeared around Tokyo Bay during the early Pleistocene by 1.6 Ma. The species content of the assemblage BS suggests that Bicornucythere and Spinileberis commonly inhabited inner-bay areas near Tokyo Bay by 1.6-1.4 Ma at the latest. This fossil fauna does not include inner-bay species of the genera Neomonoceratina and Sinocytheridea. Their absence near Tokyo Bay in the early Pleistocene is consistent with previous palaeobiogeographical findings regarding Japanese bay-dwelling ostracods. These data provide information about the route and timing of the northward or southward migrations of shallow-marine benthos along the Northwest Pacific margin during the late Cenozoic.
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Vol. 18 • No. 4