Miocene Ostracods from the Itahana Formation in the Tomioka District, Gunma Prefecture, Central Japan: Paleoenvironmental Significance and Systematics

Abstract

Sixty-two podocopid ostracod species belonging to 37 genera were identified during the analysis of 57 samples from the Miocene Itahana Formation (ca. 11.0 to 10.5 Ma) in the Tomioka district, Gunma Prefecture, central Japan. Two assemblages were recognized in the lower member of the Itahana Formation: Assemblage I is characterized by Aurila joushuensis, Loxoconcha nozokiensis, Paracytheridea neolongicaudata, Callistocythere spp., and Cythere omotenipponica; Assemblage II is characterized by Krithe sp., Acanthocythereis sp., Ambtonia takayasui, Ambtonia tamayuensis, and Argilloecia sp. Assemblage II indicates a depositional environment of muddy bottoms in the upper bathyal open sea, while for the species of Assemblage I a habitat of sandy bottoms in warm shallow open seas is indicated, suggesting that the depositional environment of the lowermost Itahana Formation was bathyal and that the ostracods of Assemblage I were transported to that environment by turbidity currents. Fifteen new species are described in this study.