1001 Fossil Mollusca of the Lower Miocene Yotsuyaku Formation in the Ninohe district, Iwate Prefecture, Northeast Japan : Part 1. General consideration of the fauna

Abstract

The fossil Mollusca of the Lower Miocene Yotsuyaku Formation in Iwate Prefecture were studied and 20 species of Bivalvia, 15 species of Gastropoda and 1 species of Scaphopoda were discriminated. They are divided into Type I and II assemblages from a paleoecological viewpoint; Type I is composed of brackish to shallow embaymental elements; Type II indicates middle to lower sublittoral muddy bottom environments. The assemblage generally changes from Type I to II in ascending order. Dosinia (Phacosoma) kawagensis Araki, Ruditapes miyamurensis (Itoigawa), Trapezium (Neotrapezium) modiolaeforme Oyama and Saka, Tateiwaia sp. nov. and Vicaryella "bacula" (Yokoyama) are reported for the first time in the formation. These species seldom occur in Northeast Japan although they have often been reported from the lower part of the First Setouchi Miocene Series. Comparison of Type I with brackish to intertidal elements of the Kadonosawa Fauna of Northeast Japan reveals that characteristic elements of the "Arcid-Potamid fauna" such as Anadara (Hataiarca) spp. are not found in the formation, and several genera are represented by different species. The Kadonosawa Fauna of Southwest Japan has been divided into the Akeyo and Kurosedani Faunas in ascending order. Similar stratigraphic differences in the shallow warm water marine molluscan fauna from upper Lower to lower Middle Miocene sequences have also been recognized in several districts in Northeast Japan. These facts support the conclusion that the molluscan fauna of the Yotsuyaku Formation corresponds to the Akeyo Fauna of Southwest Japan, and the Kadonosawa Fauna in Northeast Japan is also divided into lower and upper portions.