First Strebloceras (Mollusca: Gastropoda: Caecidae) from the middle Pleistocene of Japan

摘要

A caecid gastropod of the small genus Strebloceras recovered from the middle Pleistocene Toyohashi Formation,Atsumi Group, was newly described as S. kobayashii n. sp. It is the first record of the genus in Japan and is the geochronologically youngest fossil that fills a large time gap of approximately 5 Myr after the middle Miocene.Strebloceras kobayashii n. sp. is characterized as having a large shell that is up to approximately 5 mm long and a dorsally or ventrally depressed teleoconch. It is interpreted to be a rare, extinct fossil species that lived exclusively in the warm-temperate conditions during interglacial periods in the middle Pleistocene. Although the geochronologicai occurrence of Strebloceras apparently matches well with the hypothesis of the eastward migration from the Tethys to the Pacific in Miocene times, the origins of the Pacific members may be explainable by overlap of both the Tethys and original Pacific lineages.