Two Relict Species of Thracia (Bivalvia) from the Pliocene Ogikubo Formation in Nagano Prefecture, Central Japan

Abstract

Two Miocene species, Thracia kamayasikiensis Hatai and T. higasinodonoensis Oinomikado were found as the relict species from the Pliocene Ogikubo Formation in Nagano Prefecture, Central Japan. From this formation, Thracia kakumana (Yokoyama) which is a common species in the Pliocene to Recent boreal faunas was also occurred. In spite of the extinction of the most Miocene thraciids by the latest Miocene cooling, the above two species survived in the studied area by the Middle Pliocene, owing to the cold embayment conditions.