Paleobiogeographic significance of Trominina hokkaidoensis (Hayasaka and Uozumi)(Gastropoda : Buccinidae) from the basal part of the Tanami Formation (Oligocene) of the Kii Peninsula, southern Japan

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The basal part of the Tanami Formation, in the southern part of the Kii Peninsula, southwest Honshu, southern Japan. These include Malletia poronaica (Yokoyama), Portlandia (Portlandella) watasei (Kanehara), and Trominina hokkaidoensis (Hayasaka and Uozumi). The combination of late Eocene to early Oligocene Asagai-Poronai mollusks and previously known Oligocene to early Miocene mollusks from the Tanami Formation implies that the localities discussed here are of Oligocene age. The presence of Trominina, which was widespread in the northern Pacific during Paleogene time, suggests that it migrated from northern Japan and northward to southern Japan, in accordance with the Eocene-Oligocene transition global cooling trend.