Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 6

Tropical molluscan assemblages, especially mangrove swamp assemblages in the Japanese Miocene are studied under the paleoecological and paleobiogeographical point of view.

Three themes are presented.

1) Molluscan assemblages and ecological conditions in present mangrove swamps

Two examples in Singapore are shown in detail. In the Seletar River, the Telescopium-Cerithideopsilla assemblage is recognized from the mouth to the upper stream with some difference in salinity. Assemblages in the beach of Semakau Island, a small island which is 10 km south of the main island, are different in places in connection with bottom character of each habitat. Close relationships in distribution among mangrove swamp, sea grass and algal reef belts are recognized.

2) Distribution of tropical molluscs in Japanese Middle Miocene

Twenty-two genera of which living representatives in present seas are defined to the south of South Kyushu in distribution, are recognized in Japanese Middle Miocene sequences. The Bihoku, the Yatsuo and the Mizunami Groups have representative assemblages such as the Geloina-Telescopium-Terebralia (mangrove swamp assemblage), the Globulaηia and the Cavilucina-Glycymeris (both sandy bottom assemblage). Presence of the tropical condition comparable to those in the area as south of Iriomote Island in the Ryukyu Islands is assumed in the Bihoku and the Yatsuo, sequences in the coast of the Sea of Japan. The Mizunami shows a little weak influence of the tropical current as the proto-Kuroshio. Paleogeography of that time, of which paleoposition is reconstructed by paleomagnetic data is shown in Fig. 4 with distribution of tropical evidence.

3) Discussion on the Arcid-Potamid fauna

In connection with character of mangrove swamp assemblages, the Arcid-Potamid fauna, a well-known molluscan fauna in the Japanese Middle Miocene, is reconsidered paleoecologically and paleobiogeographically. It is concluded that reexamination and redesignation of the fauna are necessary because of the character of the fauna in ecology and distribution is wide-ranged and complicated.