Holocene Fossil Assemblages of Minute Land Molluscs from Sand Dunes on Kikaijima Island in the Amami Archipelago, Japan

Abstract

The Nakazato and the Tobiyozaki sand dunes, located on the Kikaijima Island in the Amami Archipelago, intercalate a layer of paleosol. Each paleosol layer was formed at the same time 2, 000-3, 000 yrs ago. The Nakazato land molluscan fossil assemblage, which occurs in the paleosol layer of the Nakazato sand dune, consists of 15 species of fossilized land molluscs. The Tobiyozaki land molluscan fossil assemblage in the Tobiyozaki sand dune consists of 13 species. While minute fossils occurred abundantly, large fossils are few in number but have remained intact. This indicates that these fossil assemblages are autochthonous. Each fossil assemblage does not include four extinct species of land molluscs which occur in three paleosol layers of the late Pleistocene Suitengu sand dune on the Kikaijima Island. They became extinct from 27, 000 to 2, 000 yrs ago. In particular, Cochlicopa lubrica and Pupilla (Gibbulinopsis) cryptodon, are now distributed chiefly in comparatively cool regions, such as the Japanese mainland, the Chinese Continent and the Korean Peninsula. These two species are considered to have been extinct in the postglacial hypsithermal interval. In contrast, each fossil assemblage includes Allopeas pygula, Allopeas clavulium kyotoense and Succinea sp., which are not found in the assemblage of the late Pleistocene. They may have been carried by human activities. The Nakazato land molluscan fossil assemblage is composed largely of Gastrocopta (Sinalubinula) armigerella and Tornatellides boeningi, which now inhabit near the coast lines. The Tobiyozaki land molluscan fossil assemblage is composed largely of Georissa japonica and Nakadaella micron, which now inhabit in woodland. Therefore, it is inferred that the Nakazato land molluscan fossil assemblage was formed near the coast lines and that the Tobiyozaki assemblage was formed in and around the forests.