Subfossil land snail fauna (mollusca) of central Chichijima, Ogasawara Islands, with description of a new species

摘要

The fossil record provides useful information to estimate what island communities were like before human colonization. We examined the species composition of the subfossil land snail fauna of dune deposits at the Yatsuse River, central Chichijima, Ogasawara Islands, and compared it with the species recorded in Chichijima since the nineteenth century. The 22 species in the dune deposits included 13 species that are now extinct in Chichijima. Live specimens of 11 of these extinct species were recorded in the early twentieth century, but no living Mandarna pallasiana and Ogasawarana obtusa Chiba et al., n. sp., have ever been recorded. Age of the sediment, estimated by radiocarbon (C-14) dating, was 720 years B.P., and it is possible that these two land snail species became extinct as a result of the impact of human colonization of the island, which started in 1830. Specifically, Ogasawarana obtusa, n. sp., became extinct before the start of taxonomic studies of the land snails of Ogasawara. The sample included Hawaiia minuscula, which is generally now considered a cosmopolitan species introduced from North America. This finding suggests that Hawaiia minuscula is not alien in Ogasawara but indigenous.