IMPACT OF EARLY POLYNESIAN OCCUPATION ON THE LAND SNAIL FAUNA OF HENDERSON ISLAND, PITCAIRN GROUP (SOUTH-PACIFIC)

Henderson Island, an uninhabited raised coral atoll in the Pitcairn gr oup, has recently been designated a World Heritage Site because of its unique and relatively undisturbed ecosystem. The island is believed t o have been uplifted and subaerially exposed during the last 275 kyr. This therefore provides the maximum age for the terrestrial biota that includes several endemic taxa. Henderson today supports 16 strictly t errestrial species of snails, about half of which are endemic. Analyse s of sediments beneath Polynesian occupation horizons dated between th e eleventh and seventeenth centuries AD, have yielded II species of la nd snail present in the modern fauna, together with at least six (and possibly as many as eight) further species that no longer occur on the island. These extinct taxa are illustrated and formal descriptions pr ovided for five (Pleuropama hendersoni, Orobophana carinacosta, Minido nta macromphalus, Philonesia pyramidalis, P. weisleri); a sixth, known only from broken shells, appears to belong to the genus Hiona. The tw o remaining taxa are 'tornatellinids' that have not been recognized am ong the modern fauna. Radiocarbon dates from bones of associated extin ct land birds confirm their occurrence on Henderson before the first s igns of Polynesian settlement. The extinction of these taxa seems to c oincide with the Polynesian occupation and evidence for large-scale bu rning, at least around parts of the plateau margin, suggests that thei r demise can be linked with habitat destruction. At least three specie s, Gastrocopta pediculus, Lamellidea oblanga and Pupisoma orcula, firs t appear in Polynesian occupation horizons. Their status as prehistori c introductions is therefore confirmed, but G. pediculus no longer liv es on Henderson. Pacificella variabilis, Tornatellides oblongus paroul us and Elasmias sp., all previously thought to have been other prehist oric introductions to Henderson, were recovered from pre-Polynesian le vels and are therefore native.