FOSSIL LAND SNAIL FAUNAS OF PORTO SANTO, MADEIRAN ARCHIPELAGO: CHANGE AND STASIS IN PLEISTOCENE TO RECENT TIMES

摘要

Samples of fossil land snails were made at 14 sites on the island of Porto Santo, Madeiran archipelago. Material in stratigraphical sequences could be dated by A/I racemization calibrated against radiocarbon dating, and against Uranium/Thorium estimates based on Madeiran material. Although errors associated with the oldest samples are large, it is clear that the oldest shell-bearing deposits are at least 300 ka old, and probably much older. Some taxonomic difficulties have been resolved by morphometric studies. The geographical pattern in the fossils (in both faunal composition and morphometrics) resembles closely that seen today, the southwest of the island being particularly distinctive. Of 58 native taxa found in the deposits, 22 are extinct on the island, though four still survive on Madeira. Extinction has been greater in the southwest than elsewhere, and early fossil faunas there are richer than later ones. As on Madeira, more than half of this extinction seems likely to be the consequence of human disturbance, which has also reduced and fragmented the ranges of other species. The overall pattern is of distributional stasis in many species, reduction of range or extinction in many others, but very few cases of range expansion. Neither extinctions nor changes in apparent abundance can be related to known changes in global climate over the period involved. This relative stasis is in marked contrast to the situation on Madeira, only 40 km away, where there are temporal shifts in the fauna, and evidence of colonization events. Possible reasons for this difference are discussed.