Systematic review of Late Pleistocene turtles (Reptilia : Chelonii) from the Ryukyu Archipelago, Japan, with special reference to paleogeographical implications

摘要

The Quaternary terrestrial turtle fauna of the Ryukyu Archipelago was reviewed on the basis of recently excavated fossils, as well as literature information. As a result, five extinct species (four geoemydids [Cuora sp., Geoemyda amamiensis, Mauremys sp., and another species with undetermined generic and specific status] and one testudinid [Manouria oyamai]) were recognized from Late Pleistocene cave and fissure deposits. Two of the three turtles currently occurring in this archipelago (C. flavomarginata and G. japonica) were also recognized from comparable deposits on islands, including those where they do not occur at present. These records indicate that the terrestrial turtles of the Ryukyus were much more diverse during the Late Pleistocene than at present, and that extinction has occurred during the last few tens of thousands of years not only for those five fossil species but also for some island populations of the extant species. Distributions of three of the extinct species (G. amamiensis, Cuora sp., and the geoemydid [genus and species undetermined]), confined to the central Ryukyus, are concordant with the currently prevailing hypothesis of Ryukyu paleogeography, which assumes a relatively long isolation of this region and much more recent insularization of the southern Ryukyus. In contrast, distributions of the remaining two extinct species (Man. oyamai and Matt. sp.) must be explained by some ad hoc scenario or, other-wise, drastic modification of the current hypothesis.