Series: Bulletin of the British Museum (Natural History) Zoology Volume: 29/2
32 pages, 1 b/w line drawing, 2 b/w maps, 4 tables
The present reptiles of Aldabra comprise only the giant tortoise, Geochelone gigantea, two geckoes, Phelsuma abbotti and Hemidactylus mercatorius, and the skink, Cryptoblepharus boutonii, but a richer and quite different fossil fauna has recently been discovered. Giant tortoises occur in most terrestrial deposits on the atoll and remains of crocodiles and lizards have been found in two: the Bassin Cabri Calcarenites (undated, but considerably older than 125 000 years B.P.) and cavity fillings in the Aldabra limestone at Point Hodoul formed since 100 000 years B.P. The Bassin Cabri Calcarenites contain remains of a crocodile similar to Crocodylus niloticus and of an iguanid lizard of the genus Oplurus. These also occur in the Point Hodoul deposits together with five other kinds of lizards, two or three of which are geckoes (a Paroedura similar to P. sanctijohannis and P. stumpffi, a Geckolepis close to G. maculata and what is possibly a Phelsuma) and two skinks (a ‘Scelotes’ similar to ‘S.’ johannae and a Mabuya very like M. maculilabris). The Point Hodoul lizard remains may be the food residue of a predator, perhaps an owl.
The reptile fauna of Aldabra is much more similar to that of Madagascar and the Comores Islands than to that of East Africa or of the Seychelles. Composition of the Point Hodoul fauna and the presumed requirements of some of its members suggest that conditions on Aldabra at that time may have been rather similar to those now occurring on the Comores. It is likely that the ecological requirements of the fossil forms were sufficiently different for them all to be able to coexist. Some of the fossils clearly differ in size from their closest modern relatives, both the Geckolepis and the Oplurus being very large; possible reasons for this are discussed. Aldabra was completely submerged after the laying down of the Bassin Cabri Calcarenites, but not since the Point Hodoul deposits were formed. Extinction of the reptiles found in the latter may have been largely caused by transient or permanent loss of ecological resources, although competition from the species existing on the atoll today could have been a minor factor. The possible importance of invading predators is difficult to assess. All the reptiles known from Aldabra seem to have been well adapted to the problems of transmarine colonization. There is evidence that the giant tortoises reached the island three times and the crocodile and Oplurus at least twice.
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