Temperature-Dependent Sex Determination (TSD): Crocodilian Survivorship

Abstract

It is a fascinating subject why some species become extinct and others do not. Why, for example, have the three families of crocodilia (alligators, crocodiles and gavials) not become extinct during the past 100 million or so years? They have survived essentially unchanged for around 63 million years after the dinosaurs became extinct and clearly have great survival powers. As pointed out by Benton (1997), however, those that have survived are only a small group of around the 150 fossil genera of crocodilians that have been documented. Crocodiles and alligators were around in the later part of the Cretaceous (63 to 135 million years ago). As several have noted, such as Benton (1997), they were very much more widespread than they are now, with fossils found as far north as Sweden and Canada. Colbert and Morales (1991) point out that the late Cretaceous was the peak of their evolution with the largest genus being the Deinosuchus with the most impressive jaws of any reptiles of the period: one fossil had a skull 6 feet in length which suggests it must have had a total length of 40 to 50 feet—certainly a predator to take notice of. Carroll (1988) suggests that the decline of the genera was probably due to climatic deterioration from around the beginning of the Caenozoic (63 million years ago) era. In spite of the massive extinctions, the families that have survived are rightly viewed as living fossils. Meyer (1984) gives a general overview of them while Pooley and Gans (1976) focus on the Nile crocodile and describe, among other things, its unique biology and social behaviour which have contributed so much to its long survival.