© 1989 by The Society for Integrative and Comparative Biology
Historical Zoogeography of the Eusuchian Crocodilians: A Physiological Perspective1
Queensland National Parks and Wildlife Service, Northern Regional Centre Marlow St., Pallarenda, Qld, 4810, Australia
School of Biological Sciences, Zoology A08, University of Sydney NSW, 2006, Australia
Correspondence: 2 Address for reprints.
The historical zoogeography of eusuchian crocodilians has rarely been reviewed in any detail and yet is of increasing interest to students of crocodilian biology as large amounts of comparative information on a wide range of species come to hand. Previous interpretations of crocodilian zoogeography have been based on one or another of two assumptions–that the major continental land masses have remained more or less fixed in position, and that the eusuchians have had only very limited powers of dispersal across marine barriers. Both of these assumptions are inappropriate in light of our present knowledge of continental drift and crocodilian physiology.
In this paper we attempt a reinterpretation of eusuchian zoogeography based on new information on their systematic relationships, physiological capacity for marine dispersal, and fossil history. We postulate that anatomical and physiological adaptations to a marine existence have played an important role in eusuchian history. We propose that Gavialis and Tomistoma, now restricted to fresh waters, may have been derived secondarily from ancestors adapted to salt water. In the case of Tomistoma, similarities in lingual gland and buccal cavity anatomy to the true crocodiles (Crocodylus and Osteolaemus) suggest that marine adaptations predated the divergence of tomistomine and crocodyline stocks. The buccal morphology of Gavialis suggests it also has a marine ancestry. Its systematic affinities are uncertain, lying perhaps with Tomistoma or, on other interpretations, with the Mesosuchia. In both cases, the fossil record is not inconsistent with this possibility.
Palaeontological information now available is inadequate to reconstruct the evolutionary history of the Eusuchia in detail. However, saltwater adapted eusuchians are more common in the fossil record than is widely recognized and the likelihood of dispersal across marine "barriers" by non-alligatorid crocodilians cannot be ignored.