Origin of Salt-Water Tolerance In Crocodylians

Crown clade crocodylians exhibit a variety of responses to saline environments. Alligatorids cannot drink saline water, but occasionally spend time foraging in brackish, estuarine, and even marine waters. Crocodylids are better adapted to saline habitats; several species spend a substantial amount of time in marine environments. Even the crocodylids that do not exploit marine ecosystems have special lingual salt glands for excreting excess salt and they can drink salt water if necessary. The narrow-snouted taxa, the tomistomines and gavialoids, are more problematic. Both are currently restricted to freshwater, however, they have lingual salt glands and biogeographic distributions that suggest at least some salt tolerance. We have used stable isotopes of carbon and oxygen as proxies for marine resource use in modern reptiles; here we extend this approach to study fossil individuals found in near shore marine environments. Testing members of stem Alligatoridae, Tomistominae, and Gavialoidea, as well as some outgroup taxa, we find evidence of marine resource use. We interpret the results in the context of paleobiogeography, and possible past oceanic crossings. Additionally, the isotopic data help to pinpoint the time of origin of salt-water tolerance in clade Crocodylia, and provide independent constraints for phylogenetic debates concerning crocodylians.