Today most crocodilians (crocodiles, alligators, gavials, and their kin) live in fresh to brackish water. Only a few crocodilians are able to tolerate marine water using special salt-excreting glands to maintain osmotic balance. Yet, the geographic distribution of crocodilians indicates that some groups crossed major ocean basins in their evolutionary history, leading some workers to conclude that they may have been more tolerant of salt water in the past. Paleontologists have tried to identify trends in morphology or relationships that would illuminate the origin and evolution of salt-water tolerance in crocodilians, but these studies have been largely inconclusive. Isotopic methods offer powerful tools for exploring the diet, habitat preferences, and physiology of modern and fossil animals. We will study the origin and evolution of salt-water tolerance in crocodilians using naturally occurring variations in stable isotopes. We will use carbon and strontium isotope variations to monitor the habitat in which crocodilians were feeding, and oxygen isotope variations to determine whether they were drinking fresh or marine water. Reptiles have not been studied extensively using isotopic methods. Therefore, we will take a close look at modern crocodilians to ensure that our studies of ancient crocodilians are on a solid foundation. First we will study captive crocodilians in Florida. In this controlled setting, we can measure the stable isotope composition of all the water and food ingested by the animal. This will let us determine the contribution of each source to the tooth enamel of a crocodilian. We will then study wild crocodilians at a coastal wildlife refuge in Louisiana. Here we will measure the isotopic composition of crocodilians and their prey to better understand how much "noise" is introduced by a natural environment. These two modern experiments will inform our study of ancient crocodilians, and serve as a basis for other workers using isotopes to study modern and fossil reptiles. Finally, we will use these isotopic methods to assess the salt-water tolerance and adaptations of different fossil crocodilians. In most cases, these fossils come from coastal settings, so they may potentially have used saline habitats. We will analyze fossil species that are classified near the origins of the major groups of living crocodilians. This approach will shed light on the ecology of these groups as they began to diversify and radiate. Our ultimate goal is to determine if the crocodilians are a primitively salt-water group that has become more restricted to freshwater, or a primitively fresh-water group with a few derived groups that tolerate salt water.
