Understanding saltwater tolerance and marine resource use in the Crocodylia: A stable isotope approach

Today, crocodylians are primarily freshwater-adapted today, though estuarine populations of Crocodylus acutus and C. porosus are notable exceptions. But beginning in the 1980s scientific work involving phylogeny, biogeography and osmoregulatory physiology suggested that Crocodylia had been more adept in coastal and marine environments in the past. Here I use stable isotopes as natural tracers of inputs to modern and fossil crocodylians to explore their current and past reliance on marine resources. Using carbon and oxygen isotope ratios from the carbonate portion of tooth bioapatite, I estimate the marine resource use of modern coastal populations of C. acutus and Alligator mississippiensis by comparing them to marine reptiles and inland populations of A. mississippiensis . Coastal A. mississippiensis and C. acutus feed from marine foodwebs in roughly equal percentages (∼60% and ∼70% respectively). I estimate the amount of seawater ingestion for C. acutus to be 80% on average (A. mississippiensis is an obligate freshwater drinker). Thus, many of the osmoregulatory behaviors assumed to be necessary for C. acutus (such as actively seeking out fresh drinking water) may not be necessary.;I examine the long-term diet of A. mississippiensis from Rockefeller Wildlife Refuge, Louisiana, by measuring carbon and nitrogen stable isotope ratios in alligator prey items and the bone and tooth collagen of the alligators themselves. I use a Bayesian mixing model to estimate the percent contribution of various prey to alligator tooth collagen and, separately, alligator bone collagen. Because reptiles constantly replace their teeth, tooth collagen is a much more recent record of dietary input than is bone collagen. I take advantage of these different time frames to examine differences between adult and juvenile diet. My work documents a movement to a more diverse and generalized diet (incorporating more brackish water taxa and terrestrial mammals) when alligators reach a total length of about 1.3 m. Results point the importance of nutria, potentially when alligators are growing quickly, and the possibility of seasonal prey items being of importance.;To estimate marine resource use and to pinpoint the origin of saltwater tolerant physiology in fossil crocodylians, I analyzed the carbonate and phosphate portions of tooth enamel from crocodylians and their close relatives for carbon and oxygen isotope ratios. I find an origin for saltwater tolerance at least as old as the common ancestor of Crocodylia + Dyrosauridae (Cretaceous) and perhaps as old as the common ancestor of Crocodylia + Metriorhynchidae (Jurassic), depending upon the phylogenetic hypothesis employed. I also found strong evidence of marine food dependence for several crocodilians, including seagrass ecosystem dependence for tomistomines and pelagic resource consumption in the dyrosaurids. Though modern crocodylians are largely freshwater focused, the group as whole had strong ties to the marine realm in the past. This physiology easily allows for the possibility of long-range oceanic dispersals to explain the biogeography of many lineages of crocodylians.