HOW MIGHT SPINOSAURIDS (DINOSAURIA:THEROPODA) HAVE CAUGHT FISH? TESTING BEHAVIORAL INFERENCES THROUGH COMPARISONS WITH MODERN FISH-EATING TETRAPODS

Since the 1983 discovery of the unusual dinosaur Baryonyx walkeri (Theropoda: Spinosauridae) there has been great interest in the feeding behavior of spinosaurids as the group exhibits various physical traits suggestive of piscivory, such as elongate crocodile-like jaws; retracted nares; weakly-serrated conical teeth; large digit I manual unguals; and large, robust forelimbs. However, despite a general consensus over spinosaurid piscivory, there have been few detailed behavioral comparisons with similarly adapted extant piscivorous tetrapods. An exception is the eponymous “heavy claw” of Baryonyx, suggested to have been used in gaffing (hooking) fish out of the water in behavior akin to that of grizzly bears. However, this hypothesis is problematic as grizzlies do not gaff fish in this manner. To better characterize spinosaurid piscivory, we reviewed piscivorous adaptations and related behaviors in extant fish-eating tetrapods. We then assessed which of these behaviors were possible in spinosaurids based on their morphology. The antero-posterior head darting strategy employed by herons (genus Ardea) was found to be unlikely as spinosaurs lack a strong sigmoid neck curve and their orbits are poorly positioned for binocular vision. Rather, as in the Indian gharial (Gavialis gangeticus), the jaws may have instead been used in swift lateral sweeps to seize fish. Like grizzly bears (Ursus arctos) and fishing cats (Prionailurus viverrinus), spinosaurids may have employed their powerful forelimbs to stamp down on large fish, impaling them on the manual claws against the river or lake bottom, from which the prey could be manipulated with the jaws. As spinosaur teeth lack prominent serrations, they do not appear to be well-suited for prey dismemberment; it is likely that most fish would have been swallowed whole, as in the majority of longirostrine piscivores. However, the use of the forelimbs in breaking up prey items cannot yet be ruled out. These findings not only furnish insight into feeding behavior in spinosaurs, contributing to our understanding of these bizarre and highly specialized dinosaurs, but also provide a window into the evolution of piscivory in tetrapods.