The evidence for and implications of an invertebrate diet in Jurassic pliosaurs (Reptilia: Sauropterygia)
The Callovian pliosaurs found in the Peterborough Member of the Oxford Clay Formation are relatively poorly understood. New reconstructions of the key taxa Peloneustes, Liopleurodon and Simolestes show divergent cranial, mandibular and dental specialisations. The cranial morphology of Peloneustes is elongate, narrow and low£ in Liopleurodon the skull is shorter and wider, but still relatively depressed£ but in Simolestes the cranium is short, and exceptionally wide and deep. Comparison to modern analogues indicates diet varied between the pliosaurian taxa, with Peloneustes a piscivore, Liopleurodon a predator on large hard-boned prey (thus confirming previous suggestions), but that Simolestes should be reinterpreted as predominantly consuming invertebrates, probably cephalopods. However, the consumption of invertebrates, which impart a high salt load on the consumer, has important physiological implications for reptiles, which are unable to produce urine stronger than the blood plasma£ indeed it has been suggested that life in water is impossible for reptiles that cannot solve the problem of salt. It is therefore suggested that large salt-secreting glands were present in Simolestes due to its predominantly invertebrate diet, and present but smaller in other pliosaur genera. Due to the close packing of the tooth bases (‘roots’) and other organs in the skull, the salt glands of pliosaurs were probably located within the orbits, as seen in modern marine turtles.