Plesiosaurs were a group of marine reptiles common in the shallow seas that covered Britain during the Jurassic period. There were two different types of plesiosaurs - long-necked plesiosaurs and short-necked plesiosaurs called pliosaurs. Plesiosaurs make a good case study for showing how palaeobiologists work out what prehistoric animals ate.
Skeleton of a typical plesiosaur from the UK
Fossil droppings - precoprolites and coprolites
'Precoprolites' is a term describing gut contents and vomit. Exceptional conditions are needed for the preservation of such soft stomach contents, illustrating the importance of taphonomy (how fossils form) in palaeoecology. In the stomach of one pliosaur fossil, little 'hooklets' from the arms of squid-like animals are common and fish and reptile teeth are also documented. A plesiosaur from Northern Japan has the jaws of an ammonite, (a Jurassic shelled animal), in its stomach. Of course, plesiosaurs may have eaten other things too. Fossil droppings (coprolites) also provide evidence for diet, but how do we know what made the dung? No plesiosaur coprolites have been positively identified because they must be found in close association with the fossil animal to determine 'who did it'.
Bite marks on bones
Another way of identifying what an animal ate comes from bite marks. Pliosaur bite marks are known on small marine reptile bones including limb bones and vertebrae.