Functional analysis of the jaws and teeth
We can also look at the jaws and teeth to learn about the diet of plesiosaurs. Pliosaurs with their massive heads and throats could eat large prey. They were not fussy eaters! Their heads and jaws were also very strong allowing them to clamp onto large prey with big deep-rooted teeth and then twist off lumps of flesh by rolling their whole body! These pieces would be of suitable size to be swallowed whole. This method of feeding is called twist feeding and is used by crocodiles. Isolated fossil bones are often found that may have been dropped by the pliosaurs during twist feeding. The teeth of some pliosaurs are unusually curved backwards, suggesting they were used to pull struggling prey into the mouth.
Functional analysis of the body
Because you must catch your meal before you can eat it, swimming speed is important for aquatic carnivores. By measuring the shape of the body, and other properties of the body that affect swimming, the speed for plesiosaurs has been estimated. Long necked plesiosaurs were estimated at 5.1 miles per hour, the speed of a slow ambush predator, and pliosaurs were slightly faster, indicating that they chased their prey rather than ambushing it.
Lookalikes - using living animals
Similarities between the teeth of marine reptiles and modern large marine carnivores provide us with clues as to the diet in extinct marine reptiles. Today killer whales (Orcinus orca) eat large mammals such as seals, sea lions and dolphins. Their teeth show many similarities with pliosaurs so they are likely to have relied on similar prey. Of course, there were no mammals then, so they must have eaten similar sized animals that were about then, such as ichthyosaurs and marine crocodiles. The teeth of plesiosaurs show similarities with the piscivorous (fish eating) gharial, also known as the gavial, a long-snouted crocodile. Both of these animal's teeth interlock, so plesiosaurs probably ate fish too.
A meat eater is not always a predator
Being a meat eater (carnivore) does not necessarily mean an animal was a predator. Stomach contents containing dinosaurs provide evidence that pliosaurs scavenged dinosaur corpses that floated out to sea. This is also evidence that large pliosaurs were opportunistic, eating whatever they came across. Cryptoclidus, a common plesiosaur in Britain during the Jurassic, had many thin needle-like teeth and may be similar to the living crabeater seal, which has sieve-like teeth for capturing krill, small shrimp-like organisms. A 'rushing upwards' style of attack is inferred for large pliosaurs like modern great white sharks, because this is an efficient way for large animals to hunt.
Combining the evidence
By combining all this evidence, we have been able to build up a picture of exactly what plesiosaurs ate.