Plesiosaurs were large carnivorous marine reptiles that lived during the Mesozoic period. Despite being large reptiles, they were not a type of dinosaur.
There are two major kinds of plesiosaur. One type is characterized by having a large head and a short neck, and are known as pliosaurs. The largest pliosaurs, such as Kronosaurus, had skulls 3 metres long with huge pointed teeth. Pliosaurs were predators that fed on other marine reptiles.
The other group of plesiosaurs were small-headed and long-necked. Of the latter group there are three kinds: the plesiosaurid, the cryptoclidids and the elasmosaurs. Most fed on fish and molluscs.