Plesiosaurs were marine reptiles which spent much of their time in the open seas. They swam by use of oar-like paddles and powerful tails. They appeared in the Triassic seas of Europe, and later spread worldwide. During the Cretaceous, some species became very large.

Plesiosaurs' diet consisted of cephalopods, fish, and other reptiles. They apparently swam near the surface and captured their prey with quick thrusts of the head.