Plesiosaurs have been found with fossils of belemnites (squid-like animals), and ammonites (giant nautilus-like molluscs) associated with their stomachs. But plesiosaurs could not crack shells. Instead, they probably swallowed them whole. In the belly of a plesiosaur were "stomach stones", which are called "gastroliths." These stones moved around in the plesiosaur's stomach and cracked or crushed the shells of the animals it ate. One plesiosaur fossil found in South Dakota had 253 gatroliths weighing a total of 29 pounds.