The stomach stones or gastroliths found in the
plesiosaurs made it possible for them to swallow their food whole, letting
the gastrolith grind the food. The stones may also have served as ballast
or weight for deep diving.
The skeletal remains of plesiosaurs are rare. Late Cretaceous
plesiosaurs can be
classified into three main groups
the long-necked Elasmosaurs
the giant-headed Pliosaurs
the short-necked Polycotylids.