Plesiosaurs were carnivorous marine (mostly marine) reptiles. The common name plesiosaur is applied both to the true plesiosaurs (Suborder Plesiosauroidea) and to the bigger taxonomic rank of Plesiosauria, which includes together long-necked (elasmosaurs) and short-necked (polycotylid) forms.
Short-necked, large-headed plesiosaurs are more correctly called pliosaurs. There were numerous species of plesiosaurs and not all of them were as big as Liopleurodon, Kronosaurus or Elasmosaurus.
Plesiosaurs (sensu Plesiosauroidea) first appeared at the very initiate of the Jurassic Period and thrived until the K-T extinction, at the end of the Cretaceous age. While they were Mesozoic reptiles that lived at the identical time as dinosaurs however they were not dinosaurs.
Plesiosaurs ranged in size from 8-46 feet lengthy (2.5-14 metres). They had four flippers, spiky teeth in strong jaws and small, pointed tails. Plesiosaurs may have evolved from the Nothosaurs or Pistosaurus, a mid-Triassic reptile. They had a wide body from which their long neck extended.
Plesiosaurs lived in the open sea and breathed air. Some Plesiosaurs have been found with little stones in their stomachs - these may have been used to help grind up their food, or as counterweight, to help them dive. They most likely laid eggs in beach sand (like modern-day ocean turtles).
Plesiosaurs may have laid eggs in nest that they dug into the sand, much as contemporary sea turtles do.
Plesiosaurs (with the exemption of the Polycotylidae) were most likely relatively slow swimmers. It is probable that they cruised slowly below the surface of the water, using their long supple neck to move their head into position to snap up unwary fish or cephalopods. Their unique, four-flippered swimming version may have given them exceptional manoeuvrability, so that they could quickly rotate their bodies as an aid to catching their prey.
Contrary to many reconstructions of plesiosaurs, it would have been not possible for them to lift their head and long neck above the surface, in the swan-like pose that is frequently shown. Even if they had been capable to bend their necks upward, to that degree seriousness would have tipped their body forward and kept most of the weighty neck in the water.