Plesiosaurs

Plesiosaurs — Plesiosaurs Greek: plesios meaning ’near’ or ’close to’ and sauros meaning ’lizard’) were carnivorous aquatic (mostly marine) reptiles. After their discovery, they were somewhat fancifully said to have resembled “a snake threaded through the shell of a turtle”, although they had no shell. The common name ’plesiosaur’ is applied both to the ’true’ plesiosaurs (Suborder Plesiosauroidea) and to the larger taxonomic rank of Plesiosauria, which includes both long-necked (elasmosaurs) and short-necked (polycotylid) forms. Short-necked, large-headed plesiosaurs are more properly called pliosaurs. There were many species of plesiosaurs and not all of them were as large as Liopleurodon, Kronosaurus or Elasmosaurus.

Plesiosaurs (sensu Plesiosauroidea) first appeared at the very start of the Jurassic Period and thrived until the K-T extinction, at the end of the Cretaceous Period. While they were Mesozoic reptiles that lived at the same time as dinosaurs, they were not dinosaurs.

Typical plesiosaurs had a broad body and a short tail. They retained their ancestral two pairs of limbs, which evolved into large flippers. Plesiosaurs evolved from earlier, similar forms such as pistosaurs or very early, longer-necked pliosaurs. There are a number of families of plesiosaurs, which retain the same general appearance and are distinguished by various specific details. These include the Plesiosauridae, unspecialised types which are limited to the Early Jurassic period; Cryptoclididae, (e.g. Cryptoclidus), with a medium-long neck and somewhat stocky build; Elasmosauridae, with very long, inflexible necks and tiny heads; and the Cimoliasauridae, a poorly known group of small Cretaceous forms. According to traditional classifications, all plesiosaurs have a small head and long neck but, in recent classifications, one short-necked and large-headed Cretaceous group, the Polycotylidae, are included under the Plesiosauroidea, rather than under the traditional Pliosauroidea.

Unlike their Pliosaurian cousins, Plesiosaurs (with the exception of the Polycotylidae) were probably relatively slow swimmers. It is likely that they cruised slowly below the surface of the water, using their long flexible neck to move their head into position to snap up unwary fish or cephalopods. Their unique, four-flippered swimming adaptation may have given them exceptional maneuverability, so that they could swiftly rotate their bodies as an aid to catching their prey.

Contrary to many reconstructions of plesiosaurs, it would have been impossible for them to lift their head and long neck above the surface, in the ’swan-like’ pose that is often shown. Even if they had been able to bend their necks upward to that degree (which they could not), gravity would have tipped their body forward and kept most of the heavy neck in the water.