Plesiosaurs - (pronounced plee-zee-oh-SAWRS)

Plesiosaurs (meaning 'near lizard') were carnivorous aquatic (mostly marine) reptiles. 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 however they were not dinosaurs.

Plesiosaurs ranged in size from 8-46 feet long (2.5-14 metres). They had four flippers, sharp teeth in strong jaws and short, pointed tails. Plesiosaurs may have evolved from the Nothosaurs or Pistosaurus, a mid-Triassic reptile. They had a broad body from which their long neck extended.

Plesiosaurs lived in the open oceans and breathed air. Some Plesiosaurs have been found with small stones in their stomachs - these may have been used to help grind up their food, or as ballast, to help them dive. They probably laid eggs in beach sand (like modern-day sea turtles).

Plesiosaurs may have laid eggs in nest that they dug into the sand, much as modern-day sea turtles do.

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 manoeuvrability, 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 (they could not), gravity would have tipped their body forward and kept most of the heavy neck in the water.

The order 'Plesiosauria' included two types:

Plesiosaurus:

Plesiosauroids - had long, snake-like necks, tiny heads and wide bodies. They ate small sea creatures, probably using their long necks like a snake to catch their prey. They included:
Plesiosaurus - 7.6 feet (2.3 metres) long - with a long neck, 4 wide, paddle-shaped flippers, and a tapered body. From England and Germany during the early Jurassic period.

Cryptocleidus - 13 feet (4 metres) long with curved, interlocking teeth and large flippers. From England during the late Jurassic period.

Muraenosaurus - 20 feet (6 metres) long - with a very long neck, and a wide body. From England and France during the late Jurassic period.

Woolungosaurus - 26-33 feet (8-10 metres) long - with a very long neck. From Queensland, Australia, during the early Cretaceous period, about 110 million years ago.

Elasmosaurus - 46 feet (14 metres) long with an extremely long neck that was up to half of its length. It had and had 71 vertebrae, 28 of which were in its neck. It had four very long paddle-like flippers, and a short, pointed tail. From Japan and Kansas, USA, during the late Cretaceous period.

Thalassomedon - 40 feet (12 metres) long with a very long neck (the neck had 63 vertebrae). From Colorado, USA, during the late Cretaceous period.

Pliosauroids:

These had large heads with very strong jaws, short necks and resembled modern-day whales. They ate larger sea creatures. They included:

Macroplata - 15 feet (4.5 metres) long with long, toothed jaws and a long neck (with 29 vertebrae). From England during the early Jurassic period.

Peloneustes - 10 feet (3 metres) long with a big head, streamlined body, long head, and relatively few, not very sharp teeth. The neck had only about 20 vertebrae. Fossilized stomach containing suckers from cephalopods (e.g., squids) have been found. From England and eastern Europe during the late Jurassic period.

Scientific Classification:

Kingdom: Animalia

Phylum: Chordata

Class: Sauropsida

Superorder: Sauropterygia

Order: Plesiosauria

Suborder: Plesiosauroidea

Families:
Cimoliasauridae
Cryptoclididae
Elasmosauridae
Plesiosauridae
Polycotylidae