ANATOMY

Thalassomedon, photo by Jim Puckett

Plesiosaurs were large, marine reptiles from the Mesozoic Era that ranged in size from 8-46 feet long (2.5-14 m). 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.

No one knows how the long-necked plesiosaurs held their necks; either they held the neck horizontally, or else it was supple and could move readily.

WHEN PLESIOSAURS LIVED
Plesiosaurs lived during the Mesozoic Era, from the early Jurassic to the end of the Cretaceous.

BEHAVIOR
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).

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

DIET
Plesiosaurs ate fish and other swimming animals. They had strong jaws and sharp teeth.

LOCOMOTION
Plesiosaurs swam using their four paddle-like flippers in a manner similar to that of modern turtles. They might have been able to move a little bit on land, as modern seals do.

CLASSIFICATION
Plesiosaurs are reptiles, but not dinosaurs. They are:

• Kingdom Animalia (animals)
• Phylum Chordata
• Subphylum Vertebrata (vertebrates)
• Superclass Tetrapoda (four-legged animals)
• Class Reptilia (reptiles) or
• Sauropterygia (which includes both plesiosaurs and nothosaurs).
• Order Plesiosauria (Plesiosaurs) - the pointed-tailed flippered marine-adapted reptiles which 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 m) 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 m) long - with curved, interlocking teeth and large flippers. From England during the late Jurassic period.
    • Muraenosaurus - 20 feet (6 m) 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 m) long - with a very long neck. From Queensland, Australia, during the early Cretaceous period, about 110 million years ago.
    • Elasmosaurus - 46 feet (14 m) 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 m) long with a very long neck (the neck had 63 vertebrae). From Colorado, USA, during the late Cretaceous period
  • Pliosauroids - 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 m) long with long, toothed jaws and a long neck (with 29 vertebrae). From England during the early Jurassic period.
    • Peloneustes - 10 feet (3 m) 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.
    • Kronosaurus - 30 feet (9 m) long with a short neck and huge head and jaws. The flat-topped head was up to 9 feet (2.7 m) long, about 1/4 of the entire length of the body. From Queensland, Australia during the early Cretaceous period.
    • Liopleurodon - 39-49 feet (12-15 m) long with a large head, short neck, powerful jaws and teeth, and four long, wide, strong flippers. It had a whale-like appearance. From England, France, Germany, and eastern Europe during the late Jurassic period.
    • Pliosaurus - 33-40 ft (10-12 m) long and the crocodile-like skull was 5-6.6 ft (1.5-2 m) long. From England and South America during the Jurassic period. May be the same as Liopleurodon.