Plesiosaurs (meaning "near lizard") had flippers and they were marine reptiles from the Mesozoic Era- they were not dinosaurs. Plesiosaurs are divided into two groups: the Plesiosauroids with long, snake-like necks, tiny heads, and wide bodies, and the Pliosauroids, which had large heads with very strong jaws and short necks.

Plesiosaurs were large, marine reptiles from the Mesozoic Era that ranged in size from 8-46 feet long. They had four flippers, sharp teeth in strong jaws, and short, pointed tails.

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).