Plesiosaur, << PLEE see uh sawr, >> was a large ocean reptile that is now extinct. Plesiosaurs lived in the Mesozoic Era, from about 200 million to 65 million years ago. They were large reptiles rather than dinosaurs, though they lived at the same time. Plesiosaurs evolved (developed gradually) from reptile ancestors that lived on land, but they lived in the water. Like modern whales, plesiosaurs had to surface to breathe air, and they likely gave birth to live young. Plesiosaurs were predators (hunting animals). They fed on fish and other ocean animals.
There were many different species (kinds) of plesiosaurs. All plesiosaurs had a round, stiff trunk and a tail. They had two pairs of flippers instead of arms and legs. Unlike most ocean animals, plesiosaurs did not use their tails to swim. Instead, they flapped their flippers to move through the water, much as penguins do today. Plesiosaurs are unique among ocean animals in using two pairs of flippers to swim. Other ocean animals that flap their flippers to swim depend on just one pair.
Plesiosaur species varied greatly in size. The smallest plesiosaurs were less than 6 feet (2 meters) long. The largest reached more than 40 feet (12 meters) long.
Plesiosaurs also varied in the length of the neck and size of the head. One group of plesiosaurs, the elasmosaurs, had long necks and small heads. Elasmosaurs probably fed on small fish or squid while slowly cruising through the water. Another group, the pliosaurs, had short necks, large heads, and huge teeth. Pliosaurs, such as Liopleurodon, probably fed on large fish and such ocean reptiles as ichthyosaurs and other plesiosaurs. Pliosaurs were fast, powerful swimmers. They were the apex predators (highest predators on the food chain) in the ocean for millions of years.
