Plesiosaurus was a large marine sauropterygian reptile that was about 3 to 5 meters long! It lived during the early Jurassic Period and its fossilized, nearly complete skeletons were discovered in England and Germany. The distinguishing feature of the Plesiosaurus was its small head, slender neck, broad body, short tail and two pairs of elongated turtle-like paddles.

Plesiosaurus means 'near lizard' and was named so by William Conybeare because of the normal reptile like structure, resembling those that had been found in the same rock strata. The snout of the Plesiosaurus was short and the jaws had a series of conical socket teeth. The neck, though long and slender, seemed rather stiff. This was probably due to the fact that vertebrae were observed to be flat-ended. The short tail of the Plesiosaurus was straight and tapered rapidly.

The pectoral and pelvic girdles were expanded and the pectoral arch was quite similar to that of the turtle. Its limbs were paddles and each had five digits. The skin of the Plesiosaurus was smooth. It is not yet known whether the Plesiosaurus laid eggs on land, like the turtles. The other possibility would be giving live birth in water. It is possible that the young ones could have lived in estuaries till they were mature enough to venture out.