Plesiosaur

Plesiosaurs were marine reptiles that dominated Earth's oceans from the early Jurassic to late Cretaceous. They came in two types, the Pliosaurs, which had short necks and enormous skulls and the Plesiosaurs themselves, which had long necks and tiny skulls. The true Plesiosaurs were peaceful fish-eaters who could clamber onto land to escape predators while the Pliosaurs were blood-thirsty carnivores that devoured anything in their way.

Appearance
Plesiosaurs looked like seals, with big bodies, two flippers on each side, and a tail at the end. The true Plesiosaurs had long necks and tiny skulls while the Pliosaurs had short necks and enormous skulls.

Eating habits
The true Plesiosaurs ate mostly fish, squid, belemnites and ammonites while Pliosaurs ate anything they could sink their teeth into, mostly fish, and other marine reptiles.

Appearances in the films
The Plesiosaurs have not appeared in any of the Jurassic Park books, films or video games except for Jurassic Park III: Park Builder but there are high hopes they will make an appearance in the second Jurassic Park trilogy Joe Johnston has planned.