Nothosaurus was a marine reptile from the Triassic Period, about 240 to 210 million years ago. As a nothosaur, it was a close relative and ancestor to the later marine reptile group Plesiosauria. Nothosaurus was semi-aquatic, often hauling out of the water like a sea lion in order to rest on the shore.
Wikipedia has a more detailed and comprehensive article on Nothosaurus
Nothosaurus appears as an enemy in Jurassic Park for the Game Gear and Master System in Stage Two (Brachiosaurus). Nothosaurus is seen running on its hind legs.