Plesiosaurs were predators of the Jurassic seas. They were not dinosaurs but aquatic reptiles which were abundant in the seas 245-65 million years ago.
The first skeleton of a plesiosaur was described in 1719 by William Stukeley (a famous archaeologist). His fossil was a partial skeleton found at Fulbeck, Nottinghamshire. However he had no idea that it was an unknown animal, suggesting it might be part of a porpoise or crocodile. Not until a century later in the 1820s were plesiosaurs first recognised as a distinct group of reptiles.
Plesiosaurs had a small head and very long neck, two sets of broad, tapering flippers and a stout, barrel shaped body. The remains of these animals are found in marine sediments.