Plesiosaur attack, artwork. Plesiosaurs were marine reptiles that inhabited the seas throughout the Jurassic and Cretaceous periods from around 200 to 65 million years ago. They were not dinosaurs, despite living at the same time. It is thought that they fed mainly on fish. They breathed air, like modern turtles, and may have returned to beaches to lay their eggs. The largest plesiosaurs reached up to 15 metres in length. Plesiosaur fossils have been found in Europe, Australia and North America.