Jurassic plesiosaur fossil, 19th-century illustration. 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 and other marine prey such as squid. They breathed air, like modern turtles, and gave birth to live young. The largest plesiosaurs reached up to 15 metres in length. Plesiosaur fossils have been found in Europe, Australia and North America. This specimen, Plesiosaurus macrocephalus, dates from the Jurassic. This artwork was published in an 1864 edition of 'The World Before the Deluge' by Louis Figuier.