Artwork of a plesiosaurus. This extinct animal lends it name to an entire order of animals collectively called the plesiosaurs. They were marine reptiles that roamed the oceans of the Mesozoic Era. Plesiosaurus specifically lived during the early Jurassic period in the region now associated with England. It was around 3 metres long. Like other plesiosaurs, plesiosaurus had a long neck, a relatively short tail, and two sets of broad paddles.