Ichthyosaurus (left) and Plesiosaurus (right) reptiles, 19th-century artwork. These extinct marine reptiles co-existed in the Early Jurassic Period (200 to 176 million years ago). They are examples of ichthyosaurs and plesiosaurs. The largest ichthyosaurs were over 10 metres long. The largest plesiosaurs reached up to 15 metres in length. The two were often depicted in battle. The water spouts and blowholes on the Ichthyosaurus are now known to be inaccurate. Artwork from the 1865 English edition of 'The World before the Deluge', first published in 1863 by Louis Figuier, with illustrations by Edouard Riou.