Shonisaurus marine reptile. Artwork of a Shonisaurus hunting a shoal of belemnites. This extinct reptile lived 215 million years ago and belonged to the ichthyosaur group of dolphin-like marine reptiles. It measured 21 metres in length, making it the largest marine reptile that ever lived. Adults did not have teeth and fed by sucking small prey through its mouth. It had an eye socket measuring one metre in diameter, the largest eye of any animal.