Styxosaurus extinct marine reptile. Artwork of Styxosaurus (Styxosaurus snowii) hunting a squid underwater. This extinct reptile lived 85-70 million years ago and belonged to the plesiosaur group of long-necked marine reptiles. It measured around 12 metres in length, half of which was neck length. It had sharp interlocking teeth design to grip and hold onto slippery prey such as fish, squid and belemnites (extinct group of cephalopods).