skeleton of long-necked plesiosaur (thalassomedon haningtoni) plesiosaurs were carnivorous aquatic (mostly marine) reptiles. after their discovery, they were somewhat fancifully said to have resembled a snake threaded through the shell of a turtle , although they had no shell. plesiosaurs appeared at the start of the jurassic period and thrived until the end of the cretaceous period. while they were mesozoic reptiles that lived at the same time as dinosaurs, they were not dinosaurs. royal tyrrel