skeleton of short-necked plesiosaur (trinacromerum bonneri). plesiosaurs were carnivorous aquatic (mostly marine) reptiles. the common name 'plesiosaur' is applied both to the 'true' plesiosaurs (suborder plesiosauroidea) which includes both long-necked (elasmosaurs) and short-necked (polycotylid) forms. the pliosaurs were the short-necked, large-headed plesiosaurians that were the apex predators for much of the mesozoic. plesiosaurs appeared at the start of the jurassic period and thrived until