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 the K-T extinction, at the end of the Cretaceous Period. While they were Mesozoic reptiles that lived at the same time as dinosaurs, they were not dinosaurs. Royal Tyrrell Museum, Drumheller, Alberta, Canada.