1) Animal order 2) Any of the plesiosaurs 3) Suborder plesiosauria 
Plesiosaurs first appeared in the latest Triassic Period, possibly in the Rhaetian stage, about 205 million years ago. They became especially common during the Jurassic Period, thriving until their disappearance due to the Cretaceous–Paleogene extinction event at the end of the Cretaceous Period, about 66 million years ago. They had a worldwide ...
• (n. pl.) An extinct order of Mesozoic marine reptiles including the genera Plesiosaurus, and allied forms; -- called also Sauropterygia.
Ple`si·o·sau'ri·a noun plural [ New Latin See Plesiosaurus .] (Paleon.) An extinct order of Mesozoic marine reptiles including the genera Plesiosaurus, and allied forms; -- called also Sauropterygia . 
[n] - any of the plesiosaurs
suborder Plesiosauria noun any of the plesiosaurs