The Plesiosauria or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia.
Image: Paleo Hall at HMNS plesiosaur
Image: Peloneustes philarchus Tubingen
First published plesiosaur skeleton, 1719 (specimen NHMUK PV R.1330)
As this illustration shows, Conybeare by 1824 had gained a basically correct understanding of plesiosaur anatomy.
The Mesozoic Era is the second-to-last era of Earth's geological history, lasting from about 252 to 66 million years ago, comprising the Triassic, Jurassic and Cretaceous Periods. It is characterized by the dominance of gymnosperms and of archosaurian reptiles, such as the dinosaurs; a hot greenhouse climate; and the tectonic break-up of Pangaea. The Mesozoic is the middle of the three eras since complex life evolved: the Paleozoic, the Mesozoic, and the Cenozoic.
Plateosaurus (a prosauropod)
Sericipterus
Stegosaurus
Tylosaurus (a mosasaur) hunting Xiphactinus