Pliosauroidea is an extinct clade of plesiosaurs, known from the earliest Jurassic to early Late Cretaceous. They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive toothed jaws, commonly known as pliosaurs. More primitive non-thalassophonean pliosauroids resembled plesiosaurs in possessing relatively long necks and smaller heads. They originally included only members of the family Pliosauridae, of the order Plesiosauria, but several other genera and families are now also included, the number and details of which vary according to the classification used.
Pliosauroidea
Liopleurodon ferox
Macroplata
Kronosaurus
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.