The Pliosaurs were aquatic mesozoic reptiles, from the Jurassic and Cretaceous Periods. They originally included members of the family Pliosauridae, in 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. The name is derived from Greek: πλειω from the verb 'to sail' or πλειων meaning 'fin' and σαυρος meaning 'lizard'.
This group was characterised by having a short neck and an elongated head, in contrast to the long-necked plesiosaurs. They were more crocodile-shaped. However, the four-paddle swimming action, using the large flipper-like limbs was shared with plesiosaurs and they were possibly better adapted to deeper waters. They were carnivorous and their long and powerful jaws carried many sharp teeth. Their prey may have been ichthyosaurs and other plesiosaurs.
Typical genera include Macroplata, Kronosaurus, Liopleurodon, Pliosaurus and Peloneustes. Fossil specimens have been found in England, South America, and the Arctic region near Norway.
Many very early (from the Rhaetian (Latest Triassic) and Early Jurassic) primitive pliosaurs were very like plesiosaurs in appearance and indeed used to be included in the family Plesiosauridae.
These aquatic reptiles were not dinosaurs.