Pareiasaurus large quadruped, about 2.5 metres (8.2 ft) long, with elephantine legs, walking in a typically reptilian posture. Its skull had several spine- and wart-like protrusions. The scales may have provided some protection against predators. Pareiasaurus's leaf-shaped teeth, ideal for biting through tough plant fibers, indicate it was a herbivore. Even the palate had teeth.
Pareiasaurus is an extinct genus of anapsid reptile from the Permian period. It was a typical member of its family, the pareiasaurs, which take their name from this genus.
Pareiasaurs appear very suddenly in the fossil record. It is clear that these animals evolved from Rhipaeosaurs to fill the large herbivore niche (or guild) that had been occupied early in the Permian period by the Caesid pelycosaurs and before then the Diadectid amphibians and Edaphosaur reptiles. In fact it may well have been the extinction of the Caesids created an ecological vacuum that enabled the Pareiasaurs to appear and suddenly diversify as rapidly as they did (within the span of only two million years).
It has been often suggested that these animals were semi-aquatic.
It has recently been argued that Pareiasaurs may have evolved into turtles. They had turtle-like skull features, and several genera had bony plates in the skin, possibly the first signs of a turtle shell. However, the case for turtle ancestry is not proven.
Probably, the Pareiasaurus were the victims of Titanophoneus.