Pareiasaurs (meaning "cheek lizards") are an extinct clade of large, herbivorous reptiles (traditionally considered "parareptiles") that existed during the Permian period. Members of the group were armoured with osteoderms which covered large areas of the body. They first appeared in southern Pangea during the Middle Permian, before becoming globally distributed during the Late Permian. Pareiasaurs were the largest reptiles of the Permian, some reaching sizes over , equivalent to the largest contemporary therapsids. Pareiasaurs became extinct in the end-Permian mass extinction event.
Description
thumb|200px|left|Restoration of [[Bradysaurus]]
left|thumb|Size of [[Scutosaurus, one of the largest pareiasaurs, compared to a human]]
left|thumb|Heavily ornamented skull of [[Elginia mirabilis]]
Pareiasaurs ranged in size from long, with the largest pareiasaurs like Scutosaurus and Bradysaurus estimated to exceed in body mass. The limbs of many parieasaurs were extremely robust, likely to account for the increased stress on their limbs caused by their typically sprawling posture. Pareiasaurs were protected by bony scutes called osteoderms that were set into the skin. Their skulls were heavily ornamented with bosses, rugose ridges, and bumps. Their leaf-shaped multi-cusped teeth resemble those of iguanas, indicating a herbivorous diet. The body probably housed an extensive digestive tract. but a later 2019 study found that the bone histology provided no direct evidence of this lifestyle, a conclusion also supported by a 2014 isotopic study, which found support for a terrestrial ecology.
Evolutionary history
Among reptilian groups, pareiasaurs are thought to be most closely related to the nycteroleterids as part of the clade Pareiasauromorpha. Pareiasaurs filled the large herbivore niche (or guild) that had been occupied early in the Permian period by the caseid pelycosaurs and, before them, the diadectid reptiliomorphs. Pareiasaurs abruptly became extinct as part of the Permian-Triassic mass extinction event at the end of the Permian. Jalil and Janvier, in a large analysis of pareiasaur relationships, also found turtles to be close relatives of the "dwarf" pareiasaurs, such as Pumiliopareia. However, the discovery of Pappochelys argues against a potential pareisaurian relationship to turtles, and DNA evidence indicates that living turtles are more closely related to living archosaurs than lepidosaurs, and therefore cladistically diapsids.
Pareiasaurs are currently generally considered to be an earlier branch of the reptilian family tree that diverged before the common ancestor of living reptiles including turtles. Pareiasaurs were long historically included with many other primitive reptiles in the group "Parareptilia", but recent research has questioned the validity of this group.
Cladogram of Reptilia after Jenkins et al 2025, with orange bars highlighting taxa formerly considered parareptiles:In 2025, Jian Yi and Jun Liu described Yinshanosaurus as a new Chinese pareiasaur based on two well-preserved, nearly complete skulls and an incomplete, partially articulated skeleton. Their publication included a phylogenetic analysis of pareiasaurs, the results of which are displayed in the cladogram below:
References
Further reading
- Carroll, R. L., (1988), Vertebrate Paleontology and Evolution, W.H. Freeman & Co. New York, p. 205
- Kuhn, O, 1969, Cotylosauria, part 6 of Handbuch der Palaoherpetologie (Encyclopedia of Palaeoherpetology), Gustav Fischer Verlag, Stuttgart & Portland