Pareiasaurs were a clade of parareptiles comprising the familyPareiasauridae. They were large herbivores that flourished during the Permian period. Parareptiles form a sister clade to reptiles and birds.
Description
Pareiasaurs ranged in size from long, and may have weighed up to. They were stocky, with short tails, small heads, robust limbs, and broad feet. The cow-sized species Bunostegos, which lived 260 million years ago, is the earliest known example of a tetrapod with a fully erect posture as its legs were positioned directly under its body. Pareiasaurs were protected by bony scutes called osteoderms that were set into the skin. Their heavy skulls were ornamented with multiple knobs and ridges. The leaf-shaped multi-cusped teeth resemble those of iguanas, caseids, and other reptilian herbivores. This dentition, together with the deep body, which may have housed an extensive digestive tract, are evidence of a herbivorous diet. Most authors have assumed a terrestrial lifestyle for pareiasaurs, but bone microanatomy suggests a more aquatic, plausibly amphibious lifestyle.
Pareiasaurs appear very suddenly in the fossil record. It is clear that these animals are parareptiles. As such, they are closely related to Nycteroleterids. Like Rhipaeosaurs, they may have filled the large herbivore niche that had been occupied early in the Permian period by the Caseid pelycosaurs and, before them, the Diadectid amphibians and Edaphosaur reptiles. They are much larger than the diadectids, more similar to the giant caseid pelycosaurCotylorhynchus. Although the last Pareiasaurs were no larger than the first types, there was a definite tendency towards increased armour as the group developed.
Classification
Some paleontologists considered that pareiasaurs were direct ancestors of modern turtles. Pareiasaur skulls have several turtle-like features, and in some species the scutes have developed into bony plates, possibly the precursors of a turtle shell. 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.
Associated clades
Hallucicrania : This clade was coined by MSY Lee for Lanthanosuchidae +. Lee's pareiasaur hypothesis has become untenable due to the diapsid features of the stem turtlePappochelys and the potential testudinatan nature of Eunotosaurus. Recent cladistic analyses reveal that lanthanosuchids have a much more basal position in the Procolophonomorpha, and that the nearest sister taxon to the pareiasaurs are the rather unexceptional and conventional looking nycteroleterids the two being united in the clade Pareiasauromorpha. Pareiasauroidea : This clade was used by Lee for Pareiasauridae + Sclerosaurus. More recent cladistic studies place Sclerosaurus in the procolophonid subfamily Leptopleuroninae, which means the similarities with pareiasaurs are the result of convergences. Pareiasauria : If neither Lanthanosuchids or Testudines are included in the clade, the Pareiasauria only contains the monophyletic family Pareiasauridae. It's a traditional linnaean term.
