Temporal range: Middle Permian-Late Permian, 265–252Ma
|Skeleton of Scutosaurus karpinskii in the American Museum of Natural History|
Pareiasaurs ranged in size from 60 to 300 centimetres (2.0 to 9.8 ft) long, and may have weighed up to 600 kilograms (1,300 lb). They were stocky, with short tails, small heads, robust limbs, and broad feet. Pareiasaurs were protected by bony scutes called osteoderms that 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 an herbivorous diet.
Pareiasaurs appear very suddenly in the fossil record. It is clear that these animals evolved from Nycteroleterids, perhaps a Rhipaeosaur-like form to fill the large herbivore niche(or guild) that had been occupied early in the Permian period by the Caseid pelycosaurs and before then the Diadectid amphibians and Edaphosaur reptiles. They are much larger than the diadectids, more similar to the giant caseid pelycosaur Cotylorhynchus. 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). Although the last Pareiasaurs were no larger than the first types (indeed, many of the last ones became smaller), there was a definite tendency towards increase of armour as the group developed.
Some paleontologists have argued that pareiasaurs include the 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 exact relationships of turtles remains controversial, and pareiasaur scutes may not be homologous with the shells of turtles.
Hallucicrania (Lee 1995), The clade Hallucicrania was coined by MSY Lee, for Lanthanosuchidae + (Pareiasauridae + Testudines). Lee's pareiasaur hypothesis is looking rather less likely following the discovery of Odontochelys, a transitional aquatic turtle with very non-pareiasaur-like teeth and whose half shell matches embryonic development in modern testudines. Recent cladistic analyses reveal that lanthanosuchids to 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 (Müller & Tsuji 2007, Lyson et al. 2010) the two being united in the clade Pareiasauromorpha (Tsuji et al. 2012).
Pareiasauroidea (Nopsca, 1928), The clade Pareiasauroidea (as opposed to the superfamily or suborder Pareiasauroidea) was used by Lee 1995 for Pareiasauridae + Sclerosaurus. More recent cladistic studies place Sclerosaurus in the procolophonid subfamily Leptopleuroninae (Cisneros 2006, Sues & Reisz 2008) which means the similarities with pareiasaurs are the result of convergences.
Pareiasauria (Seeley, 1988), If neither Lanthanosuchids or Testudines are included in the clade, the Pareiasauria only contains the monophyletic family Pareiasauridae. It's a traditional linnaean term.
- Lee, M.S.Y. (1997). "Pareiasaur phylogeny and the origin of turtles." Zoological Journal of the Linnean Society, 120(3): 197-280. doi:10.1111/j.1096-3642.1997.tb01279.x
- Jalil, N.-E. and Janvier, P. (2005). "Les pareiasaures (Amniota, Parareptilia) du Permien supérieur du Bassin d’Argana, Maroc." Geodiversitas, 27(1) : 35-132.
- deBraga, M. and Rieppel, O. (1997). "Reptile phylogeny and the interrelationships of turtles." Zoological Journal of the Linnean Society, 120: 281-354.
- Tsuji, L. A.; Sidor, C. A.; Steyer, J. - S. B.; Smith, R. M. H.; Tabor, N. J.; Ide, O. (2013). "The vertebrate fauna of the Upper Permian of Niger—VII. Cranial anatomy and relationships of Bunostegos akokanensis (Pareiasauria)". Journal of Vertebrate Paleontology 33 (4): 747. doi:10.1080/02724634.2013.739537.
- 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
- Laurin, M. (1996), "Introduction to Pareiasauria - An Upper Permian group of Anapsids"