Pareiasaurs (meaning "cheek lizards") are an extinct clade of large, herbivorous Parareptilia. Members of the group were armoured with osteoderms which covered large areas of the body. They first appeared in southern Pangaea 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 . Pareiasaurs became extinct in the end-Permian mass extinction event.
Description
Pareiasaurs ranged in size from long, with some species 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.
The cow-sized
Bunostegos differed from other pareiasaurs by having a more upright limb posture, being amongst the first amniotes to develop this trait.
Pareiasaurs were protected by bony
called
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
, indicating a herbivorous diet.
The body probably housed an extensive
digestive tract.
Most authors have assumed a terrestrial lifestyle for pareiasaurs. A 2008 bone microanatomy study suggested a more aquatic, plausibly amphibious lifestyle,
but a later 2019 study found that the bone histology provided no direct evidence of this lifestyle.
Evolutionary history
Pareiasaurs appear very suddenly in the fossil record. It is clear that these animals are
parareptiles.
As such, they are closely related to
.
Pareiasaurs filled the large herbivore
Ecological niche (or guild) that had been occupied early in the Permian period by the
Caseidae pelycosaurs and, before them, the
diadectid reptiliomorphs.
They are much larger than the diadectids, more similar to the giant caseid pelycosaur
Cotylorhynchus. 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 increased armour as the group developed. Pareiasaurs first appeared in the fossil record in the Middle Permian (
Guadalupian) of Southern Pangaea, before dispersing into Northern Pangaea and gaining a cosmopolitan distribution during the Late Permian (
Lopingian).
Classification
Some paleontologists considered that pareiasaurs were direct ancestors of modern
. 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,
and DNA evidence indicates that living turtles are more closely related to living
than
Lepidosauria, and therefore cladistically
.
Associated clades
Hallucicrania (Lee 1995): This clade was coined by MSY Lee for
Lanthanosuchidae + (Pareiasauridae +
Testudines). Lee's pareiasaur hypothesis has become untenable due to the diapsid features of the stem turtle
Pappochelys 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
(Müller & Tsuji 2007, Lyson
et al. 2010) the two being united in the clade Pareiasauromorpha (Tsuji
et al. 2012).
Pareiasauroidea (Nopcsa, 1928): This clade (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 Lanthanosuchidae or Testudines are included in the clade, the Pareiasauria only contains the monophyletic family Pareiasauridae.
Phylogeny
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:
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
External links
