Pareiasaurs (meaning "cheek lizards") are an extinct clade of large, herbivorous parareptiles. Members of the group were armoured with

osteoderms Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of Extant taxon, extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, Temnospondyli, ...

which covered large areas of the body. They first appeared in southern

Pangea Pangaea or Pangea ( ) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia (continent), Siberia during the Carboniferous period ...

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

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 ''Bunostegos'' ("knobbly kullroof") is an extinct genus of pareiasaur parareptile from the Late Permian of the Agadez Region in Niger. The type species, ''Bunostegos akokanensis'', was named from the Moradi Formation in 2003. It was a cow-sized ...

'' 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

scute A scute () or scutum (Latin: ''scutum''; plural: ''scuta'' "Scutum (shield), shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of Bird anatomy#Scales, birds. The ter ...

s called

osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amph ...

s 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

iguana ''Iguana'' (, ) is a genus of herbivorous lizards that are native to tropical areas of Mexico, Central America, South America, and the Caribbean. The genus was first described by Austrian naturalist Josephus Nicolaus Laurenti, J.N. Laurenti in ...

s, indicating a herbivorous diet. The body probably housed an extensive

digestive tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the Digestion, digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascula ...

. 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 nycteroleterids. Pareiasaurs filled the large herbivore niche (or

guild A guild ( ) is an association of artisans and merchants who oversee the practice of their craft/trade in a particular territory. The earliest types of guild formed as organizations of tradespeople belonging to a professional association. They so ...

) that had been occupied early in the Permian period by the caseid pelycosaurs and, before them, the diadectid reptiliomorphs. They are much larger than the diadectids, more similar to the giant caseid pelycosaur ''

Cotylorhynchus ''Cotylorhynchus'' is an Extinction, extinct genus of herbivorous Caseidae, caseid synapsids that lived during the late Cisuralian, Lower Permian (Kungurian) and possibly the early Guadalupian, Middle Permian (Roadian) in what is now Texas and Ok ...

''. 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 The Guadalupian is the second and middle Series (stratigraphy), series/Epoch (geology), epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico an ...

) of Southern Pangaea, before dispersing into Northern Pangaea and gaining a cosmopolitan distribution during the Late Permian (

Lopingian The Lopingian is the uppermost series/last epoch of the Permian. It is the last epoch of the Paleozoic. The Lopingian was preceded by the Guadalupian and followed by the Early Triassic. The Lopingian is often synonymous with the informal te ...

Classification

Some paleontologists considered that pareiasaurs were direct ancestors of modern

turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...

s. 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 ''Pumiliopareia'' is an extinct genus of pareiasaurid parareptile from the Permian period of South Africa. It is known from a complete skeleton with osteoderms. Description ''Pumiliopareia'' was about 50 cm (19.6 in) in length with a 12&nb ...

''. However, the discovery of ''

Pappochelys ''Pappochelys'' ( meaning "grandfather turtle" in Greek) is an extinct genus of diapsid reptile possibly related to turtles. The genus contains only one species, ''Pappochelys rosinae'', from the Middle Triassic of Germany, which was named by pal ...

'' argues against a potential pareisaurian relationship to turtles, and DNA evidence indicates that living turtles are more closely related to living

archosaur Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...

s than

lepidosaurs The Lepidosauria (, from Greek meaning ''scaled lizards'') is a Order (biology), superorder or Class (biology), subclass of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata also includes Lizard, lizards and Snake, snakes. Sq ...

, and therefore cladistically

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the araeosc ...

Associated clades

Hallucicrania (Lee 1995): This clade was coined by MSY Lee for Lanthanosuchidae + (Pareiasauridae +

Testudines Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...

). Lee's pareiasaur hypothesis has become untenable due to the diapsid features of the stem turtle ''Pappochelys'' and the potential testudinatan nature of ''

Eunotosaurus ''Eunotosaurus'' (''Latin (language), Latin'': Stout-backed lizard) is an extinct genus of amniote, possibly a close relative of turtles. ''Eunotosaurus'' lived in the late Middle Permian (Capitanian stage) and fossils can be found in the Karoo ...

''. 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 (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 ''Yinshanosaurus'' is an extinct genus of pareiasaurian parareptiles from the Late Permian Naobaogou Formation of China. The genus contains a single species, ''Yinshanosaurus angustus'', known from a partial skeleton including a skull and an ...

'' 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 In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...

of pareiasaurs, the results of which are displayed in the

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

below:

References

External links

* * * {{Taxonbar, from=Q132831