Parareptilia

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Parareptiles
Temporal range:
Ma
Survive if
neodiapsids descend from them[1]
A collage of five parareptile fossils. Clockwise from top,
procolophonid
)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Parareptilia
Olson, 1947
Orders

Parareptilia ("near-reptiles") is a subclass or

pareiasaurs). The only parareptiles to survive into the Triassic period were the procolophonoids, a group of small generalists, omnivores, and herbivores. The largest family of procolophonoids, the procolophonids, rediversified in the Triassic, but subsequently declined and became extinct by the end of the period.[2][3]

Compared to most eureptiles, parareptiles retained fairly "primitive" characteristics such as robust, low-slung bodies and large

temporal fenestration. In its modern usage, Parareptilia was first utilized as a cladistically correct alternative to Anapsida, a term which historically referred to reptiles with solid skulls lacking holes behind the eyes.[4] Nevertheless, not all parareptiles have 'anapsid' skulls, and some do have large holes in the back of the skull. They also had several unique adaptations, such as a large pit on the maxilla, a broad prefrontal-palatine contact, and the absence of a supraglenoid foramen of the scapula.[4][5]

Like many other so-called 'anapsids', parareptiles were historically understudied. Interest in their relationships were reinvigorated in the 1990s, when several studies argued that Testudines (turtles and their kin) were members of Parareptilia.[4] Although this would suggest that Parareptilia was not extinct after all, the origin of turtles is still heavily debated. Many other morphological or genetic analyses find more support for turtles among diapsid eureptiles such as sauropterygians or archosauromorphs, rather than parareptiles.[6][7][8][3]

Description

Skull

Early Permian. Unlike most parareptiles, this species lacks tabular bones and has a heterodont dentition with large caniform
teeth.

Parareptilian skulls were diverse, from

pareiasaurs. Parareptile teeth were quite variable in shape and function between different species. However, they were relatively homogenous on the same skull. While most synapsids and many early eureptiles had a caniform region of enlarged fang-like teeth in the front half of the skull, very few parareptiles possessed caniform teeth.[5]

Many amniotes have a row of small pits running along bones at the edge of the mouth, but parareptiles have only a few pits, with one especially large pit near the front of the maxilla.[4][9][7] The rest of the skull was often strongly-textured by pits, ridges, and rugosities in most parareptile groups, occasionally culminating in complex bosses or spines. The maxilla is usually low, while the prefrontal and lacrimal bones in front of the eye are both fairly large. In all parareptiles except mesosaurs, the prefrontal has a plate-like inner branch which forms a broad contact with the palatine bone of the palate.[4][7][5] A prominent hole, the foramen orbitonasale, is present at the intersection of the prefrontal, palatine, and lacrimal. Parareptilian palates also have toothless and reduced ectopterygoid bones, a condition taken to extremes in mesosaurs, which have lost the ectopterygoid entirely.[4][5]

Most parareptiles had large

infratemporal fenestra, a large hole or emargination lying among the bones behind the eye. In some taxa, the margins of such openings may include additional bones such as the maxilla or postorbital.[10][11] When seen from above, the rear edge of the skull is straight or has a broad median embayment.[5] From inside to outside, the rear edge of the skull is formed by three pairs of bones: the postparietals, tabulars, and supratemporals. Parareptiles have particularly large supratemporals, which often extend further backwards than the tabulars.[12]

Apart from the long, slender jaws of mesosaurs, most parareptile jaws were short and thick. The jaw joint is formed by the

braincase.[4][7] Jaw muscles attach to the coronoid process, a triangular spur in the rear half of the jaw. Both the tooth-bearing dentary bone and the posterior foramen intermandibularis (a hole on the inner surface of the jaw) reach as far back as the coronoid process.[4][5] The surangular bone, which forms the upper rear part of the jaw, is narrow and plate-like.[13]

Postcranial skeleton

procolophonid
from the Middle Triassic of England

There was some variation in the body shape of parareptiles, with early members of the group having an overall lizard-like appearance, with thin limbs and long tails. The most successful and diverse groups of parareptiles, the pareiasaurs and procolophonids, had massively-built bodies with reduced tails and stout limbs with short digits. This general body shape is shared with other ‘cotylosaurs’ such as captorhinids, diadectomorphs, and seymouriamorphs.[3] Another general ‘cotylosaurian’ feature in parareptiles is the ‘swollen’ appearance of their vertebrae, which have wide and convex upper surfaces.[12]

Parareptiles lacked a supraglenoid foramen on the scapula, a hole which is also absent in varanopids and neodiapsids.[5][11] Most had a fairly short and thick humerus which was expanded near the elbow. Unlike early eureptiles, the outer part of the lower humerus possessed both a small supinator process and an ectepicondylar foramen and groove.[4] The ulna generally has a poorly developed olecranon process, another trait in contrast with the earliest eureptiles.[4][5]

Most parareptiles had an ilium which was fan-shaped and vertically (rather than horizontally) oriented, an unusual trait among early amniotes.[4][9][7] The sacral ribs, which connect the spine to the ilium, were usually slender or fan-shaped, with large gaps between them.[4] The hindlimbs were typically not much longer than the forelimbs, and had thick reptilian ankle bones and short toes. There are some exceptions, such as Eudibamus, an early Permian bolosaurid with very elongated hindlimbs.[14]

History of classification

The name Parareptilia was coined by

temporal fenestrae
, such as modern turtles).

Parareptilia's usage was revived by

taxa and argued that captorhinids and turtles were sister groups, constituting the clade Anapsida (in a much more limited context than typically applied). A name had to be found for a clade of various early-diversing Permian and Triassic reptiles no longer included in the anapsids. Olsen's term "parareptiles" was chosen to refer to this clade, although its instability within their analysis meant that Gauthier et al. (1988) were not confident enough to erect Parareptilia as a formal taxon. Their cladogram is as follows:[16]

Amniota 

Synapsida

 
Sauropsida
 
 "Parareptiles

Mesosauridae

Procolophonidae

Millerettidae

Pareiasauria

 Reptilia 

paraphyletic assemblage. The cladogram of Laurin & Reisz (1995) is provided below:[4]

Amniota 

Synapsida

 
Sauropsida
 

Mesosauridae

 Reptilia 
 Parareptilia 

Millerettidae

 Procolophonia 

Pareiasauria

 Testudinomorpha 
 Eureptilia 

In contrast, several studies in the mid-to-late 1990s by

The cladogram below follows an analysis by M.S. Lee, in 2013.[23]

The cladogram below follows the analysis of Li et al. (2018).[24]

A 2020 study by David P. Ford and Roger B. J. Benson found that Parareptilia was nested within Diapsida as the sister group to

mesosaurs, which were again found to be basal among the sauropsids.[11] Some studes have found Parareptilia to be paraphyletic, with some parareptiles more closely related to diapsids that to other parareptiles, with Simões et al. (2022) using Neoreptilia for the clade containing Procolophonomorpha+Neodiapsida.[25]

Evolutionary history

The oldest known parareptiles are the bolosaur Erpetonyx and the acleistorhinid Carbonodraco from the Late Carboniferous (Moscovian-Gzhelian) of North America, which represents the only currently known Carboniferous parareptiles, indicating that the initial diversification of the group took place in the Late Carboniferous.[1] Numerous parareptile lineages appeared during the early Permian and the group reached a cosmopolitan distribution. Parareptile diversity declined towards the end of the Permian and procolophonoids, which first appeared during the Late Permian, were the only group of parareptiles to survive the Permian–Triassic extinction event. Procolophonid diversity sharply declined beginning in the Middle Triassic, with the group becoming extinct by the end of the Triassic.[26]

References

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  16. ^ Gauthier JA, Kluge AG, Rowe T (1988). "The early evolution of the Amniota". In Benton MJ (ed.). The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Systematics Association Special Volume. Vol. 35A. Clarendon Press. pp. 103–155.
  17. ^ Rieppel O (1994). "Osteology of Simosaurus gaillardoti and the relationships of stem-group sauropterygia". Fieldiana Geology. 28 (1462): i-85.
  18. ^ Rieppel O (1995). "Studies on skeleton formation in reptiles: implications for turtle relationships". Zoology. 98: 298–308.
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