Parareptilia

Extinct subclass of reptiles (306–201Ma ago)

Parareptilia ("near reptiles") is an extinct subclass or clade of basal sauropsids/reptiles, typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near the end of the Carboniferous period and achieved their highest diversity during the Permian period. Several ecological innovations were first accomplished by parareptiles among reptiles. These include the first reptiles to return to marine ecosystems (mesosaurs), the first bipedal reptiles (bolosaurids such as Eudibamus), the first reptiles with advanced hearing systems (nycteroleterids and others), and the first large herbivorous reptiles (the 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. Compared to most eureptiles, parareptiles retained fairly "primitive" characteristics such as robust, low slung bodies and large supratemporal bones at the back of the skull. While all but the earliest eureptiles were diapsids, with two openings at the back of the skull, parareptiles were generally more conservative in the extent of 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. 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. When seen from above, the rear edge of the skull is straight or has a broad median embayment. Apart from the long, slender jaws of mesosaurs, most parareptile jaws were short and thick. The jaw joint is formed by the articular (in the lower jaw) and the quadrate (in the upper jaw). In many parareptiles, the jaw joint is shifted forwards on the skull past the rear part of the braincase.

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.

The name Parareptilia was coined by Olson in 1947 to refer to an extinct group of Paleozoic reptiles, as opposed to the rest of the reptiles or Eureptilia ("true reptiles"). Olsen's term was generally ignored, and various taxa later known as parareptiles were generally not placed into exclusive groups with each other. Many were classified as 'cotylosaurs' (a wastebasket taxon of stout bodied 'primitive' reptiles or reptile like tetrapods) or 'anapsids' (reptiles without temporal fenestrae, such as modern turtles). Parareptilia's usage was revived by cladistic studies, to refer to those traditional 'anapsids' that were thought to be unrelated to turtles. Gauthier et al. (1988) provided the first phylogenetic definitions for the names of many amniote 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:

Laurin & Reisz (1995) found a slightly different topology, in which Reptilia is divided into Parareptilia and Eureptilia. They argued that Testudines (turtles) were members of Parareptilia; in fact, they explicitly defined Parareptilia as "Testudines and all amniotes more closely related to them than to diapsids". Captorhinidae was transferred to Eureptilia, while Parareptilia included turtles alongside many of the taxa named as such by Gauthier et al. (1988). There was one major exception: mesosaurs were placed outside both groups, as the sister taxon to the crown group Reptilia. Mesosaurs were still considered sauropsids, as they were closer to reptiles than to synapsids. The traditional group 'Anapsida' was rejected as a paraphyletic assemblage. The cladogram of Laurin & Reisz (1995) is provided below:

In contrast, several studies in the mid to late 1990s by Olivier Rieppel and Michael deBraga argued that turtles were actually lepidosauromorph diapsids related to the sauropterygians. The diapsid affinities of turtles have been supported by molecular phylogenies. The first genome wide phylogenetic analysis was completed by Wang et al. (2013). Using the draft genomes of Chelonia mydas and Pelodiscus sinensis, the team used the largest turtle data set to date in their analysis and concluded that turtles are likely a sister group of crocodilians and birds (Archosauria). This placement within the diapsids suggests that the turtle lineage lost diapsid skull characteristics, since turtles possess an anapsid skull. This would make Parareptilia a totally extinct group with skull features that resemble those of turtles through convergent evolution. With turtles positioned outside of parareptiles, Tsuji and Müller (2009) redefined Parareptilia as "the most inclusive clade containing Milleretta rubidgei and Procolophon trigoniceps, but not Captorhinus aguti." The cladogram below follows the analysis of Li et al. (2018). A 2020 study by David P. Ford and Roger B. J. Benson found that Parareptilia was nested within Diapsida as the sister group to Neodiapsida, with the clade containing Neodiapsida and Parareptilia dubbed Neoreptilia, which suggests that parareptiles were ancestrally diapsid. This excluded mesosaurs, which were again found to be basal among the sauropsids.

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 known Carboniferous parareptiles, indicating that the initial diversification of the group took place in the Late Carboniferous. 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.