Parareptilia ("near-reptiles") is an extinct subclass of basal sauropsids ("reptiles"). Traditionally considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds), its validity as a monophyletic clade has been disputed by modern cladistic analyses. "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 largest of which reached the sized of oxen. The only "parareptiles" to survive into the Triassic period were the procolophonoids, a group of small lizard-like 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. 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. Like many other so-called "anapsids", "parareptiles" were historically understudied. Interest in their relationships was reinvigorated in the 1990s, when several studies argued that Testudines (turtles and their kin) were members of Parareptilia. 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". Several studies from the early 2020s onwards have suggested that "Parareptilia" is not a monophyletic clade but a paraphyletic grade of primitive sauropsids, with some "parareptiles" more closely related to modern reptiles than to other "parareptilians".
Description
Skull
Parareptilian skulls were diverse, from mesosaurs with elongated snouts filled with hundreds of thin teeth, to the snub-nosed, knob-encrusted skulls of 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. 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. 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. 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. Most "parareptiles" had large orbits (eye sockets), significantly longer (from front-to-back) than the region of the skull behind the eyes. The jugal bone, which forms the lower and rear edge of the orbit, has a very thin suborbital process (front branch), usually no subtemporal process (lower rear branch), and a thick dorsal process (upper rear branch). The squamosal and quadratojugal bones, which lie behind the jugal, are quite large and are embayed from behind to accommodate the internal ears. "Parareptiles" were traditionally considered to have an "anapsid"-type skull, with the jugal, squamosal, and quadratojugal firmly sutured together without any gaps or slits between them. This principle still holds true for some subgroups, such as pareiasaurs. However, a growing number of "parareptile" taxa are known to have had an 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. When seen from above, the rear edge of the skull is straight or has a broad median embayment. 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. 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. 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. The surangular bone, which forms the upper rear part of the jaw, is narrow and plate-like.
Postcranial skeleton
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