Parareptilia

A skull diagram of Colobomycter pholeter, a probable acleistorhinid from the Early Permian. Unlike most parareptiles, this species lacks tabular bones and has a heterodont dentition with large caniform teeth.

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.[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 orbits (eye sockets), significantly longer (from front-to-back) than the region of the skull behind the eyes.[5] 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.[4][5] 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 now 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.[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 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.[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]

Skeleton of Kapes bentoni, a 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]