Proterochampsa
Proterochampsa is an extinct genus of proterochampsid archosauriform from the Late Triassic of South America. The genus is the namesake of the family Proterochampsidae, and the broader clade Proterochampsia. Like other proterochampsids, Proterochampsa are quadruped tetrapods superficially similar in appearance to modern crocodiles, although the two groups are not closely related. Proterochampsids can be distinguished from other related archosauriformes by characters such as a dorsoventrally flattened, triangular skull with a long, narrow snout at the anterior end and that expands transversally at the posterior end, asymmetric feet, and a lack of postfrontal bones in the skull, with the nares located near the midline. Proterochampsa is additionally defined by characters of dermal sculpturing consisting of nodular protuberances on the skull, antorbital fenestrae facing dorsally, and a restricted antorbital fossa on the maxilla.
The genus comprises two known species: Proterochampsa barrionuevoi and Proterochampsa nodosa. P. barrionuevoi specimens have been discovered in the Ischigualasto Formation in northwestern Argentina, while P. nodosa specimens have been found in the Santa Maria supersequence in southeastern Brazil. The two species are distinct in several characters, including that P. nodosa has larger, more well-developed nodular protuberances, a more gradually narrowing snout, and a higher occiput than P. barrionuevoi. Of the two, P. nodosa is thought to have less derived features than P. barrionuevoi.
Discovery
''Proterochampsa barrionuevoi''
All known Proterochampsa barrionuevoi specimens have been discovered in the Cancha de Bochas, La Peña, and Valle de la Luna members of the Ischigualasto Formation in the Ischigualasto-Villa Unión Basin in northwestern Argentina. The Ischigualasto Formation as a whole is well-known to paleontologists from its rich fossil record of flora and fauna, with the latter including fishes and a variety of tetrapod lineages. The record from Ischigualasto Provincial Park in the Argentinian province of San Juan has been particularly well-studied. Because of this abundant fossil record and the biodiversity it represents, the formation has been valuable in the study of the Late Triassic, particularly regarding the evolution of dinosaurs and other tetrapods.The Ischigualasto Formation is Late Triassic in age, straddling the boundary between the Carnian and Norian stages. Using the U-Pb isotopic method, the Hoyada del Cerro Las Lajas site within the formation has been dated, with an upper boundary around 221.36 million years ago and a lower boundary around 230.32 million years ago. At this site, nearly all known fossils have been discovered within the lower section of the Ischigualasto Formation, with most fossils estimated to age just under 228.97 million years old, placing them near the Carnian-Norian boundary. The more fossil-rich lower section of the formation can be divided into two biozones named for the rhynchosaurs most abundant within them: the lower portion is rich in Hyperodapedon specimens, and the upper portion is rich in Teyumbaita specimens. One P. barrionuevoi specimen was found at the Hoyada del Cerro Las Lajas site in the Teyumbaita biozone. This dating places P. barrionuevoi in an environment consisting of rhynchosaur-dominated faunas. The biostratigraphy recorded at this site also supports the ability for researchers to correlate dating to other sites within the Ischigualasto Formation and other sites outside the formation where Proterochampsa specimens have been discovered.
''Proterochampsa nodosa''
Proterochampsa nodosa specimens have all been found in the Santa Maria Supersequence within the Rosário do Sul Group in the Paraná Basin in southeastern Brazil. Similarly to the Ischigualasto Formation, the Santa Maria Formation is rich in Triassic tetrapod fossils, and well-known for this record. Although the Rosário do Sul Group represents a range of faunal stages during the Triassic, the portion of the Santa Maria Formation where P. nodosa has been found is specifically estimated to be near the Carnian-Norian boundary in age, supporting a similarity in age between P. barrionuevoi and P. nodosa specimens. P. nodosa has been found in the lower portion of the Highstand systems tract of the Santa Maria 2 sequence, near the boundary between the Santa Maria and Caturrita formations within the broader Santa Maria supersequence.History
Since their discovery starting in the early twentieth century, the taxa of the clade Proterochampsia have been assigned a number of different phylogenetic placements, including as relatives of early crocodiles, phytosaurs, or proterosuchids, eventually being recognized as non-archosaur archosauriformes. The first member of the genus Proterochampsa to be discovered was Proterochampsa barrionuevoi. In 1959, Reig described a specimen from the Ischigualasto Formation, and suggested that the species could be related to early crocodiles. When describing the same specimen in 1967, Sill supported this idea, and additionally denominated the family Proterochampsidae within the order Crocodilia to include the single known species. The first suggestion that proterochampsids had aquatic or semiaquatic lifestyles came from Romer in 1971, citing the dorsal position of the nares, which would make it easier for proterochampsids to breathe in aquatic environments, as evidence. Despite pointing out that the only character shared between proterochampsids and crocodilians was a secondary palate, Romer still used this character as supporting evidence for a potential aquatic or semiaquatic lifestyle for proterochampsids.A Proterochampsa nodosa specimen from the Santa Maria Formation was discovered and named by Barberena in 1982, who placed the species within Proterochampsa and Proterochampsidae. In 2000, Kischlat and Schultz proposed a new genus definition for P. nodosa, instead naming the taxon Barbarenachampsa nodosa. However, this new synonym is contested because it has not been formalized, and has received inconsistent use. By 2009, aquatic or semiaquatic lifestyles had been proposed for all proterochampsids by Schultz, and the group was no longer considered to be closely related to crocodiles or phytosaurs. Today, Proterochampsa and related taxa are generally considered archosauriformes, but a variety of more specific phylogenetic placements have been proposed and it remains unclear exactly where these taxa should be placed.
The Proterochampsa genus was known mainly from skulls and postcrania that did not extend posteriorly past the most anterior dorsal vertebrae until the description of a new P. barrionuevoi specimen by Trotteyn in 2011. This new specimen was much more complete, and included the skull, the complete vertebral series, pelvic girdle, right hindlimb, and portions of the pectoral girdle and two other limbs. This allowed for an amended and more complete definition than the original for the species.