Proterochampsa

Proterochampsa; Temporal range: Carnian ; ~235–222 Ma PreꞒ Ꞓ O S D C P T J K Pg N
	The holotype skull of Proterochampsa nodosa (MCP 1694) in dorsal view
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Archosauriformes
Clade:	† Proterochampsia
Family:	† Proterochampsidae
Genus:	† Proterochampsa ; Reig 1959
Species
	†P. barrionuevoiReig 1959 (type); †P. nodosaBarberena 1982;
Synonyms
	†Barberenachampsa nodosaBarberena 1982;

Last updated July 14, 2024

Proterochampsa is an extinct genus of proterochampsid archosauriform from the Late Triassic (Carnian-Norian boundary) 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.^[1] 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.^[2]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.^[3] The genus comprises two known species: Proterochampsa barrionuevoi and Proterochampsa nodosa. P. barrionuevoi specimens have been discovered in the Ischigualasto Formation in northwestern Argentina,^[3] while P. nodosa specimens have been found in the Santa Maria supersequence in southeastern Brazil.^[4] The two species are distinct in several characters, including that P. nodosa has larger, more well-developed nodular protuberances,^[1] a more gradually narrowing snout, and a higher occiput than P. barrionuevoi.^[2]^[1] Of the two, P. nodosa is thought to have less derived features than P. barrionuevoi.^[5]

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.^[3] 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.^[3] 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.^[3] At this site, nearly all known fossils have been discovered within the lower (and thus older) 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.^[3] 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.^[4] 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.^[3]^[4]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.^[6] 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.^[7] 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.^[7]

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.^[7] 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.^[4] 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.^[7]

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.^[2]

Description

Skull

Both Proterochampsa barrionuevoi and Proterochampsa nodosa are recognizable by their distinctly triangular, dorsoventrally flattened skulls. Proterochampsa specimens have an average skull length of 50 centimeters.^[8] The skull extends laterally at the posterior end to form a large temporal region, then narrows more anteriorly to form a long, narrow snout that comprises around two-thirds of the skull lengthwise.^[8] The orbits, external nares, and antorbital and temporal fenestrae all face dorsally.^[8]^[9] Proterochampsids have serrated, conical, and laterally compressed marginal teeth, resembling those of modern gharials. Proterochampsa has fewer teeth than other proterochampsids.^[9] There are a number of other synapomorphies present in the skull that distinguish the genus Proterochampsa from others, including a reduced antorbital fossa, a ventral lamina on the angular, a divergent occipital margin, and the lack of a fossa around the supratemporal fenestra.^[8] The description of a P. barrionuevoi braincase by Trotteyn and Haro in 2009 examined several additional neurocranial features specific to the species, including a V-shaped ridge around the basisphenoidal fossa with convex branching, and a ventrolaterally exposed semilunar depression on the parabasisphenoid.^[10] Both species within the Proterochampsa genus have prominent dermal sculpturing in the form of pits, ridges and nodular protuberances on a variety of cranial bones.^[8]^[9] This feature, which distinguishes Proterochampsa from other proterochampsids, presents distinctly in each species. In P. barrionuevoi, the nodular protuberances are smaller and more sporadically positioned, while in P. nodosa, they are more organized, larger, and more well-developed.^[2]^[1] Other features that distinguish the two species include a higher occiput, a more gradually narrowing snout,^[2] and a less pronounced anterior depression on the antorbital fenestra in P. nodosa.^[7]^[1]

Postcrania

Historically, knowledge of Proterochampsa postcrania did not extend posteriorly past the most anterior dorsal vertebrae, until a more complete P. barrionuevoi specimen was described in 2011 by Trotteyn.^[2] However, a fully complete skeleton of either species has not yet been found. Proterochampsa are known to be quadrupedal, while the posture of proterochampsians as a whole is unclear due to their intermediate tarsal type that lies between crurotarsal and mesotarsal. They are an exception to many other archosauriformes, for which tarsal type is often a clearer indicator of posture.^[9] The available postcrania of P. barrionuevoi shows 24 presacral vertebrae with the neural spines located on the posterior portions, and development of the posterior portions of the neural arches.^[2] In contrast to other proterochampsids, which have a single row of compact,^[11] rounded osteoderms along their back, with some variation in size and positioning compared to the vertebrae,^[9] the genus Proterochampsa do not have osteoderms.^[2]^[9]

Paleobiology

Life restoration of Proterochampsa barrionuevoi Proterochampsa.jpg — Life restoration of *Proterochampsa barrionuevoi*

The lifestyle of Proterochampsa and other proterochampsids has been contested, and a number of suggestions having been proposed. Many early descriptions of proterochampsids assumed aquatic or semiaquatic lifestyles similar to modern crocodiles, due to the superficial similarities shared between the two groups, including dorsoventrally flattened skulls and orbits and external nares that face dorsally.^[9] However, some more recent studies have called into question this assumption, and have proposed terrestrial/amphibious or distinctly terrestrial lifestyles for some taxa. Several of these studies use the histology of bones or osteoderms in order to infer lifestyle. From bone histology indicating compactness comparable to modern terrestrial squamates, a terrestrial lifestyle has been suggested for the proterochampsid Chanaresuchus bonapartei, and by extension, several other related proterochampsids sharing similar morphology. Notably, Proterochampsa is distinct enough from these other taxa that it could have potentially had a semiaquatic lifestyle despite this new discovery.^[12]

A number of notable proterochampsid features either tentatively suggest a terrestrial lifestyle if compared to modern reptile taxa, such as tail morphology, or are too ambiguous to definitively suggest one kind of lifestyle over another, such as a simple secondary palate, palatal teeth, an asymmetric foot, and osteoderms.^[9] The gharial-like teeth in proterochampsids have been suggested to be indicative of a piscivorous diet and thus a semiaquatic lifestyle, but due to Proterochampsa having fewer teeth than other proterochampsids, the genus may potentially be excluded from this assumption.^[9] The compact nature of proterochampsid osteoderms has been suggested by some to be evidence of a semiaquatic lifestyle due to a higher bone mass, despite the small size and low number of osteoderms not supporting a large increase in bone mass overall.^[11] This is another debate that may not be relevant to Proterochampsa, as osteoderms are not present in either species within the genus.^[9] Overall, this leaves Proterochampsa in particular with few features that can be used as definitive evidence of lifestyle, and some paleontologists have called specifically for deeper analysis of lifestyle for the genus.^[12]

Studies of bone histology in proterochampsids have used the discovery of highly vascular and fibrolamellar bone tissue as evidence for rapid growth rates.^[9]^[12] Notably, there is variation within some species,^[12] and inconsistencies found in some studies suggest that proterochampsids may have had developmental plasticity, meaning their growth rates were variable and could respond to environmental changes.^[9] Like other proterochampsids, the two Proterochampsa species are thought to be predatory.^[1]

Paleoecology

P. nodosa lived in an environment with increasing humidity, when a brief system of anastomosing rivers and lakes was gradually giving way to an enduring system of braided rivers.^[4] Using Hyperodapedon-defined biozones similarly to in the Ischigualasto Formation, paleontologists have suggested that P. nodosa would have also lived in a rhynchosaur-dominated environment, with rhynchosaur specimens accounting for 90% of Carnian specimens in the Santa Maria Formation.^[4] Other proterochampsian species have been found within the Rosário do Sul Group, including Rhadinosuchus gracilis, Cerritosaurus binsfeldi, and Chanaresuchus bonapartei.^[4]

Related Research Articles

Venaticosuchus is a genus of pseudosuchian archosaurs from the family Ornithosuchidae. Known from a single species, Venaticosuchus rusconii, this genus is described based on an incomplete skull and jaw collected from the Late Triassic (Carnian) Ischigualasto Formation in the Ischigualasto-Villa Unión Basin in northwestern Argentina, which was deposited around 230 million years ago. This fossil material has been termed the holotype specimen PVL 2578. Venaticosuchus incorporated a myriad of features present in the other two genera of ornithosuchids, Ornithosuchus and Riojasuchus. However, it also had several unique traits, relating to the lower jaw.

<i>Gracilisuchus</i> Genus of fossil reptiles

Gracilisuchus is an extinct genus of tiny pseudosuchian from the Late Triassic of Argentina. It contains a single species, G. stipanicicorum, which is placed in the clade Suchia, close to the ancestry of crocodylomorphs. Both the genus and the species were first described by Alfred Romer in 1972.

Hyperodapedon is an extinct genus of rhynchosaur reptiles which lived during Late Triassic period. Like other rhynchosaurs, it was an heavily built archosauromorph, distantly related to archosaurs such as crocodilians and dinosaurs. Hyperodapedon in particular was part of the subfamily Hyperodapedontinae, a specialized rhynchosaurian subgroup with broad skulls, beaked snouts, and crushing tooth plates on the roof of the mouth.

<i>Saurosuchus</i> Paracrocodylomorph reptile genus from Late Triassic period

Saurosuchus is an extinct genus of large loricatan pseudosuchian archosaurs that lived in South America during the Late Triassic period. It was a heavy, ground-dwelling, quadrupedal carnivore, likely being the apex predator in the Ischigualasto Formation.

Doswellia is an extinct genus of archosauriform from the Late Triassic of North America. It is the most notable member of the family Doswelliidae, related to the proterochampsids. Doswellia was a low and heavily built carnivore which lived during the Carnian stage of the Late Triassic. It possesses many unusual features including a wide, flattened head with narrow jaws and a box-like rib cage surrounded by many rows of bony plates. The type species Doswellia kaltenbachi was named in 1980 from fossils found within the Vinita member of the Doswell Formation in Virginia. The formation, which is found in the Taylorsville Basin, is part of the larger Newark Supergroup. Doswellia is named after Doswell, the town from which much of the taxon's remains have been found. A second species, D. sixmilensis, was described in 2012 from the Bluewater Creek Formation of the Chinle Group in New Mexico; however, this species was subsequently transferred to a separate doswelliid genus, Rugarhynchos. Bonafide Doswellia kaltenbachi fossils are also known from the Chinle Formation of Arizona.

The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It is primarily Carnian in age, and is notable for its fossils of cynodonts, "rauisuchian" pseudosuchians, and early dinosaurs and other dinosauromorphs, including the herrerasaurid Staurikosaurus, the basal sauropodomorphs Buriolestes and Saturnalia, and the lagerpetid Ixalerpeton. The formation is named after the city of Santa Maria in the central region of Rio Grande do Sul, where outcrops were first studied.

The Ischigualasto Formation is a Late Triassic geological formation in the Ischigualasto-Villa Unión Basin of southwestern La Rioja Province and northeastern San Juan Province in northwestern Argentina. The formation dates to the late Carnian and early Norian stages of the Late Triassic, according to radiometric dating of ash beds.

<span class="mw-page-title-main">Proterochampsidae</span> Extinct family of reptiles

Proterochampsidae is a family of proterochampsian archosauriforms. Proterochampsids may have filled an ecological niche similar to modern crocodiles, and had a general crocodile-like appearance. They lived in what is now South America in the Middle and Late Triassic.

Luperosuchus is an extinct genus of loricatan pseudosuchian reptile which contains only a single species, Luperosuchus fractus. It is known from the Chañares Formation of Argentina, within strata belonging to the latest Ladinian stage of the late Middle Triassic, or the earliest Carnian of the Late Triassic. Luperosuchus was one of the largest carnivores of the Chañares Formation, although its remains are fragmentary and primarily represented by a skull with similarities to Prestosuchus and Saurosuchus.

<i>Chanaresuchus</i> Extinct genus of reptiles

Chanaresuchus is an extinct genus of proterochampsid archosauriform. It was of modest size for a proterochampsian, being on average just over a meter in length. The type species is Chanaresuchus bonapartei was named in 1971. Its fossils were found in from the early Carnian-age Chañares Formation in La Rioja Province, Argentina. Chanaresuchus appears to be one of the most common archosauriforms from the Chañares Formation due to the abundance of specimens referred to the genus. Much of the material has been found by the La Plata-Harvard expedition of 1964-65. Chanaresuchus is the most well-described proterochampsid in the subfamily Rhadinosuchinae.

The Chañares Formation is a Carnian-age geologic formation of the Ischigualasto-Villa Unión Basin, located in La Rioja Province, Argentina. It is characterized by drab-colored fine-grained volcaniclastic claystones, siltstones, and sandstones which were deposited in a fluvial to lacustrine environment. The formation is most prominently exposed within Talampaya National Park, a UNESCO World Heritage Site within La Rioja Province.

<i>Rhadinosuchus</i> Extinct genus of reptiles

Rhadinosuchus is an extinct genus of proterochampsian archosauriform reptile from the Late Triassic. It is known only from the type species Rhadinosuchus gracilis, reposited in Munich, Germany. The fossil includes an incomplete skull and fragments of post-cranial material. Hosffstetter (1955), Kuhn (1966), Reig (1970) and Bonaparte (1971) hypothesized it to be synonymous with Cerritosaurus, but other characteristics suggest it is closer to Chanaresuchus and Gualosuchus, while it is certainly different from Proterochampsa and Barberenachampsa. The small size indicates it is a young animal, making it hard to classify.

Doswelliidae is an extinct family of carnivorous archosauriform reptiles that lived in North America and Europe during the Middle to Late Triassic period. Long represented solely by the heavily-armored reptile Doswellia, the family's composition has expanded since 2011, although two supposed South American doswelliids were later redescribed as erpetosuchids. Doswelliids were not true archosaurs, but they were close relatives and some studies have considered them among the most derived non-archosaurian archosauriforms. They may have also been related to the Proterochampsidae, a South American family of crocodile-like archosauriforms.

Gualosuchus is an extinct genus of proterochampsian archosauriform from the Middle Triassic Chañares Formation of Argentina. The type and only species is Gualosuchus reigi, named by paleontologist Alfred Romer in 1971. Its skull length is 40 cm long making it quite a large proterochampsids. Gualosuchus also said to have a more robust humerus and tibia compared to other proterochampsids.

<i>Pseudochampsa</i> Extinct genus of reptiles

Pseudochampsa is an extinct genus of proterochampsid archosauriform known from the Late Triassic (Carnian) Cancha de Bochas Member of the Ischigualasto Formation of San Juan Province, Ischigualasto-Villa Unión Basin in northwestern Argentina. It contains a single species, Pseudochampsa ischigualastensis, originally named as a second species of the closely related Chanaresuchus, based on a fairly complete articulated skeleton and skull. A revision of the remains concluded that it was best to move to species to its own genus, as no traits were found to unite P. ischigualastensis and the type species of Chanaresuchus to the exclusion of other proterochampsids. A phylogenetic analysis places both species in a polytomy with Gualosuchus as the most advanced members of Proterochampsia.

Litorosuchus is a genus of armored, semiaquatic archosauriform reptile from the Middle Triassic of China, closely related to the morphologically similar Vancleavea. It contains one species, L. somnii.

Dynamosuchus is an extinct genus of pseudosuchian archosaurs from the family Ornithosuchidae. It is known from a single species, Dynamosuchus collisensis, which is based on a partial skeleton from the Santa Maria Formation of Brazil. Dynamosuchus is considered a close relative of Venaticosuchus, which is known from the Ischigualasto Formation of Argentina. Ornithosuchids are one of many groups which lived in the Santa Maria and Ischigualasto Formations, which formed at approximately the same time and were ecologically similar. As a large scavenging reptile, Dynamosuchus helps to illuminate the trophic structure of the Santa Maria Formation. It also supports the hypothesis that ornithosuchids had diversified throughout South America by the start of the Carnian, and were not originally endemic to the Ischigualasto-Villa Unión Basin.

Rugarhynchos is an extinct genus of doswelliid archosauriform from the Late Triassic of New Mexico. The only known species is Rugarhynchos sixmilensis. It was originally described as a species of Doswellia in 2012, before receiving its own genus in 2020. Rugarhynchos was a close relative of Doswellia and shared several features with it, such as the absence of an infratemporal fenestra and heavily textured skull bones. However, it could also be distinguished by many unique characteristics, such as a thick diagonal ridge on the side of the snout, blunt spikes on its osteoderms, and a complex suture between the quadratojugal, squamosal, and jugal. Non-metric multidimensional scaling and tooth morphology suggest that Rugarhynchos had a general skull anatomy convergent with some crocodyliforms, spinosaurids, and phytosaurs. However, its snout was somewhat less elongated than those other reptiles.

Incertovenator is an extinct genus of archosauriform reptile, likely an archosaur, of uncertain affinities. Its unstable position is a result of possessing a number features found in both the bird-line avemetatarsalian archosaurs and the crocodylian-line pseudosuchians. The type and only known species is I. longicollum, which is known from single specimen discovered in the Late Triassic Ischigualasto Formation of Argentina. Incertovenator is known almost entirely by its vertebral column. This indicates that it had a relatively long neck, leading to its uncertain classification due to the convergent evolution of elongated neck vertebrae in both avemetatarsalian and pseudosuchian archosaurs.

<i>Stenoscelida</i> Genus of proterochampsid archosauriforms

Stenoscelida is a genus of proterochampsid archosauriforms from the Late Triassic Santa Maria Supersequence of Rio Grande do Sul, Brazil. The genus contains a single species, S. aurantiacus, known from a right hind limb.

References

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