''Proterochampsa'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of proterochampsid

archosauriform Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles encompassing archosaurs and some of their close relatives. It was defined by Jacques Gauthier (1994) as the clade stemming from the last common anc ...

from the

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...

(

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227.3 ...

Norian The Norian is a division of the Triassic geological period, Period. It has the rank of an age (geology), age (geochronology) or stage (stratigraphy), stage (chronostratigraphy). It lasted from ~227.3 to Mya (unit), million years ago. It was prec ...

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 A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...

superficially similar in appearance to modern

crocodiles Crocodiles (family Crocodylidae) or true crocodiles are large, semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term "crocodile" is sometimes used more loosely to include all extant member ...

, 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 The Ischigualasto Formation is a Late Triassic geological formation in the Ischigualasto-Villa Unión Basin of southwestern La Rioja Province, Argentina, La Rioja Province and northeastern San Juan Province, Argentina, San Juan Province in northw ...

in northwestern

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

, while ''P. nodosa'' specimens have been found in the Santa Maria supersequence in southeastern

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

. 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

in the Ischigualasto-Villa Unión Basin in northwestern

. 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 Ischigualasto Provincial Park (), also called ''Valle de la Luna'' ("Valley of the Moon" or "Moon Valley"), due to its moon-like appearance, is a provincial protected area in the north-east of San Juan Province (Argentina), San Juan Province, nor ...

in the Argentinian province of

San Juan San Juan, Spanish for Saint John (disambiguation), Saint John, most commonly refers to: * San Juan, Puerto Rico * San Juan, Argentina * San Juan, Metro Manila, a highly urbanized city in the Philippines San Juan may also refer to: Places Arge ...

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

, particularly regarding the evolution of dinosaurs and other tetrapods. The Ischigualasto Formation is Late Triassic in age, straddling the boundary between the

and

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 (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 ''Hyperodapedon'' (from , 'above' and , 'pavement') is an extinct genus of rhynchosaur reptiles which lived during Triassic, Late Triassic period. Like other rhynchosaurs, it was an heavily built Archosauromorpha, archosauromorph, distantly rel ...

'' 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 The Santa Maria Formation is a sedimentary geological formation, rock formation found in Rio Grande do Sul, Brazil. It is primarily Carnian in age (Late Triassic), and is notable for its fossils of cynodonts, "rauisuchian" pseudosuchians, and ear ...

within the Rosário do Sul Group in the

Paraná Basin The Paraná Basin (, ) is a large cratonic sedimentary basin situated in the central-eastern part of South America. About 75% of its areal distribution occurs in Brazil, from Mato Grosso to Rio Grande do Sul states. The remainder area is distribu ...

in southeastern

. Similarly to the Ischigualasto Formation, the

Santa Maria Formation The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It is primarily Carnian in age (Late Triassic), and is notable for its fossils of cynodonts, " rauisuchian" pseudosuchians, and early dinosaurs and othe ...

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 CaturritaProterochampsia have been assigned a number of different phylogenetic placements, including as relatives of early

, phytosaurs, or proterosuchids, eventually being recognized as non-archosaur

archosauriformes Archosauriformes (Ancient Greek, Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles encompassing Archosaur, archosaurs and some of their close relatives. It was defined by Jacques Gauthier (1994) as the clade stemmin ...

. The first member of the genus ''Proterochampsa'' to be discovered was ''Proterochampsa barrionuevoi''. In 1959, Reig described a specimen from the

, and suggested that the species could be related to early crocodiles.Trotteyn, María Jimena. ''Revisión Osteologica, Análisis Filogenético y Paleoecología de Proterochampsidae (Reptilia - Arcosauriformes)'' (PhD). Universidad Nacional de Cuyo. When describing the same specimen in 1967, Sill supported this idea, and additionally denominated the family Proterochampsidae within the order

Crocodilia Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchia ...

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 The secondary palate is an anatomical structure that divides the nasal cavity from the oral cavity in many vertebrates. In human embryology, it refers to that portion of the hard palate that is formed by the growth of the two palatine shelves med ...

, Romer still used this character as supporting evidence for a potential aquatic or semiaquatic lifestyle for proterochampsids. A ''Proterochampsa nodosa'' specimen from the

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 The postcranium ("behind the cranium"; plural: postcrania) or postcranial skeleton in zoology and vertebrate paleontology is the skeleton apart from the skull. The postcranium encompasses the axial skeleton, which includes the entirety of the verte ...

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.

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. 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. The

orbits In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificia ...

, external nares, and antorbital and

temporal fenestrae Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles. Temporal fenestrae are commonly ( ...

all face dorsally. Proterochampsids have serrated, conical, and laterally compressed marginal teeth, resembling those of modern gharials. ''Proterochampsa'' has fewer teeth than other proterochampsids. 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. The description of a ''P. barrionuevoi''

braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...

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. 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. 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. Other features that distinguish the two species include a higher occiput, a more gradually narrowing snout, and a less pronounced anterior depression on the antorbital fenestra in ''P. nodosa''.

Postcrania

Historically, knowledge of ''Proterochampsa''

did not extend posteriorly past the most anterior dorsal vertebrae, until a more complete ''P. barrionuevoi'' specimen was described in 2011 by Trotteyn. 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 A crurotarsal joint is one that's situated between the bones of '' crus'', i.e. shin (tibia and fibula) and the proximal tarsal bones, i.e. astragalus and calcaneum. The ankle joint of therian mammals ( marsupials and placentals) is a crurotarsa ...

and mesotarsal. They are an exception to many other

, for which tarsal type is often a clearer indicator of posture. The available postcrania of ''P. barrionuevoi'' shows 24 presacral vertebrae with the

neural spines Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

located on the posterior portions, and development of the posterior portions of the neural arches. In contrast to other proterochampsids, which have a single row of compact, rounded

osteoderms Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of Extant taxon, extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, Temnospondyli, ...

along their back, with some variation in size and positioning compared to the vertebrae, the genus ''Proterochampsa'' do not have osteoderms.

Paleobiology

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

, due to the superficial similarities shared between the two groups, including dorsoventrally flattened skulls and

and external nares that face dorsally. 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 Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...

of bones or

in order to infer lifestyle. From bone histology indicating compactness comparable to modern terrestrial

squamates Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles; most members of which are commonly known as lizards, with the group also including snakes. With over 11,991 species, it is also the second-largest order ...

, 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. A number of notable proterochampsid features either tentatively suggest a terrestrial lifestyle if compared to modern

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

taxa, such as tail morphology, or are too ambiguous to definitively suggest one kind of lifestyle over another, such as a simple

, palatal teeth, an asymmetric foot, and osteoderms. The

gharial The gharial (''Gavialis gangeticus''), also known as gavial or fish-eating crocodile, is a crocodilian in the family (biology), family Gavialidae and among the longest of all living crocodilians. Mature females are long, and males . Adult males ...

-like teeth in proterochampsids have been suggested to be indicative of a

piscivorous A piscivore () is a carnivorous animal that primarily eats fish. Fish were the diet of early tetrapod evolution (via water-bound amphibians during the Devonian period); insectivory came next; then in time, the more terrestrially adapted rept ...

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. 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. This is another debate that may not be relevant to ''Proterochampsa'', as osteoderms are not present in either species within the genus. 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. Studies of bone histology in proterochampsids have used the discovery of highly vascular and fibrolamellar bone tissue as evidence for rapid growth rates. Notably, there is variation within some species, 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. Like other proterochampsids, the two ''Proterochampsa'' species are thought to be

predatory Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

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. 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. Other proterochampsian species have been found within the Rosário do Sul Group, including '' Rhadinosuchus gracilis'', '' Cerritosaurus binsfeldi'', and '' Chanaresuchus bonapartei''.