Proterochampsid

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.

Description

Proterochampsids have long, crocodile-like skulls. The posterior portion of the skull is wide while the snout is very narrow. Most proterochampsids also have downturned snouts. Like many early archosauriforms, they also have dermal armour.

Proterochampsids have small holes called dorsal fenestrae at the top of their skulls. Unlike other early archosauromorphs, they do not have a parietal foramin, which in many reptiles holds a parietal eye. The postorbital bones behind the eye sockets have thick, jagged crests. As another diagnostic feature of the group, the holes that allow the passage of the internal carotid artery through the braincase open at the sides of a bony projection called the basipterygoid process. Proterochampsids are primitive in that they have simple plate-like pelvises, but they lack small bones in the vertebra called intercentra that are common in earlier reptiles.

As in most archosaurs, distinguishing features can be seen in the shape of the ankle bones. A projection on the calcaneum bone called the calcaneal tuber is narrow and positioned downward relative to other lateral projections on the bone. The calcaneum also has a facet that attaches to both the fibula bone of the leg and another tarsal, or ankle bone. A hemicylindrical facet on the calcaneum attaches to another bone in the ankle called the astragalus. The astragalus has facets that attach to the tibia and fibula that are adjacent to each other.

Classification

Proterochampsidae was named in 1966 by A.S. Romer in his book ''Vertebrate Paleontology, 3rd edition''. The clade was defined as a stem-based taxon by María Trotteyn in 2011 as the most inclusive clade containing ''Chanaresuchus bonapartei'' and ''Proterochampsa barrionuevoi'', but not ''Euparkeria capensis'', '' Doswellia kaltenbachi'', ''Passer domesticus'' (the house sparrow), or ''Crocodylus niloticus'' (the Nile crocodile). Members such as ''Proterochampsa'' and ''Cerritosaurus'' had been known for several decades prior to the family's creation. Proterochampsids were originally thought to be close relatives of crocodilians based on their similar appearance. In the following years, proterochampsids were frequently associated with Proterosuchia, another group of long-snouted Triassic archosauriforms. As phylogenetic studies became more common in the 1980s and 1990s, proterochampsids were found to be a distinct group closely related to true archosaurs. Recent studies have placed Proterochampsidae as either the sister taxon of Archosauria (the closest relatives of archosaurs), or the sister taxon of Archosauria and ''Euparkeria''. In a 2011 study, the unusual Late Triassic archosauriform ''Doswellia'' has been placed as the closest relative of proterochampsids. Because doswelliids are more closely related to proterochampsids than to any other archosauriform, the two groups form their own clade.

Twelve species have been confirmed to belong to Proterochampsidae: '' Cerritosaurus binsfeldi'', '' Chanaresuchus bonapartei'', '' Gualosuchus reigi'', '' Kuruxuchampsa dornellesi'', '' Pinheirochampsa rodriguesi'', '' Proterochampsa barrionuevoi'', '' Proterochampsa nodosa'', '' Pseudochampsa ischigualastensis'', '' Retymaijychampsa beckerorum'', '' Rhadinosuchus gracilis'', '' Stenoscelida aurantiacus'', and '' Tropidosuchus romeri''. ''P. nodosa'' was assigned to its own genus ''Barberenachampsa'' in 2000, but it is generally still considered to be a species of ''Proterochampsa''. Modern studies place Proterochampsids in a larger group called Proterochampsia. Under the classification of Kischlat and Schultz (1999), ''Cerritosaurus'', ''Proterochampsa'', and ''Tropidosuchus'' are basal forms, while ''Chanaresuchus'', ''Gualosuchus'', and ''Rhadinosuchus'' form the family Rhadinosuchidae. However, more recent studies reduce Rhadinosuchidae to a subfamily level as Rhadinosuchinae to be placed within Proterochampsidae and include advanced proterochampsid, e.g. ''Chanaresuchus'' and ''Rhadinosuchus''. It was defined by Martin Ezcurra and colleagues in 2015 during the redescription of ''Rhadinosuchus'' as all archosauriforms more closely related to ''Rhadinosuchus gracilis'' and ''Chanaresuchus bonapartei'' than to '' Cerritosaurus binsfeldi'', '' Tropidosuchus romeri'', and '' Doswellia kaltenbachi''. Below is a cladogram following the phylogenetic analysis of Ezcurra (2016) that recovered Doswelliidae as the sister taxon of Proterochampsidae (within Proterochampsia). Proterochampsia was found to be the sister taxon of Archosauria, whose living representatives consist of birds and crocodilians.

Genera

Sources

* A. S. Romer. 1966. ''Vertebrate Paleontology'', 3rd edition. University of Chicago Press, Chicago 1-468

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Proterochampsia
Prehistoric reptile families
Triassic reptiles of South America
Ladinian first appearances
Norian extinctions
Taxa named by Alfred Romer