Proterosuchus

''Proterosuchus'' is an extinct genus of archosauriform reptiles that lived during the Early Triassic. It contains three valid species: the type species ''P. fergusi'' and the referred species ''P. alexanderi'' and ''P. goweri''. All three species lived in what is now South Africa. The genus was named in 1903 by the South African paleontologist Robert Broom. The well-known genus ''Chasmatosaurus'' is a junior synonym of ''Proterosuchus''.

''Proterosuchus'' was a mid-sized quadrupedal reptile with a sprawling stance that could reach a length of up to . It had a large head and distinctively hooked snout. It was a predator, which may have hunted prey such as '' Lystrosaurus''. The lifestyle of ''Proterosuchus'' remains debated; it may have been terrestrial or it may have been a semiaquatic ambush predator similar to modern crocodiles.

''Proterosuchus'' is one of the earliest members of the clade Archosauriformes, which also includes crocodilians, pterosaurs, and dinosaurs, including birds. It lived in the aftermath of the Permian–Triassic extinction event, the largest known mass extinction in the timeline of Earth's history.

Description

''Proterosuchus'' was a quadrupedal reptile with a sprawling stance. It could reach a total length of up to . ''Proterosuchus fergusi'' is the largest known proterosuchid with a skull length of and a possible body length of . Like most reptiles, ''Proterosuchus'' had scaly skin.

''Proterosuchus'' had a proportionally large head and long neck compared to its body. The most distinctive characteristic of its head was its strongly hooked snout, formed by a downturned premaxilla. The premaxilla contained up to nine teeth in adults, and the teeth in the snout tip were splayed out to the sides. The jaws of ''Proterosuchus'' contained numerous teeth, with up to 9 premaxillary, 31 maxillary, and 28 dentary teeth in each side. The teeth of ''Proterosuchus'' were recurved, labiolingually compressed, and serrated, as in most archosauriforms. They were isodont, or all equal in size and shape, in adult individuals, but in juveniles, the teeth were less strongly curved in the back of the jaw.

The skull of ''Proterosuchus'' exhibits many features characteristic of its position as a basal archosauriform. It bears a prominent antorbital fenestra, like most archosauriforms. In some specimens, the jugal and quadratojugal contact to complete the ventral margin of the lower temporal fenestra, as in other archosauriforms, but in other specimens, there is a narrow gap between the bones so that the lower temporal bar is incomplete as in non-archosauriform archosauromorphs. The lower jaw bears a small external mandibular fenestra, another characteristic of archosauriforms and their closest relatives.

Paleobiology

The lifestyle of ''Proterosuchus'' is debated. It has conventionally been depicted as a semiaquatic ambush predator similar to modern crocodiles. However, it lived in an arid environment and many aspects of its anatomy conflict with a semiaquatic lifestyle. In particular, its limbs are well-ossified, as in terrestrial animals, and the nostrils are laterally-positioned on the snout, not dorsally-positioned. The histology of its bones is reminiscent of terrestrial animals, not semiaquatic ones. However, support for a semiaquatic lifestyle comes from its brain anatomy, which resembles semiaquatic predators such as crocodiles more closely than terrestrial reptiles. The orientation of its ear canals suggests its neutral head posture had the snout angled upward, which would have raised the nostrils high enough for the animal to breathe while largely submerged. However, the utility of the orientation of the semicircular canal in determining head posture and habitat preference has been challenged. ''Proterosuchus'' was a predator, but the specifics of its diet are not known. It has been suggested to have eaten fish or the abundant contemporary dicynodont '' Lystrosaurus''.

Snout function

The function of the hooked snout in ''Proterosuchus'' is not fully known. The most likely use was in sexual or social signaling, similar to the hooked snout of male salmon. As the snout does not appear to have been sexually dimorphic, it may be an example of mutual sexual selection. The snout may have been used in a specialized method of predation, as it exhibits high resistance to dorsoventral bending. However, what this method may have been is unclear. The premaxillary teeth do not show wear facets and did not occlude with the teeth of the lower jaw, indicating that they were not used in any abrasive activities and could not have been used to grip prey. The snout tip did not have the pressure receptors present in crocodilians and ''Spinosaurus''.

Senses

''Proterosuchus'' had mesopic vision, indicating that it was adapted to see well in both bright and dim light. Mesopic vision is characteristic of cathemeral animals, which are active in both night and day, and crepuscular animals, which are active in twilight. Adaptations to see in dim light may have been ancestral to archosaurs, and ''Proterosuchus'' may have been an early example of this trend. However, ''Proterosuchus'' lived near the Antarctic Circle, so its mesopic vision may have instead been an adaptation to the highly seasonal day lengths it experienced. The hearing of ''Proterosuchus'' was likely adapted for lower frequencies, as in modern crocodiles. Due to its low-sensitivity hearing, ''Proterosuchus'' probably did not rely heavily on vocal communication and may have been relatively solitary. Based on the size of its olfactory bulbs, ''Proterosuchus'' had a strong sense of smell, similar to that of modern crocodiles. However, its olfactory bulbs were not as large as those of its relative ''Tasmaniosaurus'', suggesting different habits and potentially a more aquatic ecology in ''Proterosuchus''.

Metabolism

The metabolism of ''Proterosuchus'' is disputed. Like other crocopod archosauromorphs, ''Proterosuchus'' had a higher metabolic rate than extant ectotherms. Furthermore, ''Proterosuchus'' possessed fibrolamellar bone, indicative of a high growth rate and corresponding high metabolism. However, studies conflict on whether the metabolism of ''Proterosuchus'' was within the range of extant endotherms. Its metabolic rate was lower than most other crocopods, except for the ectothermic phytosaurs and crocodilians, which may have been an adaptation to a crocodile-like predatory strategy.

Ontogeny

''Proterosuchus'' grew quickly. It probably reached sexual maturity within a year, at roughly two-thirds of its maximum adult body size. Rapid growth rates were typical of Early Triassic archosauromorphs, and may have been an adaptation to surviving the hostile environment of the Early Triassic. Juvenile ''Proterosuchus'' may have hunted different prey from adults.

Paleoecology

''Proterosuchus'' fossils are found in the ''Lystrosaurus'' Assemblage Zone of the Beaufort Group in South Africa. ''Proterosuchus'' was the first new species to arrive in the Karoo environment after the Permian–Triassic extinction. ''Proterosuchus'' and the therocephalian ''Moschorhinus'' were the largest carnivores in the ecosystem at the time, and soon after the extinction ''Moschorhinus'' declined and went extinct while ''Proterosuchus'' thrived. The most common tetrapod in ''Proterosuchuss environment was the herbivorous dicynodont '' Lystrosaurus''. The environment was hot, semi-arid and experienced droughts.

Classification

''Proterosuchus'' is an early member of Archosauriformes, which also contains crocodilians, pterosaurs, and dinosaurs, including birds. It is the type genus of Proterosuchidae, which also contains the genus ''Archosaurus''. Proterosuchidae is, by definition, the most basal clade of archosauriforms, as Archosauriformes is defined based on their phylogenetic position. Under pre-cladistic taxonomy, ''Proterosuchus'' was classified in the order Thecodontia and suborder Proterosuchia. Both taxa are now recognized as paraphyletic groups of basal archosauriforms.

Species

Valid Species

''Proterosuchus'' currently contains three valid species, all from the Lower Triassic of South Africa.

* ''Proterosuchus fergusi'' is the type species of ''Proterosuchus''. It was named in 1903 by Robert Broom based on a specimen from Tarkastad donated by John Fergus, for whom the species was named. It is known from several specimens, and the species ''Chasmatosaurus vanhoepeni'' and ''Elaphrosuchus rubidgei'' are junior synonyms of it. The holotype is poorly preserved and indeterminate, and a neotype has been suggested. It is distinguished from other species of ''Proterosuchus'' by its more strongly curved quadrate.
* ''Proterosuchus alexanderi'' was named by A. Hoffmann in 1965 based on a subadult specimen. It is currently only known from one specimen. It is distinguished from other species of ''Proterosuchus'' by its longer snout.
* ''Proterosuchus goweri'' was named by Martín D. Ezcurra and Richard J. Butler in 2015, based on a specimen that had originally been described as a specimen of ''Chasmatosaurus vanhoepeni''. It is currently only known from one specimen. It is distinguished from other species of ''Proterosuchus'' by a deep horizontal process of the maxilla, a sinusoidal ventral margin of the maxilla, and a gap in the tooth row between the premaxilla and maxilla.

Other Species

Several species have been assigned to ''Proterosuchus'' or its junior synonym ''Chasmatosaurus'' in the past that are either no longer valid or no longer assigned to ''Proterosuchus''.

* '' Ankistrodon indicus'' was named in 1865 by Thomas Henry Huxley, based on a specimen from the Induan-age Panchet Formation of India. ''Ankistrodon'' has been regarded as a synonym of ''Proterosuchus'' or ''Chasmatosaurus'' in the past, but if this synonymy were correct, ''Ankistrodon'' would have priority over the other names. It is now considered a ''nomen dubium''.
* ''Chasmatosaurus vanhoepeni'' is the type species of ''Chasmatosaurus''. It was named in 1924 by Haughton. The species name honors E. C. N. van Hoepen, who collected and prepared the holotype. It is now considered a junior synonym of ''Proterosuchus fergusi''. Like all ''P. fergusi'' specimens, it is from the ''Lystrosaurus'' Assemblage Zone of the Beaufort Group of South Africa.
* ''Chasmatosaurus yuani'' was named by C. C. Young in 1936, based on specimens from the Induan-age Jiucaiyuan Formation of China. It is considered a valid species of proterosuchid, but is not formally assigned to ''Proterosuchus''. It is considered to be in need of taxonomic revision. It is more closely related to ''Proterosuchus goweri'' than to other species of ''Proterosuchus''.
* ''Elaphrosuchus rubidgei'' was named by Robert Broom in 1946. It is now considered a junior synonym of ''Proterosuchus fergusi'', with the holotype being a juvenile specimen thereof. Like all ''P. fergusi'' specimens, it is from the ''Lystrosaurus'' Assemblage Zone of the Beaufort Group of South Africa.
* ''Chasmatosaurus ultimus'' was named by C. C. Young in 1964, based on a specimen from the Anisian-age Ermaying Formation of China. It was long believed to be the geologically youngest species of proterosuchid, as it would be the only one from the Middle Triassic. However, it is no longer considered to be a proterosuchid and is now considered to be a suchian archosaur. It is now considered a ''nomen dubium''.

References

{{Taxonbar, from=Q134377

Prehistoric archosauriforms
Prehistoric reptile genera
Early Triassic reptiles of Africa
Triassic South Africa
Fossils of South Africa
Fossil taxa described in 1903