Arganasuchus

''Arganasuchus'' is an extinct genus of "rauisuchian" (loricatan) archosaur. It is known from a single species, ''Arganasuchus dutuiti''. Fossils of this genus have been found in Upper Triassic rocks of the Argana Basin, Morocco. Though its remains were initially referred to ''Ticinosuchus'' when discovered during the 1970s'','' in 2007 it was identified as a distinct genus with unique features of the pubis and maxilla. ''Arganasuchus'' also had several anatomical details in common with ''Batrachotomus'', ''Fasolasuchus'', and ''Postosuchus,'' though its relations with other loricatans remains unresolved. ''Arganasuchus'' is considered a carnivore due to its large, knife-shaped teeth.

Discovery

Fossils of ''Arganasuchus'' were first reported by Jean-Michel Dutuit in 1979, who referred a maxilla, dentary, femur, and fibula to ''Ticinosuchus''. These fossils were found in the lower part of unit T5 (the Irohalene Member) of the Timezgadiouine Formation. This geological formation, which is found in the Argana Basin of Morocco, may have been deposited in the Carnian stage of the Late Triassic period.

''Arganasuchus dutuiti'' was named and described in 2007 by Nour-Eddine Jalil and Karin Peyer. Its name translates to "Dutuit's Argana crocodile" in honor of its discoverer and place of discovery. The genus was based on the holotype AZA 904, a partial pubis bone found at a fossil site near the village of Azarifen. Other ''Arganasuchus'' fossils from Azarifen include ribs and a chevron (AZA 903-1-3, and 407-1-5), teeth (AZA 408-1-3), a femur (AZA 900), a fibula (AZA 901), part of a cervical vertebra (AZA 902), part of a neural arch (AZA 905), and part of a tibia (AZA 906). Skull bones have been found at a different locale, the Alili n’yifis site near the village of Alma. These skull bones include a maxilla (ALM 1), and several lower jaws (ALM 2, 3, 5, 6, and 7). One of the jaws, ALM 2, connected to a small angled bone tentatively identified as a quadratojugal. The referral of the skull fossils to ''Arganasuchus'' is uncertain but likely considering their "rauisuchian" identity and similar size and occurrence.

Description

Skull

The maxilla has a straight lower edge with at least 12 large, knife-like teeth set in deep sockets and covered by interdental plates. A longitudinal ridge runs along the inner portion of the maxilla and is continuous with a low facet for the palatine bone. This continuity is seemingly unique to ''Arganasuchus''. The maxilla as a whole is Y-shaped like ''Batrachotomus'' and ''Fasolasuchus''. It has a long and tapering posterior process (rear branch) and a shorter ascending process (upper branch) separated by a triangular antorbital fenestra which forms a 40 degrees angle. The maxilla also has an unusually long anterior process (forward branch), forming the "stem" of the Y shape. It is uncertain whether the front edge of the maxilla formed part of the naris (like ''Batrachotomus'') or contacted the premaxilla (like other loricatans). The possible quadratojugal fossil is thick and sharply angled, apparently contacting a long jugal but not the quadrate, unlike its relatives. The front of the lower jaw is narrow and taper towards the chin, resembling that of ''Saurosuchus'' in most respects. Although the rear of the lower jaw is unknown, enough of the front half was preserved to indicate that at least 14 teeth were present. ''Arganasuchus'' has an alternating tooth replacement scheme, like many other archosaurs.

Postcrania

The single known vertebra is a spool-shaped cervical (neck) centrum with a low keel running along its underside. A partial neural arch is also preserved, though the low inclination of the postzygapophyses indicates that it may belong to a dorsal (back) vertebra. The rib is similar to those of ''Saurosuchus'' (thick, two-headed, with a longitudinal ridge) while the chevron is similar to those of ''Ticinosuchus'' (thickened at its tip). The pubis is among the most unusual and unique bones in ''Arganasuchus''. The acetabulum (hip socket) expands down the shaft of the pubis and is delineated from the front by a thick ridge which projects out as a tuberosity at its lower extent. The size and orientation of the acetabulum on the pubis is an autapomorphy (unique defining feature) of ''Arganasuchus''. The femur is large and robust, with a prominent knob-like fourth trochanter overlooking a smaller knob which may be an area of diseased bone. The tibia is thick but incomplete while the fibula is thinner and has a pronounced ''iliofibularis'' muscle scar almost halfway down its shaft.

Classification

''Arganasuchus'' is universally considered a "rauisuchian", a grouping of large carnivorous pseudosuchians (crocodile-line archosaurs) from the Triassic. However, "Rauisuchia" is currently considered to be a paraphyletic grade of archosaurs incrementally closer to crocodilians, rather than a clade (a natural grouping defined by shared relations). The most recent cladistic interpretations of archosaur classification are mainly inspired by an analysis by Nesbitt (2011). He split "rauisuchians" (relabeled as Paracrocodylomorpha) into two branches: the unusual poposauroids and a more diverse branch called Loricata, which includes crocodylomorphs and ancestral forms like ''Postosuchus'' and ''Saurosuchus''. Although most frequently compared with loricatans, ''Arganasuchus'' has yet to be properly incorporated into this newer understanding of crocodilian ancestry, and as a result its classification relative to other "rauisuchians" is poorly understood.

The original description by Jalil & Peyer (2007) made many comparisons between the anatomy of ''Arganasuchus'' and other loricatans. The shape of the maxilla was similar to ''Batrachotomus'' and ''Fasolasuchus'', the vertebrae were similar to ''Postosuchus'', the femur was comparable to ''Prestosuchus'', and the pubis had a few similarities with both ''Batrachotomus'' and ''Postosuchus''. The question of ''Arganasuchus'''s relations was left unresolved, partly because its describers were aware of the uncertainty within rauisuchian taxonomy as a whole.

Some phylogenetic analyses have attempted to deal with ''Arganasuchus,'' although there is no consensus on its precise affinities. Brusatte ''et al.'' (2010) found weak support for a small clade of basal "rauisuchoids" (loricatans) containing ''Arganasuchus'', ''Fasolasuchus'', ''Stagonosuchus'', and ''Ticinosuchus''. França ''et al.'' (2011) found that ''Arganasuchus'' was an unstable "wildcard" taxon in their analysis, though a connection to ''Decuriasuchus'', ''Prestosuchus'', and ''Batrachotomus'' was one possibility.

References

{{Taxonbar, from=Q4162558

Rauisuchids
Late Triassic reptiles of Africa
Fossil taxa described in 2007
Prehistoric pseudosuchian genera