| Arganasuchus | |
|---|---|
 | |
| Left mandible, specimen ALM 2 | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Clade: | Pseudosuchia |
| Clade: | Loricata |
| Genus: | †Arganasuchus Jalil & Peyer, 2007 |
| Type species | |
| Arganasuchus dutuiti Jalil & Peyer, 2007 | |
Arganasuchus is an extinctgenus of "rauisuchian" (loricatan)archosaur. It is known from a singlespecies,Arganasuchus dutuiti.Fossils of this genus have been found inUpper Triassic rocks of theArgana Basin,Morocco. Though its remains were initially referred toTicinosuchus when discovered during the 1970s, in 2007 it was identified as a distinct genus with unique features of thepubis andmaxilla.Arganasuchus also had several anatomical details in common withBatrachotomus,Fasolasuchus, andPostosuchus, though its relations with other loricatans remains unresolved.Arganasuchus is considered acarnivore due to its large, knife-shaped teeth.[1][2]
Fossils ofArganasuchus were first reported byJean-Michel Dutuit in 1979, who referred amaxilla,dentary,femur, andfibula toTicinosuchus. These fossils were found in the lower part of unit T5 (the Irohalene Member) of theTimezgadiouine Formation. This geological formation, which is found in theArgana Basin ofMorocco, may have been deposited in theCarnian stage of theLate Triassic period.[1]
Arganasuchus dutuiti was named and described in 2007 byNour-Eddine Jalil andKarin Peyer. Its name translates to "Dutuit's Argana crocodile" in honor of its discoverer and place of discovery. The genus was based on theholotype AZA 904, a partialpubis bone found at a fossil site near the village of Azarifen. OtherArganasuchus fossils from Azarifen include ribs and achevron (AZA 903-1-3, and 407-1-5), teeth (AZA 408-1-3), a femur (AZA 900), a fibula (AZA 901), part of acervical vertebra (AZA 902), part of a neural arch (AZA 905), and part of atibia (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 aquadratojugal. The referral of the skull fossils toArganasuchus is uncertain but likely considering their "rauisuchian" identity and similar size and occurrence.[1]
Themaxilla has a straight lower edge with at least 12 large, knife-like teeth set in deep sockets and covered byinterdental plates. A longitudinal ridge runs along the inner portion of the maxilla and is continuous with a low facet for thepalatine bone. This continuity is seemingly unique toArganasuchus. The maxilla as a whole is Y-shaped likeBatrachotomus andFasolasuchus. It has a long and tapering posterior process (rear branch) and a shorter ascending process (upper branch) separated by a triangularantorbital 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 thenaris (likeBatrachotomus) or contacted thepremaxilla (like otherloricatans). The possiblequadratojugal fossil is thick and sharply angled, apparently contacting a longjugal but not thequadrate, unlike its relatives. The front of the lower jaw is narrow and taper towards the chin, resembling that ofSaurosuchus 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.[1]
The single knownvertebra is a spool-shapedcervical (neck) centrum with a low keel running along its underside. A partial neural arch is also preserved, though the low inclination of thepostzygapophyses indicates that it may belong to a dorsal (back) vertebra. The rib is similar to those ofSaurosuchus (thick, two-headed, with a longitudinal ridge) while thechevron is similar to those ofTicinosuchus (thickened at its tip). Thepubis is among the most unusual and unique bones inArganasuchus. Theacetabulum (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) ofArganasuchus. Thefemur is large and robust, with a prominent knob-likefourth trochanter overlooking a smaller knob which may be an area of diseased bone. Thetibia is thick but incomplete while thefibula is thinner and has a pronouncediliofibularis muscle scar almost halfway down its shaft.[1]
Arganasuchus is universally considered a "rauisuchian", a grouping of large carnivorouspseudosuchians (crocodile-line archosaurs) from the Triassic. However, "Rauisuchia" is currently considered to be aparaphyleticgrade of archosaurs incrementally closer tocrocodilians, rather than aclade (a natural grouping defined by shared relations). The most recentcladistic interpretations of archosaur classification are mainly inspired by an analysis byNesbitt (2011). He split "rauisuchians" (relabeled asParacrocodylomorpha) into two branches: the unusualpoposauroids and a more diverse branch calledLoricata, which includescrocodylomorphs and ancestral forms likePostosuchus andSaurosuchus.[3][2] 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 ofArganasuchus and other loricatans. The shape of the maxilla was similar toBatrachotomus andFasolasuchus, the vertebrae were similar toPostosuchus, the femur was comparable toPrestosuchus, and the pubis had a few similarities with bothBatrachotomus andPostosuchus. The question ofArganasuchus's relations was left unresolved, partly because its describers were aware of the uncertainty within rauisuchian taxonomy as a whole.[1]
Somephylogenetic analyses have attempted to deal withArganasuchus, although there is no consensus on its precise affinities.Brusatteet al. (2010) found weak support for a small clade ofbasal "rauisuchoids" (loricatans) containingArganasuchus,Fasolasuchus,Stagonosuchus, andTicinosuchus.[4] Françaet al. (2011) found thatArganasuchus was an unstable "wildcard" taxon in their analysis, though a connection toDecuriasuchus,Prestosuchus, andBatrachotomus was one possibility.[5]
