Luperosuchus

Luperosuchus (meaning "vexing" or "difficult crocodile") is an extinct genus of loricatan pseudosuchian reptile (historically known as a "rauisuchian") which contains only a single species, Luperosuchus fractus. It is known from the Chañares Formation of Argentina, within strata belonging to the latest Ladinian stage of the late Middle Triassic, or the earliest Carnian of the Late Triassic. Luperosuchus was one of the largest carnivores of the Chañares Formation, although its remains are fragmentary and primarily represented by a skull with similarities to Prestosuchus and Saurosuchus.

Description
Luperosuchus is known only from a single incomplete skull, with an associated atlas intercentrum representing the only known postcranial material. However, it is likely that it was a large quadruped, similar to other basal loricatans. Isolated osteoderms from the same region were initially attributed this genus based on their size and similarity to those of other "rauisuchian" osteoderms, however they were later found to be belong to the contemporary erpetosuchid Tarjadia. The presence of osteoderms in Luperosuchus is therefore left ambiguous. The length has been estimated to be 4.1 – based on rauisuchids and 3.7 m based on Prestosuchus.

Skull
The skull belongs to a large animal, with the preserved portions measuring 54.5 cm and an estimated complete length of approximately 60 cm, resembling those of other basal loricatans. The material largely consists of the skull roof, including the nasals, maxilla and part of the premaxilla. The snout is narrow and pointed, with a tall maxilla. Only a small, rounded front portion of the antorbital fenestra is preserved, though it was likely triangular based on the height of the skull. The antorbital fossa, the basin surrounding the antorbital fenestra, was deep. The premaxilla has a characteristically long posterodorsal process that articulates with the nasals, excluding the maxilla from the external naris, which tapers posteriorly. A narrow slit was identified between the premaxilla and maxilla, as in other loricatans, however this feature is likely to be from postmortem distortion rather than anatomical. An unusual feature found in Luperosuchus is the tall, mediolaterally compressed crest that sits on the front of the snout. This structure is formed by the dorsally arching nasals, and has been described as a 'roman-nose' following Romer's initial description. A similar structure is found in other loricatans, particularly Prestosuchus and Batrachotomus, although it is most prevalent in Luperosuchus.

The orbit is tall and has smooth boundaries, without the "keyhole" shape of many other loricatans. The upper part of the infratemporal fenestra was also tall and narrow, although it may have been much wider near the lower rear corner of the skull, which was not preserved. The prefrontal bone had a pronounced finger-like process which stretched forwards above the lacrimal bone, although it did not stretch as far as the front edge of the frontals which were adjacent to it. A palpebral bone was situated above the orbit, though it was small and fused to the frontal and postfrontal. The postorbital bone had two distinguishing features: a large rounded knob on its upper portion and an elongated lower branch.

Discovery and naming
The holotype material of Luperosuchus, specimen PULR 04, was collected by Ruth Romer, wife of palaeontologist Alfred S. Romer, on January 17, 1965. This specimen is stored at the La Plata Museum in La Plata, Argentina. It was found near the remains of a large dicynodont at a locality about 5 km northeast of where the Chañares River emerges onto the Plano de Talampaya. The site is believed to represent the top of the lower member of the Chañares Formation, based on similarities in preservation and the composition of the surrounding matrix to other fossils known from this time interval. This would be positioned below the strata recently dated by Marsicano et al., providing a minimum age of 236.1 +/- 0.6 Ma for the Luperosuchus locality. Ezcurra et al. (2017) assigned the Luperosuchus specimen to the base of the formation, within the relatively fossil-poor Tarjadia Assemblage Zone below the much more prolific Massetognathus-Chanaresuchus Assemblage Zone.

Romer published his description of Luperosuchus in 1971, as part of a series of articles describing the numerous "thecodonts" he had collected from the Chañares Formation. The binomial name, Luperosuchus fractus, refers to the fragmentary and "perplexing" nature of the fossil, and is derived from the Greek lyperos ("vexing", "difficult", "troublesome") and souchos ("crocodile"), with the specific name is from the Latin fractus, meaning "broken".

Romer had collected additional fragments of bone from the holotype locality, but was unsure if they belonged to Luperosuchus or the dicynodont collected with it. After rediscovering and examining these fragments, Nesbitt & Desojo determined that many of the identifiable pieces were consistent with loricatan archosaurs, and so referred them to the holotype of Luperosuchus. These additional pieces include portions of the maxilla, quadrate, fragments of the braincase and the atlas intercentrum, the only postcranial material for Luperosuchus.

A second specimen, PULR 057, was reported in 2009 by Desojo & Arcucci and was referred to this genus. This specimen however was a third smaller than the type specimen, and differed from it in a number of aspects, including a less developed 'roman-nose', but these were interpreted as ontogenetic variation as the material appeared to exhibit juvenile features (including loose sutures and less developed ornamentation of the bones). The referral of this material to Luperosuchus was questioned by Nesbitt & Desojo in 2017, who suggested that the differences are taxonomic, rather than ontogenetic. The holotype PULR 04 remains the only known material of Luperosuchus, with the taxonomic identity of the former referred specimen PULR 057 currently unknown.

Classification
Romer identified Luperosuchus as a member of the family Rauisuchidae in 1971, however at the time of his description the relationships of Rauisuchidae to other archosaurs were poorly understood and a detailed examination of its relationships was not performed. Luperosuchus was largely ignored in later phylogenetic analyses of Triassic pseudosuchians, likely due to the poor preservation of its remains and lack of identifying characteristics, as well as an unclear understanding of the interrelationships of close relatives. As such, it was typically only ever provisionally assigned to "Rauisuchia" based on general similarities, without further justification.

Luperosuchus was incorporated into a phylogenetic analysis for the first time by Nesbitt & Desojo in 2017, where it was recovered in a clade with two other South American loricatans, Saurosuchus and Prestosuchus, at the base of Loricata. They found two equally plausible hypotheses for the least inclusive position of Luperosuchus; one as a sister taxon to both Saurosuchus and Prestosuchus, and another as a sister taxon to Saurosuchus, the former of which is reproduced here:

A study on new Chañares vertebrates (including new specimens of Tarjadia) published in 2017 incorporated Luperosuchus and several other suchians into an archosauromorph dataset used by Ezcurra (2016). This study found Luperosuchus as the sister taxon to Decuriasuchus based on a prominent vertical peg on the supraoccipital bone of the braincase. The Luperosuchus + Decuriasuchus clade was found outside of a more restrictive Paracrocodylomorpha, crownward of Ticinosuchus and stemward of Prestosuchus.

Nesbitt & Desojo (2017) was one of several phylogenetic analyses combined during the course of Da-Silva et al. (2019)'s study of a new Prestosuchus specimen. This study did not fully support the Luperosuchus + Saurosuchus + Prestosuchus clade, although it did note that the clade was only barely less optimal than their most parsimonious tree (MPT). Their MPT found these three taxa in a series leading up to more crownward loricatans, with Prestosuchus occupying the bottom (stemward) rung, Saurosuchus in the middle, and Luperosuchus at the top (crownward) rung just below Batrachotomus.

Histology and growth
The skull of Luperosuchus has very few visible sutures, partly due to the material's poor preservation but also from the addition of consolidates to the surface of the specimen that obscure finer anatomical details. Romer struggled to identify the suture pattern, but identified fused sutures which he interpreted as a possible indicator that the specimen was mature. Later examination by Nesbitt & Desojo confirmed that many of the sutures in the skull of Luperosuchus were obliterated, with the bones in the skull fully fused, in addition to well developed sculpting of the bones.

In 2008, Ricqlès et al. reported that they performed a histological study on a limb bone belonging to Luperosuchus, identified as specimen MCZ 4077. However, as Luperosuchus is only definitively known from the holotype material, the referral of this bone to Luperosuchus is dubious. MCZ 4077 was later referred to Tarjadia ruthae.

Palaeoecology
Luperosuchus is one of the largest animals in the Chañares Formation, and indeed was one of the few large carnivores known from the formation. In the Tarjadia Assemblage Zone of the early part of the formation, Luperosuchus shared its environment with large herbivorous dicynodonts, small burrowing cynodonts, herbivorous rhynchosaurs, and the large predatory erpetosuchid Tarjadia. Other indeterminate taxa include the small loricatan represented by PULR 057, as well as specimens of very large paracrocodylomorphs, possibly representing the largest predatory archosaurs in the ecosystem. The early part of the Chañares Formation corresponds to a braided river system with periodic flooding events. The environment later shifts to encompass deposits of pyroclastic and volcaniclastic materials, such as tuff and ash, indicating a complex history of regional volcanism in the environment. Volcanic mass mortality events may have contributed to the abundance of well-preserved skeletal material within the Massetognathus-Chanaresuchus Assemblage Zone near the stratigraphic middle of the Chañares Formation.