Luperosuchus

Luperosuchus (meaning 'vexing' or 'difficult crocodile') is an extinct genus of large 'rauisuchian' pseudosuchian. It lived between the Ladinian stage of the late Middle Triassic to the earliest Carnian of the Late Triassic, and is known from the Chañares Formation of Argentina. It contains only a single type species, Luperosuchus fractus. 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 4.1-4.4 meters (13.5-14.4 ft) based on rauisuchids and 3.7 meters (12.3 ft) based on Prestosuchus. The skull belongs to a large animal, with the preserved portions measuring 54.5 centimetres (21.5 in) and an estimated complete length of approximately 60 centimetres (2.0 ft), 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 Saurosuchus, although it is most prevalent in Luperosuchus. The orbit is tall and has smooth boundaries, without the 'keyhole' shape of some loricatans like Batrachotomus and Postosuchus. 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. The holotype material of Luperosuchus, specimen PULR 04, was collected by Ruth Romer, wife of palaeontologist Alfred S. Romer, on January 17, 1965. 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. The material is stored at the La Plata Museum in La Plata, Argentina. 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, and 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.