Skorpiovenator

Skorpiovenator is a genus of abelisaurid theropod dinosaur from the Late Cretaceous Huincul Formation of Argentina. The sole species of Skorpiovenator, S. bustingorryi, was named in honour of Manuel Bustingorry, the late owner of the farm on which the type specimen was discovered. Formally described in 2009, the type specimen is one of the most complete and informative abelisaurids yet known, described from a nearly complete and articulated skeleton. A tibia fragment was assigned to Skorpiovenator in 2022.
Skorpiovenator was a fairly large abelisaurid. What is preserved of the type specimen measures in length. Based on the anatomy of close relatives, it may have been between long in life, and may have weighed a little under. Its skull was short and blunt, and somewhat resembled that of Carnotaurus. Its bone texture was rugose, even by the standards of other abelisaurids, which indicates the presence of large facial scales. The skull is also covered in small pits called foramina, which suggests a strong degree of facial sensitivity.

Discovery and naming

The type specimen of Skorpiovenator bustingorryi was discovered on a farm owned by Manuel Bustingorry, northwest of Villa El Chocón, in the Neuquén Province. It consists of a nearly complete skeleton, missing only part of the nasals and premaxillae, parts of the pelvic girdle, sections of the tail, and most of the forelimb elements. The strata from which Skorpiovenator was recovered belong to the lower part of the Huincul Formation in Patagonia, dating to the late Cenomanian stage, about 95 million years ago. After discovery, they were relocated to the Ernesto Bachmann Paleontological Museum of Villa El Chocón, Patagonia, Argentina. In a paper physically published in 2009, Juan Canale, Carlos A. Scanferla, Federico L. Agnolin and Fenando E. Novas formally described the specimen, assigning to it the binomial name of Skorpiovenator bustingorryi. The generic name derives from the Latin words skorpios and venator, referring to its nature as a predator and the abundance of scorpions at the dig site. The specific name honours Manuel Bustingorry, who had died by the time the paper was released. In naming Skorpiovenator, Canale et al. erected a new abelisaurid clade, Brachyrostra, to which it was assigned. In 2022, the proximal end of a right tibia, MMCh-PV255, was assigned to Skorpiovenator.

Description

Body size

The preserved length of the excavated Skorpiovenator skeleton, measured from the premaxilla to the 12th caudal vertebra is. Since it lacks most of the tail, precise length measurements are currently impossible, but it has been estimated to have grown up to long and weighed up to.

Skull and dentition

The skull of Skorpiovenator, measured from the premaxilla to the quadrate, measured in length. It was fairly short and blunt, similar to that of Carnotaurus, though was shorter and deeper than those of Abelisaurus and Majungasaurus. The antorbital fossa is less developed than in other abelisauroids. The postorbital was large, and intersected the orbit, to the point where it almost contacted the lacrimal on the opposite side. Consequently, the orbit had a distinctive "keyhole" shape, similar to that of Carnotaurus and the giant coelurosaur Tyrannosaurus rex. The dorsal border of the postorbital was inflated and well-ornamented, similar to that of Ekrixinatosaurus. The bone texture of Skorpiovenator's skull was very rugose, even when compared with other abelisaurids. The rugosities of the snout are hummocky, which are osteological correlates for scales, and the overall pattern of cranial ornamentation is almost identical to that of Rugops. Much of the bone surface, especially on the top of the skull, is covered in small foramina, suggesting extensive innervation from the trigeminal nerve. This suggests the presence of sensitive facial tissues, akin to those seen in many other theropods, including the related Majungasaurus.
Skorpiovenator's maxilla bore 19 teeth, more than in any other abelisaurid. The shape of the tooth crowns is similar to other abelisaurids. They exhibit enamel wrinkles and serrations, and in those regards, resemble the teeth of carcharodontosaurids.

Postcranial skeleton

With the exception of essentially all of the proximal section of the tail, parts of the pelvic girdle, and most of the forelimb elements, Skorpiovenator's postcranial skeleton is well preserved, though much of the postcranial axial skeleton remains undescribed. The caudal vertebrae that are preserved have outwardly-projected transverse processes, similar to Aucasaurus and Carnotaurus. The femur is stout, and is almost straight from the front, though is somewhat concave from the side. As in all ceratosaurs, its head is triangular proximally, and is oriented anteromedially. The anterior surface of the femoral shaft has a well-developed intermuscular line that would have separated the origins of the femorotibialis internus and femorotibialis externus muscles,. The cnemial crest was very large, and the ilia had deep preacetabular and postacetabular blades, suggesting that, as in other ablelisaurids, the muscles that extended and flexed the legs were very powerful. The muscles responsible for foot pronation, such as the pronator profundus, appear to have been reduced, though remained present.

Classification

In 2009, Canale et al. published a phylogenetic analysis focusing on the South American carnotaurines. In their results, they found that all South American forms grouped together as a sub-clade of Carnotaurinae, which they named Brachyrostra, meaning "short snouts". They defined the clade Brachyrostra as "all the abelisaurids more closely related to Carnotaurus sastrei than to Majungasaurus crenatissimus". Within their topology, Canale et al. recovered Skorpiovenator as part of a sister clade to Carnotaurini, including Ekrixinatosaurus and Ilokolesia, and was the sister taxon of the former. In 2020, Hussam Zaher et al published a paper describing a basal abelisaurid, Spectrovenator ragei. In their topology, this clade was still recovered, though with Ekrixinatosaurus and Ilokolesia as each other's closest relatives. In 2021, Gianechini et al. described another abelisaurid, Llukalkan. Their phylogenetic analysis recovered the same clade, though recovered it as more basal. Whereas Carnotaurini fell within another clade, Furileusauria, in their topology, the clade consisting of Ekrixinatosaurus, Ilokelesia and Skorpiovenator was just outside it.
Below are the topologies recovered by Zaher et al. and Gianechini et al :
Topology 1: Zaher et al..
Topology 2: Gianechini et al..

Palaeoecology

The Huincul Formation, from which Skorpiovenator is known, is thought to represent an arid environment with ephemeral or seasonal streams. The age of this formation is estimated at 97 to 93.5 MYA. The dinosaur record is considered sparse here. Skorpiovenator shared its environment with the sauropods Argentinosaurus, Choconsaurus, Chucarosaurus and Cathartesaura. Three giant carcharodontosaurids, Mapusaurus, Meraxes and Taurovenator, were found in the same formation, though likely were not all coevals. Another abelisaurid, Ilokelesia, also lived in the region.
Fossilized pollen indicates a wide variety of plants was present in the Huincul Formation. A study of the El Zampal section of the formation found hornworts, liverworts, ferns, Selaginellales, possible Noeggerathiales, gymnosperms, and angiosperms, in addition to several pollen grains of unknown affinities. The Huincul Formation is among the richest Patagonian vertebrate associations, preserving fish including dipnoans and gar, chelid turtles, squamates, sphenodonts, neosuchian crocodilians, and a wide variety of dinosaurs. Vertebrates are most commonly found in the lower, and therefore older, part of the formation.