On 13 February 2007, Akiko Shinya, preparator of the Field Museum of Natural History, east of the Ezequiel Ramos Mexía Reservoir at the Rancho Violante, discovered the skeleton of a theropod new to science. In 2016, the specimen was named and described by Sebastián Apesteguía, Nathan D. Smith, Rubén Juárez Valieri and Peter J. Makovicky. The generic name is derived from the gualichu, a demon of local folklore. The specific name honours Shinya as the animal's discoverer. The holotype, MPCN PV 0001, consists of a partial skeleton lacking the skull. It contains four vertebrae of the back, three vertebrae of the middle tail, ribs, a basket of belly-ribs, the left shoulder girdle, the left forelimb, the rightlower arm, the lower ends of both pubic bones, the right thighbone, the lower end of the left thighbone, the upper ends of the right shinbone and calf bone, elements of both metatarsi and three toes of the right foot. Most bones were uncovered in their original anatomical position but much of the skeleton had been destroyed by erosion. Gualicho has been suggested to be synonymous with the megaraptoran Aoniraptor, also known from Huincul Formation and uncovered at the Violante site in view of similarities in their caudal vertebrae. However, Aoniraptor does not meet the requirements of ICZN Article 8.5.3, meaning it is an invalid nomen nudum.
Description
Like the well-known Tyrannosaurus, to which it has been compared, the Gualicho possesses reduced arms and possibly two fingered hands, although a 2020 study suggests enough of the third metacarpal is present for a third finger. The finding of Gualicho indicates that carnosaurs may have been subject to the same evolution of limb-reduction as tyrannosaurids and abelisaurids, provided that Gualicho is a carnosaur in the first place.
Classification
Phylogenetically, Gualicho presents two possibilities; that megaraptorans and neovenatorids were carnosaurs, or that megaraptorans and neovenatorids were a grade of theropods more closely related to coelurosaurs than to carnosaurs. The cladogram below follows a 2016 analysis by Sebastián Apesteguía, Nathan D. Smith, Rubén Juarez Valieri, and Peter J. Makovicky. The cladogram below follows the strict consensus of the twelve most parsimonious trees found by Porfiri et al. 's phylogenetic analysis. Although the results are different, the methodology analysis was practically identical to that of Apesteguia et al., only differing in the fact that it incorporated Tratayenia and Murusraptor, two megaraptorans not sampled in the analysis of Apesteguia et al.
