THE METATARSAL ARTICULATION IN CARCHARODONTOSAURIDS: A COMPARATIVE ANALYSIS BETWEEN ACROCANTHOSAURUS ATOKENSIS AND MERAXES GIGAS

PALOMBI, DAMIANO; CANALE JUAN IGNACIO; DIAZ MARTINEZ, IGNACIO; APESTEGUÍA, SEBASTIÁN

General Roca

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2023

The metatarsal articulation of Carcharodontosauridae (Theropoda, Tetanurae) is a pivotal but understudied aspect in understanding dinosaur locomotion and their evolutionary relationships. The recent discoveries of well-preserved specimens like Concavenator corcovatus in Spain and Meraxes gigas in Argentina have illuminated this topic, providing the opportunity to examine a fully articulated metatarsus. Specimen OMNH 10147, the paratype of Acrocanthosaurus atokensis, provided the first description of carcharodontosaurid metatarsals, but the more complete specimen NCMS 14345 is commonly employed as a comparison reference. Notably, NCMS 14345 presents an intriguing difference from the typical tetanuran condition, since it was illustrated as having a proximal portion of the third metatarsal with a posterior surface that is more expanded compared to the anterior surface. This has created difficulties in placing enigmatic taxa, such as Gualicho, which presents a metatarsal with Ceratosauria-like features but is assigned to Tetanurae, sharing this characteristic in the group only with Acrocanthosaurus. Being preserved in a slab, it is impossible to verify these aspects in the posterior and proximal views of Concavenator. Meraxes, on the contrary, shows fully preserved three-dimensional metatarsals of both pedes, allowing to support again which is the typical condition of the third metatarsal in basal Tetanurae: an "hourglass" shape in proximal aspect, with a wider anterior edge, a pinched middle section, and a posterior edge that is narrower than the anterior one. Meraxes, this way, enables a comprehensive comparison with NCMS 14345 and prompts a reassessment of the articulation of Acrocanthosaurus metatarsals, supported by comparison with the paratype OMNH 10147, overlooked in any description since the 1950s. Our analysis reveals that: 1) the metatarsal designated as right third metatarsal in Acrocanthosaurus is, in fact, a right fourth metatarsal; 2) the fifth metatarsal has been illustrated as connected with the third instead of the fourth; 3) furthermore, the fifth metatarsal, described as right, is actually a left; 4) the same applies to the first metatarsal. This analysis further enhances our understanding of the genuine articulation of carcharodontosaurid metatarsi and aids in identifying unique Tetanurae characteristics, notably the expanded anterior surface of the third metatarsal. Prominently, this study underscores the dissimilarity of the Gualicho metatarsus compared to both Acrocanthosaurus and other Tetanurae, resembling the condition observed in Ceratosauria.