A NEW CARCHARODONTOSAURID FROM THE NEUQUÉN GROUP, PATAGONIA, ARGENTINA, WITH A NEAR COMPLETE SKULL AND FORELIMBS INFORMS EVOLUTIONARY TRENDS IN THEROPOD ARM REDUCTION

MAKOVICKY, PETER J.; CANALE JUAN IGNACIO; APESTEGUÍA, SEBASTIÁN; GALLINA, PABLO ARIEL; MITCHELL, JONATHAN; SMITH, NATHAN D.; CULLEN, THOMAS M.; SHINYA, AKIKO; HALUZA, ALEJANDRO; GIANECHINI, FEDERICO A.

Toronto

Congreso; 82nd Anual Meeting of the SVP; 2022

The Carcharodontosauridae were dominant dinosaurian predators that inhabited most continents during the Early to early Late Cretaceous, reaching sizes rivaling those of later living tyrannosaurids during the Aptian–Turonian. Despite a number of important discoveries over the last three decades, critical aspects of their anatomy, especially with regard to their skulls, forelimbs, and feet, remain poorly known. A new carcharodontosaurid, Meraxes gigas, based on a specimen recovered from the Upper Cretaceous Huincul Formation (Cenomanian–Turonian) of northern Patagonia, Argentina, includes the most complete cranial, pelvic, and appendicular remains for late-diverging members of Carcharodontosauridae. Phylogenetic analysis places Meraxes among derived Carcharodontosauridae, in a clade with other massive South American species that we name Carcharodontosaurinae. Meraxes is diagnosed by the unique pattern of rugosities on its facial bones, co-ossified sacral neural spines forming an inverted yoke-like profile, and unique pneumatization of the proximal caudal neural arches and postzygapophyses. It represents the fourth carcharodontosaurine to be named from early Late Cretaceous strata of Patagonia and provides further evidence that carchardontosaurids reached peak diversity shortly before their extinction. These penecontemporaneous taxa differ in pelvic anatomy and in aspects of their facial ornamentation, which may have been linked to a social signaling role. Meraxes preserves nearly complete forelimbs which provide evidence for a statistically significant degree of convergence in forelimb reduction between carcharodontosaurines, tyrannosaurids, and abelisaurids. Similar allometric trends in these three lineages coupled with a hypothesized lower bound on forelimb reduction around a ratio of 0.4 forelimb/femur length, combined to produce the familiar short-armed bauplan of large bodied, megapredatory theropods with large skulls. We tested the correlation between skull size and relative forelimb length using multiple regression analyses while controlling for phylogeny. These analyses indicate that as skull size increases, the correlation between femur length and arm length decreases. This result supports the hypothesis that forelimb reduction is likely not the result of adaptation for a particular function, nor a direct consequence of body size increase, but rather that it tracks some other trait, which for large predatory species is skull size.