New giant carnivorous dinosaur reveals convergent evolutionary trends in theropod arm reduction

Giant carnivorous dinosaurs such as Tyrannosaurus rex and abelisaurids are characterized by highly reduced forelimbs that stand in contrast to their huge dimensions, massive skulls, and obligate bipedalism.1,2 Another group that follows this pattern, yet is still poorly known, is the Carcharodontosauridae: dominant predators that inhabited most continents during the Early Cretaceous3–5 and reached their largest sizes in Aptian-Cenomanian times.6–10 Despite many discoveries over the last three decades, aspects of their anatomy, especially with regard to the skull, forearm, and feet, remain poorly known. Here we report a new carcharodontosaurid, Meraxes gigas, gen. et sp. nov., based on a specimen recovered from the Upper Cretaceous Huincul Formation of northern Patagonia, Argentina. Phylogenetic analysis places Meraxes among derived Carcharodontosauridae, in a clade with other massive South American species. Meraxes preserves novel anatomical information for derived carcharodontosaurids, including an almost complete forelimb that provides evidence for convergent allometric trends in forelimb reduction among three lineages of large-bodied, megapredatory non-avian theropods, including a remarkable degree of parallelism between the latest-diverging tyrannosaurids and carcharodontosaurids. This trend, coupled with a likely lower bound on forelimb reduction, hypothesized to be about 0.4 forelimb/femur length, combined to produce this short-armed pattern in theropods. The almost complete cranium of Meraxes permits new estimates of skull length in Giganotosaurus, which is among the longest for theropods. Meraxes also provides further evidence that carchardontosaurids reached peak diversity shortly before their extinction with high rates of trait evolution in facial ornamentation possibly linked to a social signaling role. •Meraxes, a new, giant predatory dinosaur from Patagonia had short arms like T. rex•Meraxes is the most complete carcharodontosaurid yet from the Southern Hemisphere•It documents peak diversity of carcharodontosaurids just before they went extinct•Meraxes documents convergent evolution of short arms among megapredatory theropods. Canale et al. describe a new species of carcharodontosaurid, Meraxes gigas, from Patagonia. Meraxes adds evidence for a peak in carcharodontosaurid diversity just before their extinction in the Late Cretaceous. With short forelimbs like T. rex, Meraxes documents convergent evolution of this trait among large, predatory t