Drivers and mechanisms of convergent forelimb reduction in non-avian theropod dinosaurs

Abstract Forelimb reduction has been observed in numerous and disparate non-avian theropod dinosaurs, resulting in the hypothesis that reduced forelimbs evolved convergently. Clades with reduced forelimbs also possess high degrees of cranial robusticity and gigantic body sizes. Here, we provide a novel quantification of forelimb reduction across Theropoda, and create and implement a cranial robusticity scoring system, and analyse this dataset using bivariate and comparative phylogenetic analyses. Results indicate that forelimb reduction is strongly correlated with cranial robusticity and gigantism. Reduced/vestigial forelimbs evolved in at least five theropod lineages in concert with increased cranial robusticity and gigantism. Abelisaurids, carcharodontosaurids and tyrannosaurids show the greatest forelimb reduction relative to the skull. Repeated forelimb reduction across theropods was facilitated by increased cranial robusticity and greater body size that was potentially influenced by an upward trend in prey body size. These events resulted in a shift from subduing prey using grasping forelimbs to using powerful bites and robust skulls.