The last weaponized arm: macroevolutionary trends of megaraptorid manual claw I and forelimb integration in medium-...

From the Jurassic to the Cretaceous, most medium- to large-bodied non-avian theropods evolved towards predominantly head-driven predatory strategies. Megaraptorid dinosaurs represent a striking exception, retaining well-developed forelimbs and hypertrophied manual unguals hypothesized as a key adaptation within the clade. However, the macroevolutionary processes underlying the mode and tempo of megaraptorid claw evolution remain poorly understood. Here, we investigate the macroevolutionary trends of the megaraptorid manual claw by quantifying the shape and size of 22 manual phalanx I-2 (= manual ungual I) across different lineages of medium- to large-bodied non-avian theropods, using 2D geometric morphometric and phylogenetic comparative methods. Our results indicate that manual claw shape and size were evolutionarily coupled in Megaraptoridae, supporting a major reconfiguration of allometric trajectories during the evolution of the clade. This evolutionary coupling triggered directional shape modifications towards a distinct adaptive optimum associated with enlarged manual claws. This adaptation emerged following an early phase of rapid morphological divergence near the middle Early Cretaceous boundary and was subsequently maintained, likely under stabilizing selection throughout the Late Cretaceous, resulting in a canalized claw morphology. We further show that medium- to large-bodied non-avian theropods with well-developed forelimbs exhibit strong phylogenetic integration between distal and proximal forelimb elements. Despite this integration, the phalanx-I-2 and ulna-radius complex evolved at substantially higher evolutionary rates than the humerus. These results highlight Megaraptoridae as the only large-bodied carnivorous theropod lineage retaining a specialized forelimb-based predatory strategy until the end of the Cretaceous, with no evidence of overall claw shape convergence.