Evolutionary decoupling between cranial shape and size underlies divergent evolutionary pathways in Cretaceous apex and meso-predator theropods

Abstract

Apex and meso-predators of the Cretaceous exhibited distinct macroevolutionary trajectories,reflecting differential responses to ecological and environmental pressures. However, the role ofevolutionary process in shaping macroevolutionary trends of apex and meso-predatoryCretaceous theropods remains poorly understood. Here, I use the maxillary shape and size,combined with phylogenetic comparative methods, to elucidate distinct macroevolutionarytrends among three lineages of larger (apex-pedators) and medium-size (meso-predators)Cretaceous theropods (Carcharodontosauria, Abelisauridae, and Coelurosauria). Abelisauridsprimarily explored morphological innovation in skull shape while maintaining relatively stablebody sizes through time. Carcharodontosaurids, in contrast, followed a size-driven evolutionarypath, achieving gigantism while retaining conservative cranial morphologies. Coelurosaurs,particularly tyrannosauroids, adopted a mixed strategy, displaying early cranial modificationsduring the Jurassic, followed by an explosive increase in body size in the Late Cretaceous. Thesedivergent strategies reveal an evolutionary decoupling between cranial shape and size,characterized by an initial phase of disruptive selection promoting morphological divergence, followed by stabilizing selection and phenotypic canalization that ultimately shaped specializedadaptive responses in each clade. These results highlight how evolutionary history, ecologicalinteractions, and developmental constraints structured the diversification of apex and mesopredator theropods.