Evolutionary decoupling between cranial shape and size underlies divergent evolutionary pathways in Cretaceous apex...

Apex and meso-predators of the Cretaceous exhibited distinct macroevolutionary trajectories, reflecting differential responses to ecological and environmental pressures. However, the role of evolutionary process in shaping macroevolutionary trends of apex and meso-predatory Cretaceous theropods remains poorly understood. Here, I use the maxillary shape and size, combined with phylogenetic comparative methods, to elucidate distinct macroevolutionary trends among three lineages of larger (apex-pedators) and medium-size (meso-predators) Cretaceous theropods (Carcharodontosauria, Abelisauridae, and Coelurosauria). Abelisaurids primarily explored morphological innovation in skull shape while maintaining relatively stable body sizes through time. Carcharodontosaurids, in contrast, followed a size-driven evolutionary path, achieving gigantism while retaining conservative cranial morphologies. Coelurosaurs, particularly tyrannosauroids, adopted a mixed strategy, displaying early cranial modifications during the Jurassic, followed by an explosive increase in cranial size in the Late Cretaceous. These divergent 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 specialized adaptive responses in each clade. These results highlight how evolutionary history, ecological interactions, and developmental constraints structured the diversification of apex and meso-predator theropods.