Divergent strategy to supremacy: cranial evolution and evolutionary constraints in Cretaceous apex- and meso-predators

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 non-avian 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- and medium-size non-avian 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 body size in the Late Cretaceous. These divergent strategies suggest an initial phase of disruptive selection that promoted morphological divergence, followed by stabilizing selection and phenotypic canalization in the Cretaceous period, ultimately shaping specialized adaptive responses in each clade. Overall, these results highlight how evolutionary history, ecological interactions, and developmental constraints structured the diversification of apex- and meso-predator non-avian theropods, providing insight into the interplay between innovation, constraint, and ecological adaptation in the evolution of large and medium terrestrial carnivores.白垩纪的顶级捕食者与中型捕食者（apex and meso-predators）展现出截然不同的宏观演化轨迹，反映出它们对生态与环境压力的差异化响应。不过，演化过程在塑造非鸟类兽脚类恐龙（non-avian theropods）顶级与中型捕食者宏观演化趋势中的作用仍不甚明晰。本研究借助上颌骨形态与尺寸数据，结合系统发育比较方法（phylogenetic comparative methods），阐明了大中型非鸟类兽脚类恐龙三个演化支——鲨齿龙类（Carcharodontosauria）、阿贝力龙科（Abelisauridae）以及虚骨龙类（Coelurosauria）——间的差异化宏观演化趋势。阿贝力龙科主要在头骨形态上探索形态创新，同时在演化历程中维持相对稳定的体型。与之相反，鲨齿龙类遵循体型驱动的演化路径，在保留保守头骨形态的同时实现了巨型化。虚骨龙类尤其是暴龙超科（tyrannosauroids）则采取混合策略：侏罗纪时期率先完成头骨改造，随后在晚白垩世出现体型的爆发式增长。这些差异化策略表明，演化初期存在一轮歧向选择，推动了形态分化；白垩纪时期则转为稳定选择与表型渠化（phenotypic canalization），最终塑造了各演化支的专属适应性响应。总体而言，本研究结果揭示了演化历史、生态相互作用与发育约束如何共同塑造了非鸟类兽脚类顶级与中型捕食者的多样化进程，为理解大中型陆生食肉动物演化过程中创新、约束与生态适应之间的相互作用提供了新视角。