Beyond species recognition hypothesis: evolutionary trends in Abelisauridae cranial ornamentation

Abstract Abelisaurid dinosaurs display a remarkable diversity in cranial ornamentation, which includes several features such as rugosities, pits and grooves on the rostrum and orbital regions, midline knobs, and paired plus sculptured structures across the skull roof. To investigate macroevolutionary patterns that underlie this diversity, we tested the “species recognition” hypothesis using phylogenetic comparative methods. Our results indicate directional and ordered evolutionary patterns in cranial ornamentation that are more consistent with expectations of sexual selection than with the stochastic patterns predicted under the species recognition hypothesis. Moreover, sympatric species exhibited highly divergent ornamentation, and the degree of surface texturing and the development of certain structures (e.g., knobs) increased through ontogeny. These findings suggest that cranial ornamentation in Abelisaurids was influenced by mechanisms of sexual selection, sexual dimorphism, or mutual selection. Evolutionary modeling combined with evidence from the fossil record and phylogenetic correlation analysis reveals that the ornamentation of the rostrum and orbital regions was evolutionarily coupled and became canalized during the Early Cretaceous. In contrast, phylogenetic correlation analyses demonstrated that skull roof ornaments and rostral and orbital ornaments evolved decoupled. Neither discrete ornament traits nor the accumulation of traits over time showed significant associations with body size, indicating that ornament diversification in Abelisauridae was not constrained by allometric trends. During the Late Cretaceous, after refined ecological specialization, the subclades Carnotaurini and Majungasaurinae underwent further elaboration, acquiring features such as a prominent midline knobs, paired structures, and reinforced occipital and cervical regions. These derived morphologies may have facilitated more complex sociosexual behaviors.