Beyond Species Recognition Hypothesis: Evolutionary Trends in Abelisauridae Cranial Ornamentation

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, here using Abelisauridae ornamented discrete characters, traditionally used to estimate their phylogenetic relationships (Carrano and Sampson 2008), we aimed to: 1) determinate the model of evolution of each ornaments in Abelisauridae; 2) estimate ancestral states for each ornamentation trait; 3) evaluate the correlation between different cranial sculptured structures (rostral, orbit and skull roof) and between types of ornaments (sculptured morphology, midline and pairs lateral ornaments); 4) to assess whether the degree of ornamentation followed an allometric pattern during the evolution of Abelisauridae body size and finally; 6) test the species recognition hypothesis in Abelisauridae. Our results indicate evidence of directional and ordered evolution in these ornaments, which contradicts the random pattern expected under the species recognition hypothesis. Moreover, sympatric species exhibited highly divergent ornamentation, and the overall morphological variation within Abelisauridae was found to be greater than previously recognized. These findings suggest that cranial ornamentation in Abelisaurids was likely influenced by mechanisms of sexual selection or signals of sexual dimorphism. Evolutionary modeling combined with evidence from the fossil record and phylogenetic correlation analysis reveals that the ornamentation of the rostrum and orbital regions were evolutionary correlated and became canalized during the Early Cretaceous. In contrast, phylogenetic correlation and regression analyses demonstrated that cranial ornamentation and body size evolved largely independently. 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 tightly constrained by allometric trends. During the Late Cretaceous, the subclades Carnotaurini and Majungasaurinae underwent further elaboration, acquiring features such as a cornified integument, prominent midline knobs, paired structures, and reinforced occipital and cervical regions. These derived morphologies may have facilitated more complex socio-sexual signaling behaviors. The discovery of new fossils from the Jurassic and Early Cretaceous Abelisaurid taxa would provide critical tests for our proposed evolutionary scenario. Future research should prioritize a detailed anatomical characterization and standardized categorization of these traits to further refine macroevolutionary inferences.