The evolutionary mechanisms of cranial ornamentation in abelisaurids

Sexual selection has long been regarded as one of the major drivers of evolution alongside natural selection. It primarily influences differences in morphology and behavior among species and has contributed to the remarkable diversity of biological forms.

Ornamental structures found in males or females are generally considered products of sexual selection. However, sexual selection does not act independently; it often interacts with natural selection, together shaping the form and function of these secondary sexual characteristics. Modern birds provide many examples, having evolved diverse plumage colors, elaborate ornamental structures, and complex courtship behaviors, highlighting the important role of sexual selection in the evolution of biological form.

The same applies to extinct organisms. Dinosaurs exhibited an extraordinary diversity of ornamental structures. Many theropods possessed various bony cranial ornaments, while ornithischian dinosaurs frequently developed horns, projections, or rough ornamentation on their skulls. Ceratopsians, in particular, are famous for their elaborate cranial ornamentation, having evolved a wide variety of horns and frills among different species, making them one of the most ornamentally diverse dinosaur groups. These examples indicate that cranial ornaments evolved independently multiple times across different dinosaur lineages, producing a wide range of forms and functions.

Skulls of different abelisaurids. A, B, Eoabelisaurus. C, Llukalkan. D, Viavenator. E, F, Majungasaurus. G, H, Carnotaurus.（Image source：Seculi Pereyra, E. E et al. (2026). ， CC BY 4.0 。）

Among ornamented dinosaurs, Abelisauridae is one of the most representative groups. These carnivorous dinosaurs inhabited Gondwana from the Jurassic to the end of the Cretaceous and were widely distributed across South America, Africa, India, and Madagascar. Their skulls commonly bear rough textures, pits, and grooves, and some species evolved prominent horn-like projections, such as the large horns of Carnotaurus.

Carnotaurus（Image source：Fred Wierum， CC BY-SA 4.0 。）

Because abelisaurids possessed robust skulls, powerful neck muscles, and extremely reduced forelimbs, some researchers have suggested that they may have used their heads in pushing or head-butting behaviors. If so, these cranial ornaments may have been associated with sexual selection or visual display.

Despite the remarkable diversity of abelisaurid cranial ornamentation, it remains unclear how these structures evolved and whether they primarily functioned in sexual display, intraspecific competition, or species recognition. To address this question, a research team conducted a phylogenetic analysis to reconstruct the evolutionary history of abelisaurid cranial ornaments and evaluate whether their evolution is more consistent with sexual selection or species recognition.

The analysis showed that the rough ornamentation, pits, and grooves around the snout and orbit had already appeared by the Early Cretaceous. Once these features evolved, they were rarely lost in later lineages. The researchers suggest that these traits may have been constrained by developmental mechanisms or retained because they served important social functions. In addition, the evolution of ornamentation on the snout and around the orbit was highly correlated, suggesting that both regions may have been controlled by a shared developmental mechanism.

In contrast, ornamentation on the top of the skull first appeared in the early Late Cretaceous as a roughened skull surface, followed by the evolution of central bosses or horn-like projections and eventually paired large horns. This indicates that more complex cranial ornamentation developed progressively upon pre-existing skull surface ornamentation.

Head reconstruction of Majungasaurus, showing its prominent horn-like projection（Image source：Deviant Paleoart， CC BY-SA 3.0 。）

The analysis also found no significant correlation between ornamentation on the top of the skull and that of the snout or orbit, suggesting that these structures were influenced by different selective pressures and developmental mechanisms. This may indicate that cranial ornaments on the skull roof served more specialized display or behavioral functions and became increasingly diverse during the rapid radiation of abelisaurids in the Late Cretaceous.

Taken together, these results largely reject the species recognition hypothesis. If cranial ornamentation primarily functioned to distinguish individuals of the same species, different species would be expected to evolve ornaments rapidly, randomly, and in ways that maximized differences from one another.

However, the evolutionary pattern observed in abelisaurids is the opposite. Their cranial ornaments exhibit clear directional evolution. Once ornamentation on the snout and around the orbit evolved, it was retained over long periods, whereas ornamentation on the skull roof became progressively more elaborate in a stepwise manner. These regular evolutionary trends are inconsistent with random diversification.

Furthermore, if cranial ornaments mainly functioned in species recognition, then different species living in the same place and at the same time should have evolved distinctly different ornamentation. Instead, sympatric abelisaurids often evolved similar cranial structures. For example, species from India and Madagascar commonly possessed a central boss on the skull roof, whereas members of Carnotaurini from South America generally evolved paired horns. Rather than diverging to avoid confusion, these species frequently converged on similar ornamentation.

Moreover, the cranial ornaments of abelisaurids were structurally complex and likely required considerable physiological investment to develop and maintain, a pattern more consistent with traits shaped by sexual selection.

The researchers propose that the evolution of abelisaurid cranial ornamentation occurred in multiple stages. The earliest ornamentation on the snout and around the orbit may have conveyed only simple signals related to an individual's health, age, or physiological condition. Later, during the Late Cretaceous, the evolution of more elaborate horns and bosses on the skull roof may reflect increasingly complex display behaviors and social interactions.

In addition to visual display, these ornaments may also have been involved in physical contact and intraspecific combat, making the display system of abelisaurids more complex than simple visual signaling alone. However, this hypothesis will require further research to test.

The study also found no significant relationship between cranial ornamentation and body size. Although previous studies suggested that larger theropods were more likely to evolve prominent cranial ornaments, ornament complexity in abelisaurids appears to have been driven primarily by social behavior and sexual selection rather than by increasing body size alone.

The diversity of abelisaurid cranial ornamentation may have been even greater than previously recognized. Different regions and evolutionary lineages independently evolved their own distinctive ornamental structures. Future discoveries of Jurassic and Early Cretaceous abelisaurid fossils, together with more detailed studies of skull surface texture, vascular traces, and soft-tissue attachment sites, will help reconstruct the origin and evolutionary history of these ornaments.

Finally, the case of abelisaurids demonstrates that different ornamental structures on the same individual may be shaped by different evolutionary mechanisms and selective pressures. This not only improves our understanding of abelisaurid evolution but also provides an important example of how complex ornaments, sexual selection, and developmental constraints interact to shape biological diversity.

（Author: Bai Leng）

Reference：

Seculi Pereyra, E. E., Soto, I. M., Perez, D. E. (2026). Beyond species recognition hypothesis: evolutionary trends in Abelisauridae cranial ornamentation. Paleobiology