Let them fight: using finite element analysis to determine combative evolutionary drivers of ceratopsian (Dinosauria: Ornithischia) horns

The intensely-researched fossil record of ceratopsian dinosaurs makes them an ideal clade to test hypotheses of morphological evolutionary drivers. These dinosaurs are best known for their extravagant headgear, including an assortment of cranial horns and often elaborate frills. While the frills have been suggested as display structures, their horns have been hypothesized as intraspecific combat weapons with various fighting styles from modern analogs providing ample baselines for testing. However, while direct fossil evidence of intraspecific combat does exist, an absence of biomechanical data makes determining whether different ceratopsid taxa were best suited to specific combat styles very difficult. Here we used 3D finite element analysis to model differences in biomechanical performance in simulated combat behaviors of five ceratopsian taxa ( Psittacosaurus , Protoceratops , Centrosaurus , Spiclypeus , and Triceratops ). Size-corrected results indicate that Psittacosaurus and Protoceratops skulls, which lack elaborate horns, exhibit high and low stress cranial stress magnitudes respectively. The nasal horn of Centrosaurus was well-suited for accommodating vertical and longitudinal stresses but much less so for mediolateral ones. The supraorbital horns of Spiclypeus and Triceratops demonstrate consistently low-to-moderate stresses across most tests, correlating with various fighting behaviors. These results suggest that differing intraspecific combat behaviors may be an evolutionary driver of ceratopsian cranial weaponry, with strong skulls being a result of phylogenetic inertia from earlier members of the clade. This study helps ascertain the effectiveness of cranial weaponry in ceratopsids for intraspecific combat and fills a notable knowledge gap in our understanding of these dinosaurs.