A thick-skulled troodontid theropod from the Late Cretaceous of Mexico

Dinosaurs repeatedly evolved adaptations for sexual selection over their 150-million year history, including adaptations for display and intraspecific combat. Adaptations for intraspecific combat have not previously been described in non-avian maniraptorans. We report a troodontid from the Campanian Cerro del Pueblo Formation of Coahuila, Mexico, showing a thickened and domed skull roof. The cranium is domed and bones are extremely thick, a morphology convergent on that of Pachycephalosauridae. Referred specimens show less thickening or doming, suggesting ontogenetic changes or perhaps sexual dimorphism. The holotype shows fusion of the frontal midline suture and tightly interdigitating sutures between skull bones, and a rugose skull roof. The specializations seen here suggest adaptation for intraspecific combat, specifically head-butting as hypothesized for pachycephalosaurids and pachyrhinosaurin ceratopsids. Repeated evolution of elaborate weapons and display features in the Cretaceous suggests that sexual selection became increasingly important in dinosaur evolution during the Cretaceous.