A team of paleontologists from Mexico and the United States has identified a new species of bird-like dinosaur with an unusually thick and domed skull, suggesting it may have used head-butting during combat with members of its own species.
Life reconstruction of Xenovenator espinosai. Image credit: Connor Ashbridge / CC BY 4.0.
The newly-described dinosaur species lived around 73 million years ago during the Late Cretaceous epoch.
Named Xenovenator espinosai, it belonged to the Troodontidae, a group of agile theropod dinosaurs closely related to birds.
The holotype and paratype specimens were discovered during surface collecting in the Cerro del Pueblo Formation of Coahuila, northern Mexico, in the 2000s.
Troodontids are known for their relatively large brains and keen senses, but the new species stands out for its remarkably thickened skull roof.
The holotype specimen preserves much of the braincase, including the frontals and parietals, which are strongly domed and can reach up to 1.2 cm in thickness.
CT scans show that the skull bones are densely constructed, with tightly interlocking sutures and a rugose, striated outer surface.
This architecture closely resembles the reinforced skulls of dome-headed pachycephalosaurid dinosaurs, despite the two groups being only distantly related.
According to the paleontologists, such features are consistent with adaptations for intraspecific combat, particularly head-butting behavior.
While display structures and weapons are common among many dinosaur groups, adaptations specifically linked to combat have not previously been documented in non-avian maniraptoran theropods.
The paratype specimens of Xenovenator espinosai show less pronounced skull thickening and doming.
This variation may reflect differences in age or sex, with the most extreme skull reinforcement developing later in life or possibly only in one sex.
“The thick, modified cranium of Xenovenator espinosai is unique among maniraptorans, and its function is not immediately obvious,” said lead author Dr. Hector Rivera-Sylva from the Museo del Desierto and colleagues.
“Many features that lack an obvious adaptive value in terms of improving their owners’ survival — cranial horns, crests, bosses, frills, etc. — are sexually selected.”
“In modern mammals and birds, such features can function in courtship, for display, or as weapons.”
“Given the evidence found here — the thickening of the skull, the cranial dome, cranial rugosity, and the elaborate sutures — it seems likely that the doming of the skull in Xenovenator espinosai was an adaptation for intraspecific combat,” they added.
“If so, it is the first known example of modification of the skull for intraspecific combat in a paravian.”
“Curiously, troodontine frontals are sometimes rugose, and rugosity is also seen on the maxillae and nasals.”
“This hints that intraspecific combat may have been widespread in Troodontinae, but that such combat was particularly intense than in Xenovenator espinosai.”
The team’s phylogenetic analyses place Xenovenator espinosai within a clade of large-bodied North American troodontids, but its thickened and domed skull represents a unique specialization within the group.
The repeated evolution of elaborate weapons and display features during the Cretaceous suggests sexual selection played an increasingly important role in dinosaur evolution.
The discovery expands the known diversity of troodontid dinosaurs in southern Laramidia and provides rare evidence that even small, lightly built theropods evolved specialized structures for physical combat.
“The presence of the related Xenovenator robustus in New Mexico suggests they represent a distinct clade of thick-skulled troodontids endemic to the Southwest, emphasizing the endemicity and diversity of southern Laramidian faunas,” the researchers concluded.
“Sexual selection, including adaptations for display and combat, was a widespread phenomenon in Late Cretaceous dinosaurs.”
The discovery is described in a paper in the journal Diversity.
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Hector E. Rivera-Sylva et al. 2026. A Thick-Skulled Troodontid Theropod from the Late Cretaceous of Mexico. Diversity 18 (1): 38; doi: 10.3390/d18010038
