Abstract
BackgroundPachycephalosaurs were bipedal herbivorous dinosaurs with bony domes on their heads, suggestive of head-butting as seen in bighorn sheep and musk oxen. Previous biomechanical studies indicate potential for pachycephalosaur head-butting, but bone histology appears to contradict the behavior in young and old individuals. Comparing pachycephalosaurs with fighting artiodactyls tests for common correlates of head-butting in their cranial structure and mechanics.Methods/Principal FindingsComputed tomographic (CT) scans and physical sectioning revealed internal cranial structure of ten artiodactyls and pachycephalosaurs Stegoceras validum and Prenocephale prenes. Finite element analyses (FEA), incorporating bone and keratin tissue types, determined cranial stress and strain from simulated head impacts. Recursive partition analysis quantified strengths of correlation between functional morphology and actual or hypothesized behavior. Strong head-strike correlates include a dome-like cephalic morphology, neurovascular canals exiting onto the cranium surface, large neck muscle attachments, and dense cortical bone above a sparse cancellous layer in line with the force of impact. The head-butting duiker Cephalophus leucogaster is the closest morphological analog to Stegoceras, with a smaller yet similarly rounded dome. Crania of the duiker, pachycephalosaurs, and bighorn sheep Ovis canadensis share stratification of thick cortical and cancellous layers. Stegoceras, Cephalophus, and musk ox crania experience lower stress and higher safety factors for a given impact force than giraffe, pronghorn, or the non-combative llama.Conclusions/SignificanceAnatomy, biomechanics, and statistical correlation suggest that some pachycephalosaurs were as competent at head-to-head impacts as extant analogs displaying such combat. Large-scale comparisons and recursive partitioning can greatly refine inference of behavioral capability for fossil animals.
Highlights
Many animals strike with their heads at conspecifics, in ritualized flank-butting, head-to-head shoving matches and head-butting combat
We introduce an extension of recursive partition analysis (RPA) called correlate disruption, which examines how strongly adult Stegoceras morphology would suit it for combat relative to extant head-butting taxa
Extant combative ungulates vary in adaptations for headbutting, and pachycephalosaurs possessed a combination of their respective traits
Summary
Many animals strike with their heads at conspecifics, in ritualized flank-butting, head-to-head shoving matches and head-butting combat. Dome-shaped crania of pachycephalosaurian dinosaurs have been hypothesized as appropriate for head- or flank-butting, but internal histology appears to contradict such capability in young and old individuals [7] unless a thick keratinous covering protected the osseous dome [8]. Dome function in pachycephalosaurs has been controversial, with trabeculae within the dome interpreted as developmental traces inconsistent with head-butting [7], or as structures that would halt or absorb strain during collisions [9]. Pachycephalosaurs were bipedal herbivorous dinosaurs with bony domes on their heads, suggestive of headbutting as seen in bighorn sheep and musk oxen. Previous biomechanical studies indicate potential for pachycephalosaur head-butting, but bone histology appears to contradict the behavior in young and old individuals. Comparing pachycephalosaurs with fighting artiodactyls tests for common correlates of head-butting in their cranial structure and mechanics
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