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Stegoceras head-butting in PLoS ONE

From: Ben Creisler

Snively, E. & Theodor, J.M. (2011)
Common Functional Correlates of Head-Strike Behavior in the
Pachycephalosaur Stegoceras validum (Ornithischia, Dinosauria) and
Combative Artiodactyls. 
PLoS ONE 6(6): e21422. 


Pachycephalosaurs 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 Findings
Computed 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.

Anatomy, 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.

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