ScienceDaily (June 29, 2011) — Llamas can't really manage it. Giraffes aren't very good at it and while big horn sheep and muskox excel at it, it turns out a small plant eating dinosaur -- the pachycephalosaur Stegoceras validum -- was probably even better at it: head butting.
Relative surface densities of cranial bone in Stegoceras validum (UA 2): External densities of the cranium of Stegoceras validum, in dorsal (A), ventral (B), lateral (C, D), anterior (E) and posterior (F) views. Note high densities of cranial ornamentation, and numerous neurovascular canals (correlates of a keratinous pad) exiting onto the cranial roof. (Credit: Eric Snively, Jessica M. Theodor. Common Functional Correlates of Head-Strike Behavior in the Pachycephalosaur Stegoceras validum (Ornithischia, Dinosauria) and Combative Artiodactyls. PLoS ONE, 2011; 6 (6): e21422 DOI: 10.1371/journal.pone.0021422)
Researchers surveyed the heads of a large number of modern animals as well as one of the world's best dinosaur fossils, the Stegocerasspecimen from the University of Alberta. They found that the bony anatomy of some pachycephalosaur domes are better at protecting the brain than in any modern head butter. The results of their research is published in PLoS ONE.
"Pachycephalosaur domes are weird structures not exactly like anything in modern animals. We wanted to test the controversial idea that the domes were good for head butting," says co-author Dr. Eric Snively, University of Calgary alumnus and post-doctoral researcher in biomedical engineering at Ohio University.
"Finding out brings us closer to their social lives: were pachycephalosaurs more likely just showing off their domes like peacocks with their tails, or were they also cracking their heads together like musk oxen?"
Using CT scanning and a new statistical method for diagnosing behavior in fossil animals, the researchers compared the bony-headed dinosaur with modern ungulates (hoofed animals) that engage in different kinds of combat.
"Our analyses are the closest we can get to observing their behavior. In a way, we can get "inside their heads" by colliding them together virtually. We combined anatomical and engineering analyses of all these animals for a pretty thorough approach," says Snively. "We looked at the actual tissue types in the skulls and heads of the animals."
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Common Functional Correlates of Head-Strike Behavior in the Pachycephalosaur Stegoceras validum (Ornithischia, Dinosauria) and Combative Artiodactyls
1 Department of Mechanical Engineering, Ohio University, Athens, Ohio, United States of America, 2 Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada
Abstract
Background
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.
Conclusions/Significance
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.
Citation: Snively E, Theodor JM (2011) Common Functional Correlates of Head-Strike Behavior in the Pachycephalosaur Stegoceras validum (Ornithischia, Dinosauria) and Combative Artiodactyls. PLoS ONE 6(6): e21422. doi:10.1371/journal.pone.0021422
Editor: Kenneth Carpenter, Utah State University-College of Eastern Utah, United States of America
Received: February 9, 2011; Accepted: June 1, 2011; Published: June 28, 2011
Copyright: © 2011 Snively, Theodor. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: University of Calgary Alberta Ingenuity Russ College of Engineering Canada Foundation for Innovation National Science Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
* E-mail: es180210@ohio.edu
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