title: Estimating impact forces of tail club strikes by ankylosaurid dinosaurs.
abstract: BACKGROUND
  It has been assumed that the unusual tail club of ankylosaurid dinosaurs was used actively as a weapon, but the biological feasibility of this behaviour has not been examined in detail. Ankylosaurid tail clubs are composed of interlocking vertebrae, which form the handle, and large terminal osteoderms, which form the knob.
  
  
  RESULTS
  Computed tomographic (CT) scans of several ankylosaurid tail clubs referred to Dyoplosaurus and Euoplocephalus, combined with measurements of free caudal vertebrae, provide information used to estimate the impact force of tail clubs of various sizes. Ankylosaurid tails are modeled as a series of segments for which mass, muscle cross-sectional area, torque, and angular acceleration are calculated. Free caudal vertebrae segments had limited vertical flexibility, but the tail could have swung through approximately 100 degrees laterally. Muscle scars on the pelvis record the presence of a large M. longissimus caudae, and ossified tendons alongside the handle represent M. spinalis. CT scans showed that knob osteoderms were predominantly cancellous, which would have lowered the rotational inertia of the tail club and made it easier to wield as a weapon.
  
  
  CONCLUSIONS
  Large knobs could generate sufficient force to break bone during impacts, but average and small knobs could not. Tail swinging behaviour is feasible in ankylosaurids, but it remains unknown whether the tail was used for interspecific defense, intraspecific combat, or both.
authors.affiliation: Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada. arbour@ualberta.ca
authors.forename: Victoria Megan
authors.initials: VM
authors.lastname: Arbour
mesh_terms: D000818:Animals; D025061:Dinosaurs; D005580:Fossils; D013623:Tail; D014057:Tomography, X-Ray Computed
references.citation: World J Surg. 2007 Nov;31(11):2215-21
references.pmid: 17726630
references.citation: PLoS Biol. 2006 Jul;4(8):e248
references.pmid: 16817695
references.citation: J Morphol. 2008 Apr;269(4):398-422
references.pmid: 17960802
references.citation: J Morphol. 2007 Aug;268(8):701-14
references.pmid: 17514722
references.citation: J Appl Physiol (1985). 2001 Mar;90(3):865-72
references.pmid: 11181594
references.citation: Science. 1999 Apr 30;284(5415):798-800
references.pmid: 10221910
references.citation: Science. 2006 Jul 14;313(5784):213-7
references.pmid: 16840697
references.citation: Exp Brain Res. 1997 Jun;115(2):257-66
references.pmid: 9224854
references.citation: Anat Rec A Discov Mol Cell Evol Biol. 2006 Jul;288(7):782-93
references.pmid: 16779820
references.citation: Med Sci Sports Exerc. 2000 Jul;32(7):1307-13
references.pmid: 10912898
references.citation: Acta Physiol Scand. 2001 Aug;172(4):249-55
references.pmid: 11531646
references.citation: J Exp Biol. 2007 Aug;210(Pt 16):2811-8
references.pmid: 17690228
references.citation: Physiol Rev. 1972 Jan;52(1):129-97
references.pmid: 4256989
references.citation: J Morphol. 2007 Nov;268(11):986-1020
references.pmid: 17823914