Estimating Impact Forces of Tail Club Strikes by Ankylosaurid Dinosaurs

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. METHODOLOGY/PRINCIPAL FINDINGS: 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/SIGNIFICANCE: 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

CT scan images of sagittal slices through UALVP 47273 handle in left lateral view, dorsal is up.

<p>A) Mid-width of the club. Most of the centra appear to be fused at the anterior and proximal faces (arrow with open head), although one joint does not appear fused (arrow with closed head), with B) position in specimen, oblique dorsal view, anterior is to the right. C) Mid-width of the left half of the club, with D) position in specimen. The neural canal extends to the anterior terminus of the minor plates at the distal end of the knob (arrow). The three narrow, vertically stacked structures at the anterior of the handle are ossified tendons. Scale bar equals 10 cm. Three-dimensional reconstructions in B and D created in Mimics.</p

Actual and ideal values for dimensions of the centra in ROM 784, in mm.

<p>Actual and ideal values for dimensions of the centra in ROM 784, in mm.</p

Impact velocities, impulses, forces, and stresses for the AMNH 5245/ROM 788 composite tail.

<p>Impact velocities, impulses, forces, and stresses for the AMNH 5245/ROM 788 composite tail.</p

Diagrammatic representation of composite tails used in this study.

<p>A) ROM 784 (<i>Dyoplosaurus</i>)/UALVP 47273 (<i>Euoplocephalus</i>) composite tail. ROM 784 elements indicated by light grey. UALVP 47273 elements indicated by dark grey. The black vertebra represents the transitional vertebra in ROM 1930. Its presence is inferred by the gap at this location in ROM 784. The light purple area represents the free caudal tail frustum, and the dark purple area represents a single free caudal tail segment. The orange area represents the transitional tail frustum, and the pink area represents the handle volume. B) UALVP 16247 reconstructed tail. Only the knob is preserved (dark grey); the rest of the tail is reconstructed from measurements of ROM 784 (black). C) AMNH 5245/ROM 788 composite tail. AMNH 5245 elements are light grey, ROM 788 elements are dark grey, and elements reconstructed from ROM 784 are black.</p

Impact velocities, impulses, forces, and stresses for the UALVP 16247 reconstructed tail.

<p>Impact velocities, impulses, forces, and stresses for the UALVP 16247 reconstructed tail.</p

Morphology of ankylosaurid tail clubs.

<p>A) UALVP 47273, dorsal view. B) ROM 784 dorsal view and C) posterior view, D) UALVP 16247 dorsal view, E) AMNH 5245 dorsal view, and F) ROM 788 ventral view.</p

Cross-sectional reconstructions of ankylosaurid caudal musculature.

<p>A) Anterior free caudal vertebra, modified from TMP 85.26.70 (<i>Euoplocephalus</i>). M. transversospinalis is not divided into its subunits. The relative sizes of all muscles are speculative, especially M. iliocaudalis and M. ischiocaudalis. B) More muscular reconstruction, with muscles bulging past neural spine, haemal spine, and transverse processes. This reconstruction is 43% larger than the reconstruction in A. C) Posterior free caudal vertebra, reconstructed from TMP 2007.20.100. M. iliocaudalis may not have extended very far posteriorly along the tail, in which case M. ischiocaudalis may have occupied the area reconstructed as M. ischiocaudalis here. D) Musculature of the handle, reconstructed from a CT scan image of UALVP 47273 at the midlength of the club. M. transversospinalis and M. longissimus caudae are represented by ossified tendons in many tail club specimens. The size of M. iliocaudalis is speculative. The width of M. longissimus caudae is equivalent to the maximum space between the major osteoderms of the knob. Scale equals 5 cm.</p

Summary of results of sensitivity analyses for ROM 784/UALVP 47273– angular accelerations, velocities, and impulses.

<p>Summary of results of sensitivity analyses for ROM 784/UALVP 47273– angular accelerations, velocities, and impulses.</p

Ossified tendons in ROM 784, oblique right lateral view.

<p>M. spinalis is represented by the inner set of imbricated tendons, and M. longissimus caudae is represented by the outer set of parallel to braided tendons. The ossified tendons continue underneath the knob osteoderms (arrowhead). Scale bar equals 10 cm.</p