Were ankylosaurs’ clubbed tails weapons?

I’m going to dare to step way outside my research expertise on this post, and look at a paper just because I love me some dinosaurs. I particularly love ankylosaurs; there’s one within arm's reach in my office. Ankylosaurs are often depicted in art locked in combat with a savage meat eater like Tyrannasaurus rex, mainly relying on its armor, but wielding one massive weapon: a huge bony club at the end of its tail.

I was disappointed to learn that the ankylosaurs I had as a kid (and that I still have on my desk) didn’t really exist, but were composites of many different species. Now, will Victoria Arbour destroy another childhood memory with her analysis of whether ankylosaurs could really use their clubbed tails as weapons?

The first couple of paragraphs made me quaver in my decision to try understanding this paper. I have no idea what “postzygapophyses” are, except that they’re some part of skeletal anatomy. But I march on to the descriptions of the X-rays slices they did of some skeletons.

Arbour estimated of the placement of tail muscles by looking at tendons that fossilized, and using crocodiles as a model. She suggests that the tail could be bent sideways, but says nothing about swinging it up, as is often depicted in art.

To figure out the forces the animal might have been able to generate by swinging the tail, Arbour crunches some numbers using estimates of muscle mass, inertia, and so on. Unfortunately for a casual reader like me, the number presented are not linked to anything that I might reasonably be able to relate to. That comes in the discussion, fortunately, where the $64,000 question starts to take shape: Could these clubbed tails do damage?

For some of the ankylosaurs with small clubs, Arbour argues, probably not. But some of the animals with larger clubs probably could break bones. Since the shear forces needed vary from bone to bone, Arbour suggests future studies might try to quantify the strength of leg bones of meat eating dinosaurs as well as ribs of ankylosaurs.

Arbour is more interested in the latter possibility, in fact: she suggests that the size of the clubs is such that juveniles probably did not have very large clubs, which she argues means they are unlikely to be defensive weapons. Instead, she thinks the clubs may have been used in ankylosaur on ankylosaur competition. This may be testable. Recent research on ceratopsian dinosaurs (like Triceratops) showed injuries consistent with the horns being used for competition. If ankylosaurs were clubbing each other, the breaks in the bones should be preserved.

So the notion of ankylosaurs using their tails as weapons may not be completely wrong, although the hypothesis has taken a bit of a beating here. (Pun fully intended!)

Reference

Arbour, V. (2009). Estimating Impact Forces of Tail Club Strikes by Ankylosaurid Dinosaurs PLoS ONE, 4 (8) DOI: 10.1371/journal.pone.0006738