Sunday, March 6, 2011

I like ankylosaur butts, and I cannot lie.

Yes, I seem to have this recurring fascination for the derrière of these dinosaurs. In 2009 I published a paper (with my fellow grad students Mike Burns and Robin Sissons) on Dyoplosaurus, in which we argued that the pelvis (specifically, the sacral ribs) is different than that of Euoplocephalus. I reconstructed the muscles of the tail and pelvis in my 2009 PLoS ONE paper on tail clubbing. Twice now I have spent significant portions of my summer preparing the pelvis of two different ankylosaur specimens. And now this month I have another paper (again with Mike Burns and my supervisor Phil Currie) on the ankylosaur pelvis, this time on the armour of the pelvic region.

The lovely behind of Aletopelta, from my visit to the San Diego Natural History Museum in 2009. It is an excellent museum, you should check it out!

It has been recognized for a long time now that some ankylosaurs were doing really weird things with the armour over their pelvis. In some taxa, the osteoderms fuse together to form a carapace-like shield over the hips. Previously, the presence or absence of this shield has been used in phylogenetic analyses to examine whether or not a third group of ankylosaurs, the Polacanthidae, is a valid taxon (in addition to the Nodosauridae and Ankylosauridae). However, it is not just the presence or absence of the pelvic shield, but the way that the pelvic shield is constructed, that may be important.

In this paper I and my coauthors propose a revised way of looking at the pelvic shield that breaks shields up into three categories: 1. fused, rosette pattern, 2. fused, uniform-sized polygons, and 3. not fused. “But Victoria, if the osteoderms aren’t fused, then why call it a pelvic shield?” you may ask. Well, after looking at the AMNH’s Sauropelta and the BMNH’s Euoplocephalus, I noticed that although the osteoderms of the pelvis weren’t coossified, there also weren’t any transverse bands segmenting the body in that region (see image below of BMNH R5161, the holotype of Scolosaurus discussed in this post). Although the osteoderms aren’t fused together, they still form a continuous shield over the pelvis.

The always exciting headless, clubless, BMNH R5161, modified from Nopsca's 1928 paper and as seen in Arbour et al. 2011.

More interesting, though, is what happens when we look at the other two categories – rosettes vs. uniform polygons – in a stratigraphic and geographic context. Rosettes are restricted to the Jurassic and Lower Cretaceous, whereas uniform polygons are primarily Upper Cretaceous. Uniform polygon shields are only found in North America (with the possible exception of Antarctopelta...which is from Antarctica). We did not run a phylogenetic analysis for this paper, but I plan to incorporate this data into subsequent analyses stemming from my PhD thesis on the phylogenetic relationships and biogeography of the Ankylosauridae. I will be very interested to see if any clades show up that reflect these stratigraphic and geographic patterns.

Here’s the paper! (behind a paywall, unfortunately...):

Arbour VM, Burns ME, Currie PJ. 2011. A review of pelvic shield morphology in ankylosaurs (Dinosauria: Ornithischia). Journal of Paleontology 85:298-302.
More papers about ankylosaur pelves!
Arbour VM, Burns ME, Sissons RL. 2009. A redescription of the ankylosaurid dinosaur Dyoplosaurus acutosquameus Parks, 1924 (Ornithischia: Ankylosauria) and a revision of the genus. Journal of Vertebrate Paleontology 29:1117–1135.
Arbour VM. 2009. Estimating impact forces of tail club strikes by ankylosaurid dinosaurs. PLoS ONE 4: e6738.


  1. Coombs doubted the tail club was used for intraspecific combat. As for interspecific defense, well, it may say something that the biggest clubs, those of T. gigantea and A. magniventris, coincided with the largest known tyrannosaurs. :)

  2. The only known tail club of Ankylosaurus actually has nothing on a couple of large Euoplocephalus tail clubs (ROM 788 and AMNH 5245)! The large Tarchia tail club (on display in Ulaanbaatar, but I think it still has a ZPAL number) is currently the largest I have measured and that I know of.

  3. Well, the relatively small size of the AMNH 5214 club can be explained by the immaturity of the specimen. If only CNM 8880 included a club I think it would surpass those of E. tutus and T. gigantea. Even the Tarchia club is consistent with increasing size to confront bigger tyrannosaurs, considering that Tarbosaurus was bigger than Campanian relatives.
    Oh btw, I understand another A. magniventris club is known from the Ferris.

  4. Oh, I didn't twig that you were involved with renaming the specimen of the Euoplocephalus to Dyoplosaurus until just now! I've been trying to get the people in my office to call our animatronic Dyoplosaurus by the right name ever since I read the articles and confirmed with Royal Tyrrell that our model was based on the one you were researching. It's such a great story.

    I started following your blog because of the Hornby Island pterosaur(bravo)and it is part of my every day readings along with my first cup of coffee when I'm not out on the road.