Know Your Ankylosaurs: North American Odds and Ends Edition

I've covered many of the North American ankylosaurs in my previous papers and blog posts. In 2013, I argued that what we thought was Euoplocephalus was more likely 4 taxa – Anodontosaurus, Dyoplosaurus, Scolosaurus, and Euoplocephalus proper. Then in 2014 we described a newankylosaurid, Ziapelta, from New Mexico. There are a few other taxa that had previously been proposed to be ankylosaurids, so let's take a look at them here.

Aletopelta, Stegopelta and Glyptodontopelta

Aletopelta is one of the more tantalizingly enigmatic ankylosaurs from North America. It's from a weird place – California – which may have been much further south 75 million years ago compared to its current position. It was also found in marine sediments, and the decaying carcass had formed a little reef, with oysters encrusting the ribs. The only known specimen of Aletopelta is relatively complete, all things considered, with the osteoderms in situ over part of the pelvis, the legs partially articulated, and with various odds and ends like osteoderms and vertebrae. Unfortunately, the ends of the bones are often chewed apart, and some of the material is a bit hard to interpret.

Here's the articulated pelvis and hindlimbs, and some other armour pieces, on display at the San Diego Museum of Natural History.

Regardless, Aletopelta is a very interesting ankylosaur. It has an unusual osteoderm morphology over the pelvis, with small hexagonal osteoderms closely appressed to each other. Ankylosaur pelvic armour seems to come in two major flavours: fused rosettes, like we saw in Dongyangopelta and Taohelong (and perhaps most famously in Polacanthus), and interlocking hexagons, like in Stegopelta, Glyptodontopelta, and Aletopelta. Tracy Ford suggested that ankylosaurs with these hexagonal pelvic shields might represent a clade (dubbed Stegopeltinae) of ankylosaurids. Glyptodontopelta has since typically been interpreted as a nodosaurid, as has Stegopelta, but the most recent interpretation of Aletopelta was that it was an ankylosaurid. In the revised phylogeny in my new paper, we found Stegopelta and Glyptodontopelta as nodosaurids, but Aletopelta as a very basal ankylosaurid. However, although Ford and Kirkland reconstructed Aletopelta with the typical ankylosaurid tail club, I don't think that it possessed one: the preserved distal caudal vertebrae don't show any of the lengthening or other modifications that are characteristic of ankylosaurid handle vertebrae.

An updated restoration of the known elements in Aletopelta - the main differences between this and Ford and Kirkland's reconstruction are the absence of a tail club, and uncertainty over what the head should look like.

Cedarpelta

Cedarpelta is an important taxon for understanding the biogeography and evolution of ankylosaurids, and I wish we had more specimens! I don't have many new comments to add about this taxon, since Ken Carpenter published a great description of the disarticulated skull back in 2001. Cedarpelta has been interpreted as a shamosaurine ankylosaur, as a relative of taxa like Gobisaurus and Shamosaurus (which I'll talk about in the next post) from Asia, and thus may point towards a mid Cretaceous faunal interchange between these two continents. In our revised phylogenetic analysis, we didn't find Cedarpelta as the sister taxon to either Gobisaurus or Shamosaurus, but it does come out as a basal ankylosaurid in their general neighbourhood, and I honestly wouldn't be surprised if future analyses or new taxa show support for it as a shamosaurine ankylosaur after all.

Nodocephalosaurus

Nodocephalosaurus! What a fun ankylosaur. It's really quite unlike the other ankylosaurids from North America, which typically have flat, hexagonal cranial ornamentation. Instead, Nodocephalosaurus has bulbous, conical cranial ornamentation. Bulbous cranial ornamentation is typical of Campanian-Maastrichtian Mongolian ankylosaurs like Saichania and Tarchia, but in those taxa the ornamentation is pyramidal rather than conical. The front end of the snout in Nodocephalosaurus is also unusual, because there's no obvious narial opening and instead the ornamentation has a stepped appearance. Hopefully better specimens with more complete snouts will resolve this weird morphology. I've also reinterpreted the position of the quadratojugal horn compared to Sullivan's original figures – the horn should be rotated forward so that the bottom margin of the orbit is complete.

Nodocephalosaurus holotype skull in dorsal and left lateral views.

Tatankacephalus

I don't have much to say about Tatankacephalus because I didn't look at the original material myself, but the previous phylogenetic analysis by Thompson et al. recovered it as a nodosaurid rather than an ankylosaurid as originally suggested by Parsons and Parsons, and we found the same result. Overall, Tatankacephalus is VERY similar to Sauropelta, so this is perhaps not surprising.

Up next: More odds and ends, but after I return from Utah!


Arbour VM, Currie PJ. In press. Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs. Journal of Systematic Palaeontology.

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.