Who-oplocephalus: Everything old is new again.

Over the last few posts, I've talked about why Euoplocephalus tutus is a valid genus and species, how the Horseshoe Canyon Formation ankylosaurid is really Anodontosaurus lambei, and how the headless and clubless holotype of Scolosaurus cutleri is most likely the same species as the ankylosaurid from the Two Medicine Formation. Here's a diagram summarizing some of the key points from the paper.

One ankylosaur I didn't talk about in the previous posts was Dyoplosaurus acutosquameus, an ankylosaurid long synonymized with Euoplocephalus tutus but resurrected by me and my colleagues Michael Burns andRobin Sissons back in 2009. 

ROM 784, holotype of Dyoplosaurus acutosquameus, from Arbour et al. (2009).

The holotype of Dyoplosaurus, ROM 784, is another really nice specimen, consisting mostly of the back half of the animal (although a fragmentary skull is also preserved). Dyoplosaurus differs from Euoplocephalus in the shape of the hooves on the hind foot (which are more triangular than in other ankylosaurs), and in some aspects of the pelvis. The tail club knob is very narrow compared to almost all other tail clubs from the Dinosaur Park Formation, although it's not entirely clear how much of this is related to ontogeny versus taxonomy. Do tail clubs start off narrow and become increasingly wider as the animal grows? Or do they maintain approximately the same proportions and just increase in size? The sole tail club knob for Scolosaurus is about the same width as in ROM 784, but the knob is round and about as long as wide. If knob proportions changed as they got bigger, we'd expect knobs of the same width to have about the same length:width ratio, but that isn't the case between TMP 2001.42.9 and ROM 784. We'll need more specimens in order to really test this hypothesis, but for now I'm tentatively considering the narrow tail club knob of Dyoplosaurus to be a characteristic of that genus. And because of that, there is one other isolated tail club that may be referable to Dyoplosaurus – UALVP 47273, a specimen I CT scanned and used in my biomechanics of tail clubbing project.

Digital model of UALVP 47273 from CT scans, made using Mimics. 

Going from one to four species is a big increase in diversity. Is this a reasonable conclusion based on the patterns of diversity in other dinosaurs? I think so. Euoplocephalus was already kind of weird for dinosaurs from Alberta because it occurred in the Oldman Formation, throughout the Dinosaur Park Formation, and throughout the Horseshoe Canyon Formation. Few dinosaur species are found in all three formations, and even within the Dinosaur Park Formation there are three faunal zones with distinct sets of species. Based on my research, there is now good evidence to suggest that Euoplocephalus tutus was primarily found in Megaherbivore Assemblage Zone 1 (sensu Mallon et al. 2012), which represents about the lower 30 m of the formation. Anodontosaurus lambei is almost exclusively found in the Horseshoe Canyon, with only one specimen, TMP 1997.132.1, found in the upper part of the Dinosaur Park Formation. Scolosaurus is mostly present in the Two Medicine Formation, and the holotype is either from the lowest Dinosaur Park Formation, or perhaps from the Oldman Formation. Finally, Dyoplosaurus is from the lowest Dinosaur Park Formation as well.

Campanian-Maastrichtian ankylosaurids from Alberta and Montana, from Arbour and Currie (2013).

So, where do I go from here? Well, for my PhD dissertation I'm trying to better understand the evolution of ankylosaurid dinosaurs, and so knowing who all of the players are is really essential for constructing a revised phylogenetic tree. Understanding variation in cranial ornamentation in Euoplocephalus will also help me understand which other ankylosaur species are valid or need to be synonymized, or maybe even identify some new species. I also now know that tail club shapes are at least somewhat taxonomically informative, and this leads to questions about function and behaviour. If there really are three ankylosaurid species in the lower Dinosaur Park Formation, what does that mean for niche partitioning among similar large herbivore species? Why does Scolosaurus appear in the geologically older Oldman Formation, disappear, and then reappear in the younger upper Two Medicine Formation? Does this reflect habitat preferences in ankylosaurid species? As usual in science, answering one question leads to many more.

I've also had some people asking if there's anything going on with the Albertan nodosaurid ankylosaurs. My fellow grad student Mike Burns is currently working on the nightmare of taxonomy that is Edmontonia longiceps, Edmontonia rugosidens, Panoplosaurus mirus, and various combinations thereof. I'm looking forward to seeing what he comes up with! And of course, don't forget that the Royal Tyrrell Museum collected an astonishingly good nodosaurid (probably) fossil from Early Cretaceous marine sediments near Fort McMurray, Alberta, in spring 2011. If you're visiting the Tyrrell this summer, you can see parts of it being prepared in the lab.

Tomorrow I am off to Dinosaur Provincial Park for a brief bit of fieldwork, including, hopefully, a search for the Scolosaurus quarry. Wish me luck, and hopefully there will be even more ankylosaur news over the next couple of months.

Papers!

Arbour VM. 2009. Estimating impact forces of tail club strikes by ankylosaurid dinosaurs. PLOS ONE 4:e6738.

Arbour VM, Snively E. 2009. Finite element analyses of ankylosaurid dinosaur tail club impacts. Anatomical Record 292:1412-1426.

Arbour VM, Currie PJ. 2013. Euoplocephalus tutus and the diversity of ankylosaurid dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA. PLOS ONE 8:e62421.

Hill SR. 2012. An unconvential discovery from an unconventional resource: recovery of an Early Cretaceous ankylosaur fossil from an oil sands mining operation, Fort McMurray, Alberta, Canada. Search and Discovery Article #70121, adapted from poster and extended abstract presented at AAPG Annual Convention and Exhibition, Long Beach, California, April 22-25, 2012.

Mallon JC, Evans DC, Ryan MJ, Anderson JS. 2012. Megaherbivorous dinosaur turnover in the Dinosaur Park Formation (upper Campanian) of Alberta, Canada. Palaeogeography, Palaeoclimatology, Palaeoecology 350-352:124-138.

Who-oplocephalus: The Fellowship of the Half Ring

Behold, NHMUK R5161: the extraordinary holotype of Scolosaurus cutleri. This is truly one of the most amazing dinosaur fossils that has been collected from Alberta, and is one of the best preserved ankylosaurs in the world. And the best part is that it is on display for everyone to enjoy in the galleries of the Natural History Museum in London.

(Many thanks to Angelica Torices for snapping this photo for me! NHMUK R5161 is beautiful to look at, but difficult to photograph well, and I'm afraid most of my photos from my visit in 2009, while useful to me, are not necessarily that nice to look at.)

Most of the time, when you're looking at a drawing or model of Euoplocephalus, what you're really mostly looking at is NHMUK R5161. This specimen preserves a large amount of intact skin, which means that the free-floating osteoderms are, for the most part, in their correct life positions. Osteoderms form in the dermis of the skin, and so osteoderms are usually found as isolated elements, or scattered around disarticulated or associated skeletons. Because NHMUK R5161 is so complete, it has formed the basis for most restorations of Euoplocephalus.

Yup, all of these are kind of Scolosaurus. Especially the pink one. Perhaps somewhat embarassingly, this does not represent my complete collection of ankylosaur 'scientific models'.

However, you may have noticed something important: Scolosaurus lacks a skull and tail club. So, how could I figure out if Scolosaurus was the same as Euoplocephalus, if I couldn't look at the patterns of cranial ornamentation? If you remember back to part 2 of this series, I said that the morphology of the first cervical half ring was useful for diagnosing Euoplocephalus. 

The first cervical half ring of NHMUK R5161 has some important differences when compared to CMN 0210, UALVP 31, or AMNH 5406. In NHMUK R5161, the two medial osteoderms (closest to the midline) are round and lack keels, instead having a centrally located bump. Even in specimens of Euoplocephalus that have relatively flat medial osteoderms on the cervical half rings, like AMNH 5404, the medial osteoderms always have a keel and are more oval than circular. This suggested that Scolosaurus was distinct from Euoplocephalus and Anodontosaurus, even though it didn't have a head or tail club. (I largely agree with Penkalski and Blows' (2013) assessment that Scolosaurus is a valid taxon, but for somewhat different reasons than what they present in their paper.)

This reconstruction of Scolosaurus was drawn by Alice Woodward based on NHMUK R5161, and I suspect that many subsequent images of Scolosaurus were inspired by Woodward's art.

(A quick note about tail clubs and Scolosaurus – many artists have reconstructed this animal with a short tail and spiked tail club. The tail of NHMUK R5161 is broken at about the midpoint of the tail, probably just in front of where the tail club would have started. The skin is kind of sloughed out in this area, which has led many to erroneously interpret Nopcsa's figures and drawings as showing the knob of bone at the end of the tail. The 'spikes' on the tail club are really just osteoderms present at about the midpoint of the tail – no ankylosaurs had spikes on their tail clubs.)

The collection of NHMUK R5161 is one of the more interesting stories of palaeontological collecting in Alberta. The specimen was discovered and primarily excavated by William Edward Cutler in 1914, working for the Calgary Syndicate for Prehistoric Research (an excellent name for an organization that is, sadly, defunct). During excavation, the ankylosaur block collapsed on Cutler, no doubt causing grievous injuries. One of the Sternbergs finished the excavation, and the specimen was shipped to London.

NHMUK R5161 is significant, beyond being so complete, because it may derive from the Oldman Formation of Dinosaur Provincial Park, rather than the Dinosaur Park Formation. The quarry is somewhere across the river from Happy Jack's, the University of Alberta's field camp since 2008. However, the precise locality of the quarry is somewhat up for debate, and Oldman Formation sediments crop out in this area. Next week I'll be heading to Dinosaur Provincial Park, and one of the goals is to check out some of the potential quarry locations with our crew from the UofA, as well as Darren Tanke from the Tyrrell Museum, who has been investigating the NHMUK R5161 quarry for some time. It's important to know whether or not Scolosaurus comes from the Oldman or Dinosaur Park formation, so we can know whether or not Scolosaurus lived at the same time as Euoplocephalus. We will also be on the lookout for any leftover material…like the skull and tail club.

Two Medicine Formation skulls, from Arbour and Currie (2013). Notice the long, curved squamosal horns.

For a long time I was bitterly disappointed that there was no skull known for Scolosaurus. Could some of the isolated skulls I was referring to Euoplocephalus instead belong to Scolosaurus? Or was the skull of Scolosaurus noticeably different from Euoplocephalus? I despaired that I wouldn't have an answer to that question unless we miraculously collected another Scolosaurus from Dinosaur Park during my thesis. In the meantime, I was trying to figure out the identities of "Euoplocephalus" specimens from the Two Medicine Formation of Montana, which seemed very different from the Albertan specimens. These skulls had much longer and pointier squamosal horns with a 'backswept' appearance compared to Euoplocephalus and Anodontosaurus. I thought perhaps that the Two Medicine ankylosaurids might represent a new species of ankylosaurid (as did others – Penkalski named Oohkotokia earlier this year based on MOR 433, which has this unusual squamosal horn morphology).

From left to right, the first cervical half rings of AMNH 5337, TMP 2001.42.9, USNM 7943, and NHMUK R5161. The top row shows the half rings in anterior or posterior view, and the bottom row shows the rings in dorsal view. NHMUK R5161 is in dorsal view. Modified from Arbour and Currie (2013).

One day I was flipping through my photos of the Two Medicine Formation specimens when I realized that the morphology of the first cervical half ring of TMP 2001.42.9, the only specimen that had a half ring and skull, matched that of NHMUK R5161. Both had flat medial osteoderms with a central bump, rather than a keel. And indeed, another isolated half ring from the Two Medicine formation had this morphology as well. Scolosaurus was present in Montana!

I'll just take a moment here to address Oohkotokia: while I agree with Penkalski (2013) that the Two Medicine Formation ankylosaurid differs from Euoplocephalus, I do not think it differs sufficiently from Scolosaurus for these to be considered separate species. Perhaps one day in the future we'll find another Scolosaurus from Dinosaur Park that has flat medial osteoderms on the cervical half ring, but a skull with different squamosal horns compared to the Two Medicine skulls. If that happens, then I think you could make the case that Oohkotokia is valid. Until then, Oohkotokia is a junior synonym of Scolosaurus.

With the referral of the Two Medicine ankylosaur material to Scolosaurus, Scolosaurus now had a head! TMP 2001.42.9 even has a tail club, so we know that Scolosaurus had a round tail club knob. With the in situ osteoderms and skin impressions of NHMUK R5161, Scolosaurus is now one of the best understood ankylosaurids in the world. Now, if we could just figure out for certain exactly where it was collected from…

Next time: wrapping up loose ends, and figuring out what it all means.




PAPERS!


Arbour VM, Currie PJ. 2013. Euoplocephalus tutus and the diversity of ankylosaurid dinosaurs from the Late Cretaceous of Alberta, Canada, and Montana, USA. PLOS ONE 8:e62421.

Nopcsa BF. 1928. Palaeontological notes on reptiles. V. On the skull of the Upper Cretaceous dinosaur Euoplocephalus. Geologica Hungarica, Series Palaeontologica 1:1-84.

Penkalski P, Blows WT. 2013. Scolosaurus cutleri (Ornithischia: Ankylosauria) from the Upper Cretaceous Dinosaur Park Formation of Alberta, Canada. Canadian Journal of Earth Sciences 50:171-182.

Penkalski P. 2013. A new ankylosaurid from the Late Cretaceous Two Medicine Formation of Montana, USA. Acta Palaeontologica Polonica, in press.

Who-oplocephalus: Heads for Tails.

With the identification of UALVP 31 as a specimen that could be confidently referred to Euoplocephalus based on the shape of the cervical half ring, I now had a pretty complete skull, and some postcranial material, to help expand our knowledge of Euoplocephalus. 

The Euoplocephalus skull UALVP 31. From Arbour and Currie (2013).

  

The top of the skull of UALVP 31, and the holotype CMN 0210, is covered in small flat polygons. In some ankylosaurs, like Saichania and Minotaurasaurus, these polygons bulge upwards from the skull. One of the things I wanted to know was whether or not the shapes and patterns of these polygons could be taxonomically useful. Many previous authors have said the distribution of polygons in at least some regions of the skull is random, but was it really? One problem is that there hasn't been a consistent nomenclature for these polygons, and it's also unclear what their developmental origin is. Are the polygons fused osteoderms, like in the skulls of gila monsters, or are they formed from sculpturing of the skull bones, like in crocodiles?

Ooh, labels. The 'ca' in the abbreviations usually refers to caputegulum. From Arbour and Currie (2013).

To get around the osteoderm-vs-sculpturing problem, I decided to use a term that was coined back in 2001 by William Blows, but which had not caught on in the literature: caputegulum. Caputegulum means 'skull tile', and neatly removes any developmental origin from the descriptive aspect of comparing the polygon patterns among skulls. I compared Euoplocephalus skulls to juvenile ankylosaurs which had visible cranial bone sutures, so I could figure out which bones the caputegulae probably belonged to. Some caputegulae had been given names before – like the supranarial caputegulae or the median nasal caputegulum – but others are new for this study, like the lacrimal and loreal caputegulae. By naming the caputegulae, it is easier to compare them.

This is where having a large (and by that I mean 'large') sample size for this study was essential. If wanted to understand how the caputegulae patterns and shapes varied in Euoplocephalus, I needed to see as many specimens as possible. I tried to look at pretty much every published skull referred to Euoplocephalus, and all of the more recently collected or unpublished skulls that I could find out about. Do you have a skull in your collection that I missed? Let me know, I would love to see it!

Because specimens have been collected from Alberta for a long time, Albertan dinosaurs have wound up in many museums around the world. In order to collect my dataset, I had to see specimens in the collections of the Royal Tyrrell Museum, the Royal Ontario Museum, the Canadian Museum of Nature, the American Museum of Natural History, the Smithsonian, the Natural History Museum in London, and the Museum of the Rockies. It was great fun to visit all of these interesting museums, but it can take a long time to finish a project like this – planning so many trips takes time, arranging for funding for said trips takes time, and museum trips need to be worked around teaching and fieldwork schedules. A tip for any of my aspiring palaeo friends: I am very lucky to be part of a large lab, and one way I was able to cut down costs significantly while traveling was to have people to share my hotel room with. Oftentimes, I would travel with several other Currie Lab members – we would figure out who needed to go to what museums, and coordinate our trips so we went at the same time. It makes a huge difference when you need to go to, say, New York...

Rainbow ankylosaur skulls FTW! From Arbour and Currie (2013).

When you compare the Euoplocephalus skulls, there's definitely a lot of variation in the shapes, numbers, and positions of the caputegulae. But there is also a lot of stuff that is pretty similar, or varies only slightly. For example, all of the skulls have a large hexagonal median nasal caputegulum, all of them have a single loreal and all of them have a single lacrimal caputegulum. What are some of the sources of variation? I'm not entirely sure. The number of discrete caputegulae varies a lot, as does how far back on the skull they go before they fade out in the parietal region. Maybe this is just regular variation, or maybe it has to do with ontogenetic changes.

TMP 1997.132.1, and CMN 8530. See the extra caputegulae behind the eye? From Arbour and Currie (2013).

As I looked at the skulls, I noticed that some of them had extra caputegulae at the base of the squamosal and quadratojugal horns, and some of them didn't. I thought this was pretty interesting, because the specimens with these extra caputegulae (which I called postocular caputegulae) always had extra, smaller osteoderms (interstitial osteoderms) on the cervical half rings, ringing the main osteoderms.

What turned out to be very interesting indeed was the fact that whether or not a specimen had these extra osteoderms seemed to be related to its stratigraphic position! Specimens from the Horseshoe Canyon Formation always had postocular caputegulae and interstitial osteoderms, and almost all specimens from the Dinosaur Park Formation did not. One significant exception is TMP 1997.132.1, which is from relatively high in the Dinosaur Park Formation compared to most of the other specimens.

The stratigraphic separation of cranial ornamentation features wasn't the only difference I noticed. Tail club knobs from the Horseshoe Canyon Formation were typically wider than long, and had a pointy look to them in dorsal view, whereas the ones from the Dinosaur Park Formation were usually about as wide as long, and round in dorsal view.

All together, these features seemed to indicate that "Euoplocephalus" specimens from the Horseshoe Canyon Formation were not the same species as those from the Dinosaur Park Formation. CMN 0210, the holotype of Euoplocephalus, is from the Dinosaur Park Formation. But CMN 8530, the holotype of Anodontosaurus, is from the Horseshoe Canyon Formation. And lo, the taxon that I originally thought I was least likely to resurrect became one of the best supported species in my paper. Anodontosaurus has postocular caputegulae, interstitial osteoderms on the cervical half ring, and a wide, pointy tail club.

Anodontosaurus was also probably orange. So there.

It's a shame about the name - Anodontosaurus means 'toothless lizard', which isn't really accurate at all. Ankylosaurs have very small teeth, but they are definitely present. The holotype of Anodontosaurus, CMN 8530, does not preserve teeth, but that doesn't mean they weren't there - teeth often fall out after an animal dies. Oh well! Anodontosaurus it is.

Next time: a headless and clubless wonder - how will the mystery of NHMUK R5161 be solved?

PAPERS!

Arbour VM, Currie PJ. 2013. Euoplocephalus tutus and the diversity of ankylosaurid dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA. PLOS ONE 8:362421.

Blows WT. 2001. Dermal armor of the polacanthine dinosaurs. In: Carpenter K (ed). The armored dinosaurs. Bloomington: Indiana University Press, pp. 363-385.

Coombs WP, Jr. 1995. Ankylosaurian tail clubs of middle Campanian to early Maastrichtian age from western North America, with description of a tiny tail club from Alberta and discussion of tail orientation and tail club function. Canadian Journal of Earth Sciences 32:902-912.

Sternberg CM. 1929. A toothless armoured dinosaur from the Upper Cretaceous of Alberta. Canada Department of Mines Geological Survey Bulleting (Geological Series) 54:28-33.

Who-oplocephalus: Is Euoplocephalus 'real'?

The holotype of Euoplocephalus isn't very complete, as holotypes go. So, one of the first things I had to figure out for this project was whether or not the holotype of Euoplocephalus had any diagnostic features. Euoplocephalus was certainly unique when it was named – no other dinosaurs had the pattern of flat, polygonal plates on the skull like CMN 0210 has, and Lambe wasn't even entirely sure what the cervical half ring was (he suggested it might have been the back of a frill or crest).

Here it is again: CMN 0210, holotype of Euoplocephalus tutus.

A lot of times, features that are unique or diagnostic to a species when it is named are later found to be present in other closely related dinosaur species. There's a great term for this – character obsolescence (Wilson and Upchurch 2003). Anodontosaurus, named by Sternberg in 1929, had similar flat polygonal plates on the skull. And although Ankylosaurus was named in 1908, a skull was not known for some time – but it too has flat polygonal plates on the skull. So, this pattern isn't necessarily a diagnostic or unique feature for Euoplocephalus.

Paul Penkalski looked at variation of skulls referred to Euoplocephalus in 2001, and noted that there were a couple of different morphologies represented by cervical half rings. Is the cervical half ring of CMN 0210 unique? Luckily for me, the University of Alberta has an excellent ankylosaur specimen called UALVP 31. Collected by GF Sternberg in 1921 and described by Gilmore in 1923, the skull has been on display in our paleontology museum for several decades. 

And here's UALVP 31's skull on display, plus the tail club UALVP 16247.

But more than the skull was collected – the cervical half rings, part of the hips and leg, a scapula, and other bits and pieces had been collected but never prepared! So, I dug out the half rings and other pieces and got to work.

Hop in the wayback machine, here's a picture from 2007, my first year at the U of A, working on the cervical half rings of UALVP 31.

Preparing the material took a couple of years, because I did it in bits and snags around my research and other projects. I had help from several of my fellow grad students and technicians – Robin Sissons did the scapula, Mike Burns and Kristina Barclay helped with the pelvis, and Ian Macdonald and Clive Coy assisted with some of the trickier parts like the poorly-preserved second cervical half ring.

You guys, opening up old jackets is hard! Anyway, this is Mike and Robin in 2008 being awesome weirdos. 

In 2009, I decided to supervise two high school students in the University of Alberta's WISEST Summer Research Program. WISEST (Women in Scholarship, Engineering, Science and Technology) is an awesome organization that organizes conferences for junior high and high school students, and supports grad students, postdocs, and early career professionals on campus. Their Summer Research Program places high school students into university labs for about 6 weeks during the summer, and by the end the students produce and present a poster discussing what they've been up to. In 2009 I was joined by Carmen Chornell and Idel Riemer, who were enthusiastic and excellent fossil preparators! Together we completed preparing the UALVP 31 pelvis, as well as other small bits.

Carmen works away on the pelvis and leg...

...while Idel puts the finishing touches on some osteoderms.

The UALVP 31 cervical half ring turned out pretty neat, but I needed more information to figure out if the half ring of CMN 0210 was unique. I visited lots of museums to see what the half rings of other "Euoplocephalus" specimens looked like, and to see the half rings of other species of ankylosaurs. Thankfully, it seems that the half ring of CMN 0210 is diagnostic, so Euoplocephalus is a real taxon after all. The first cervical half ring has 6 tall, keeled osteoderms, and each pair has a distinct morphology. The middle (medial) pair are tall, with a centrally located apex to the keel. The lateral pair also have a keel, but it's kind of sigmoidal (or, S-shaped). The distal pair, which cover the tips of the band, are flange-shaped. Some specimens, like UALVP 31, are missing the distal osteoderms – these don't seem to fuse on as tightly, so it's possible that they are more easily broken off before fossilization. The half ring of UALVP 31 shares the same morphology as CMN 0210, and so UALVP 31 was one of the few specimens that I could confidently refer to Euoplocephalus for a long time (another being AMNH 5406). UALVP 31 was the only one of these that had a complete skull. 

On the left, CMN 0210, in the middle is UALVP 31, and on the right is AMNH 5406.

In the next post, I'll talk about figuring out variation in the cranial ornamentation in Euoplocephalus skulls.

AND HERE ARE SOME PAPERS!

Arbour VM, Currie PJ. 2013. Euoplocephalus tutus and the diversity of ankylosaurid dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA. PLOS ONE 8: e62421.

Brown B. 1908. The Ankylosauridae, a new family of armored dinosaurs from the Upper Cretaceous. Bulletin of the American Museum of Natural History 24:187–201.

Gilmore CW. 1923. A new species of Corythosaurus with notes on other Belly River Dinosauria. Canadian Field Naturalist 37: 1–9.

Lambe LM. 1902. New genera and species from the Belly River Series (mid-Cretaceous). Geological Survey of Canada Contributions to Canadian Palaeontology 3: 25–81.

Lambe LM. 1910. Note on the parietal crest of Centrosaurus apertus and a proposed new generic name for Stereocephalus tutus. Ottawa Naturalist 14: 149–151.

Penkalski P. 2001. Variation in specimens referred to Euoplocephalus tutus. In: Carpenter K (ed.) The Armored Dinosaurs. Bloomington: Indiana University Press, 363-385.

Wilson JA, Upchurch P. 2003. A revision of Titanosaurus Lydekker (Dinosauria – Sauropoda), the first dinosaur genus with a “Gondwanan” distribution. Journal of Systematic Palaeontology 1:125-160.

Who-oplocephalus?

Today I published a revision of the North American ankylosaurid genus Euoplocephalus. I'd like to take some time to go through some of the major points of the paper over the next few days here, but today I will give a brief introduction to the motivation behind this study.

Euoplocephalus is one of those 'classic' dinosaurs. Named in 1902 (as Stereocephalus, which was preoccupied by...a beetle! of course), it wasn't known from very much material. All that Lambe had was a chunk of the snout/forehead region, and an unusual structure called a cervical half ring. Cervical half rings are totally bizarre structures that seem to be unique to ankylosaurids. They're made of an underlying yoke of fused bone segments, and topped by fused osteoderms like you see on the rest of an ankylosaur's body. 

Canadian Museum of Nature 0210, holotype of Euoplocephalus tutus. On the left, the skull chunk. On the right, the first cervical half ring. Images modified from Arbour and Currie (2013).

Over the next couple of decades, three more ankylosaurid species would be identified from Alberta (besides Ankylosaurus): Anodontosaurus, Dyoplosaurus, and Scolosaurus. Anodontosaurus is known from a complete but somewhat squished skull and a half ring, Dyoplosaurus is known from an articulated pelvis and tail, plus hindlimb and other assorted bits, and Scolosaurus is known from an exquisitely preserved skeleton with in situ osteoderms and skin impressions, but without a skull or tail club.

In the 1970s, Walter Coombs, Jr. undertook a large and important study of the ankylosaurian dinosaurs, which formed the foundation for many subsequent studies of ankylosaur anatomy and systematics. More specimens had been collected by that point, and subtle variations in the skull ornamentation among these specimens led Coombs to conclude that either every specimen must be its own distinct species, or they all represented the same species: Euoplocephalus (because it was named first). That's a pretty reasonable conclusion to have drawn with the available evidence at the time.

For my MSc thesis at the University of Alberta, I studied the biomechanics of tail clubbing in ankylosaurids, and specimens referred to Euoplocephalus formed an important part of that research. This is where I became interested in the variation that I saw in tail clubs referred to Euoplocephalus – some were small, some were large, some were skinny, some were round, some were pointy. What did it all mean? Were all of these the same species? Was I looking at ontogenetic changes? Were tail clubs just really variable in Euoplocephalus? What would this mean for how the tail clubs were used?

Wow, 2007 was a long time ago. Anyway, here's me at the Royal Ontario Museum holding the smallest tail club referred to Euoplocephalus, ROM 7761, and standing next to the largest, ROM 788. I CT scanned ROM 788 for my finite element analysis research, and you can now see it on display in the ROM's dinosaur gallery.

This was ultimately what led me to pursue my current line of research into ankylosaur systematics and evolution. I hoped that by understanding variation within Euoplocephalus, I would be better able to assess the validity of other ankylosaurid species, and perhaps identify new species. Over the next couple of posts, I'll talk about how I did this study, and what makes a Euoplocephalus a Euoplocephalus. Stay tuned!

The title for this post was shamelessly stolen from Collin Vanburen, because it is hilarious and perfect.

(But you don't have to take my word for it...: Arbour VM, Currie PJ. 2013. Euoplocephalus tutus and the diversity of ankylosaurid dinosaurs from the Late Cretaceous of Alberta, Canada, and Montana, USA. PLOS ONE 8: e62421.)