Ankylosaurs by the sea

Ankylosaurs, like probably most other dinosaurs, were landlubbing, terrestrial animals without obvious aquatic adaptations. And yet, surprisingly, their fossils are found in marine sedimentary environments more often than most other dinosaurs (except hadrosaurs). Some, like Aletopelta, wound up in shallow or lagoonal environments - Aletopelta's carcass became a reef! - but some, like the Suncor nodosaurid, wound up far away from shore.

Aletopelta! See if you can spot the oyster marks, invertebrates, and shark teeth around the pelvis and legs.

I wanted to look into this phenomenon in a bit more detail than it had been investigated previously, and with my work on ankylosaurid phylogenetics and biogeography published last year the time was right to dive into marine ankylosaurs. I added in more taxa into my ankylosaurid character matrix to check whether there were any secret ankylosaurids hiding in the taxa I didn't previously include. 

Surprisingly, Hylaeosaurus popped out as a basal ankylosaurid in this analysis, an intriguing result that I think needs further investigation but could be really interesting. Mostly everything else that should be a nodosaurid was a nodosaurid, and I got surprisingly good resolution in the Nodosauridae, but take the interrelationships there with a tiny grain of salt given that I didn't make any new characters for this analysis and the existing character set is geared towards ankylosaurids.

Next up, I downloaded a dataset of all the ankylosaur occurrences in the Paleobiology Database and went through each entry (more than 500!) to verify whether or not the specimens were ankylosaurids, nodosaurids, or if we couldn't tell for sure, and their depositional environment. With that data in hand, we wanted to know what the geographic distribution of ankylosaur marine occurrences looked like, and here it is:

Lots of ankylosaurs in the northern hemisphere, not so many in the southern hemisphere. More than half of marine ankylosaurs occur in North America, which is perhaps unsurprising given that North America had a bad habit of being underwater for a lot of the Cretaceous. When we exclude indeterminate occurrences, we get an ankylosaurid-nodosaurid marine-terrestrial split that looks like this:

In North America and Asia, the distribution of ankylosaurs in different environments is statistically significant, but it isn't in Europe (and remember, Hylaeosaurus hasn't previously been found as an ankylosaurid so there might not even be ankylosaurids in Europe).

Although there are nodosaurids and ankylosaurids in Asia, I don't think they overlapped very much in time - nodosaurids disappear from the Asian fossil record around the time that ankylosaurids appear. North America is unique in that it is the only place where we find both clades of ankylosaurs overlapping for significant chunks of time, so I wanted to know whether or not the significant marine-terrestrial dichotomy holds up in different time intervals within North America. We decided to divide up the dataset by the ebb and flow of the Western Interior Seaway, or, transgressive-regressive cycles. There were several major cycles well known to sequence stratigraphers and other geologists, and it seemed like a logical way to look at how the seaway influenced marine occurrences in ankylosaurs. How does that data look?

Like this! Unsurprisingly, a huge number of ankylosaur occurrences are clustered at the end of the Cretaceous, largely because of the Dinosaur Park Formation and its well-documented dinosaur fauna. But focus on the marine occurrences and an interesting pattern emerges - while the number of terrestrial occurrences fluctuates widely, the number of marine occurrences stays relatively steady. We think this reflects the absence of terrestrial outcrops for a chunk of the mid Cretaceous when sea levels were at their very highest - in other words, although there's a greater proportion of marine occurrences in the middle, they don't really increase in absolute terms and the proportion is being driven by the drop in terrestrial occurrences.

And here's what it looks like when we exclude indeterminate ankylosaurs. Unfortunately, this is the point at which we lose statistical power for most of the data, which means we can't really interpret most of the results with any confidence. Plus, there is a good chunk of time in the mid Cretaceous in which there are no ankylosaurids known at all, which probably represents a regional extinction for this group of dinosaurs. BUT, one time bin shows a significant difference between depositional environment and clade - the Kiowa-Skull Creek cycle in the Albian-Cenomanian.

What does it all mean? For one thing, breaking down the global ankylosaur dataset still yields enough statistical power for some analyses to be useful and give us more insight into biogeographic patterns...up to a point. Secondly, there really does seem to be something about nodosaurids that makes them wind up in marine environments more often than ankylosaurids. And this begs the question: why don't ankylosaurids like the beach? Given that ankylosaurids disappear from North America right around the time that sea levels rose to their highest point, could rising sea levels have led to the extinction of North American ankylosaurids? Did sea levels need to drop substantially before the inland, somewhat desert-dwelling Asian ankylosaurids migrated back into North America in the latest Cretaceous? Did ankylosaurids avoid the European archipelago because they didn't like to get their feet wet? I don't have definite answers yet, but I find this pattern terribly interesting and I'm sure this isn't the last we'll be looking at this weird split in environmental preferences between ankylosaurids and nodosaurids.

The paper is free until April 22! Get your PDF while it's still hot! Arbour VM, Zanno LE, Gates T. 2016. Ankylosaurian dinosaur palaeoenvironmental associations were influenced by extirpation, sea-level fluctuation, and geodispersal. Palaeogeography, Palaeoclimatology, Palaeoecology 449:289-299.

Ankylosaurs all the way down

After the SVP meeting in Dallas, I spent a couple of days working on Texan ankylosaurs at the Ft Worth Museum of Science and History, and at the collections at Southern Methodist University. It was nice to see a bit of Texas outside of downtown Dallas, so here's a few shots from my visit to Ft Worth!

You know it's going to be a pretty good museum visit when you're greeted by Dr. Suess statues on your way in! Especially when it's from your favourite Dr. Suess story, the underappreciated Yertle the Turtle.

This is probably the most interesting office space I have ever worked in. Or at least, the most intimidating. Look closely between the dueling bears and you will see...

...Pawpawsaurus! This is the holotype and only known skull of this beautiful little nodosaur. What a treat to be able to study the original in person.

Just next to me were these very interesting Katsina dolls, including my new favourite character, Squash Man. Apparently he is present in harvest stories and now I want to know all about him because he is the greatest.

The museum has a pretty nice dinosaur exhibit, which I liked a lot because it features local Texas dinosaurs rather than the standard Tyrannosaurus and Triceratops that museums of this size typically have. There was also an outdoor dig site recreating the Jones Ranch quarry that I had visited the previous week on the SVP field trip!

The dinosaur exhibit has these really great gigantic line drawings of Texas dinosaurs, which I liked a lot. They look like somebody roughed in some chalk drawings on the walls, and I find them really appealing and dynamic!

The room is dominated by the skeleton of "Paluxysaurus", more recently considered a junior synonym of Sauroposeidon. Whatever it's called, it's an interesting sauropod, representing one of the last North American sauropods before the lengthy 'sauropod hiatus' from the mid Cretaceous until the Maastrichtian.

Here's something new for me - the foot of Tenontosaurus! A cool original fossil to have on display; behind it there's a reconstructed Tenontosaurus skeleton, and there was also a slightly worse for the wear original Tenontosaurus skull. It's like Tenontosaurus central around here!

Here's a super cool interactive station! Measure the circumference of a femur, put your measurements onto the computer, and see how massive different animals were!

Given the extreme dearth of ankylosaurs in museum exhibits, I was pretty over the moon that Pawpawsaurus featured so prominently! Usually the original skull is on display in the glass case, but today they had taken it off exhibit for me to look at and replaced it with a cast. Now somebody just needs to find the rest of the skeleton so we can have a more complete picture of this important mid Cretaceous ankylosaur!

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This Way to the Dinosaurs

Welcome to the Perot Museum of Nature and Science! The Society of Vertebrate Paleontology Annual Meeting's welcome reception was held here last week. This museum is trying out some interesting and different exhibition ideas that I haven't seen too often elsewhere, so let's take a look at some highlights. One of the most interesting design elements is the visible escalators poking out the side of the building. Back in 2013 at the Korea-Mongolia International Dinosaur Project Symposium, Tony Fiorillo gave a really interesting presentation on the design of this museum, and talked about how a lot of people never make it off the first floor of a museum, which is also usually where the dinosaurs are. So at the Perot Museum, the dinosaurs are on the top floor and you are immediately shuttled upstairs (it's actually kind of hard to *not* go to the top floor first!), and then you work your way down the museum to exit. 

The dinosaur exhibits feature some interesting species that aren't found in a lot of other museums - here's a modern take on Tenontosaurus, and the still unnamed Proctor Lake 'hypsilophodontid' (somebody name that guy, already!).

My favourite exhibit in the whole museum! One of the only places where I've seen the North American-Asian faunal interchange visualized in an exhibit. Tarbosaurus is in Asia, and its close relatives are in North America (I can't remember exactly which taxon is featured here, but perhaps it is Bistahieversor based on its geographic position?). Also whoa, Beringia sure looks strange from this polar vantage point.

Another interesting thing the museum has done is to place modern animals alongside the dinosaurs for comparative purposes. Here we've got predators and prey - a mountain lion and a deer, and Tyrannosaurus and the sauropod Alamosaurus (off to the left of my photo).

A similar approach is taken in the Alaskan dinosaur corner - here's the herbivorous Edmontosaurus Ugruunaluk...

...and its extant analogue the caribou (Rangifer!). I wasn't totally sold on this approach, but I was intrigued by the mixture of extant and extinct, and of modern and ancient ecosystems, so maybe I just need to ruminate on it a little more.

I'm a sucker for Sinclair dinosaurs, what can I say.

Does the mould for the Ankylosaurus exist anywhere still??? DO WANT.

At one end of the dinosaur hall you take a set of stairs up to the bird exhibit! I liked this a lot, both because the bird exhibit had some cool interactive stuff, but also because I like the symbolism and narrative structure to traveling upwards towards birds from dinosaurs - it's like moving up the phylogenetic tree, and gaining flight.

From up in the rafters, you get a nice view of the dinosaur gallery, and a great vantage point for examining the gigantic Alamosaurus (real vertebrae are tucked down at ground level behind the skeleton from this angle). Alamosaurus is a weird and biogeographically interesting creature, representing a re-emergence of sauropods in North America after a lengthy hiatus throughout much of the mid Cretaceous. 

ELSEWHERE IN THE MUSEUM...

Rocks and minerals! With gigantic mineral shapes! (My favourite is the giant malachite clump in the back.)

Brains! There's a really fun section on medicine and human anatomy.

Phylogenies! Can you find where humans are located on this giant tree of life?

Outside the museum, we were bid farewell by these very fine green leapfrogs, which surely must be great fun to play with if you are smaller than I am.

More Texas adventures forthcoming - stay tuned!

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Burgers and Hot Dogs

Sydney Mohr is a friend and colleague of mine whose art you will have seen in the news lately, if you are inclined to read about ankylosaurs. She's done amazing reconstructions of two ankylosaurs for me in the last year - Ziapelta and Gobisaurus - and so I asked her to take a few minutes and tell us about her process for creating her palaeoart. Also this way I get to show off more of her drawings, so yay!

Sydney decided that this Gobisaurus was named Burger, and that seemed fine with me.

Once we got started on Gobisaurus, I sent Sydney a pile of photos of both Gobisaurus and its close relative Shamosaurus, and some of my own very rough sketches of what the osteoderms might have looked like. Gobisaurus isn't completely known, so we're guessing a bit on the osteoderm arrangement in the final version and using Shamosaurus for the cervical armour. Here are the earliest sketches Sydney sent me - so many great poses and personality. Also, here's a Sydney in her natural habitat (thanks John Acorn for the photo!).

  

One of the things I really like about your art is that it's obvious you are very familiar with animal anatomy and behaviour – your dinosaurs have real animal personalities. Can you tell us about some of your inspirations for your palaeoart? 

The best inspiration any artist can have when reconstructing extinct animals are...living animals! In most cases that's the best if not the only source of reference we have, at least when it comes to external appearances. Depending on what type of fossil I'm drawing, I'll try to find an extant analogue/s that may share some characteristics, like habitat and environment, diet, colour scheme, etc. For colour in particular I often mix and modify schemes from two or more animals, all the while keeping the fossil's apparent paleobiology and habitat in mind. I'll peruse images of modern animals on the web to get an idea of the posture and stance I want the fossil animal to be in, as well as the lighting and angle. A lot of a creature's emotion comes from the face, so I really like to focus on eyes. Getting the shape, depth, colour, and light just so can make a huge difference in terms of giving a drawing personality. It also isn't a bad idea to look at other artist's work, obviously not to copy directly, but you might get ideas for new methods or techniques that you can adopt and fit into your own style. 

Mr Iridescent - a beautiful take on Microraptor. So shiny and chrome.

This reminds me, I think you said the Ziapelta reconstruction you did for our paper was inspired by a photo of a bird that you took! And that in turn reminds me that you are also a pretty great bird photographer - do you find that you get a better sense for conveying personality and movement in your dinosaurs by observing birds in the wild yourself?

So I did! The proudest grackle ever! 

I can see the family resemblence! Also, when I found out the Gobisaurus was named Burger, I asked if the Ziapelta had a name. Naturally, it was Hot Dog.

And definitely, seeing any animal in their habitat first hand can create a narrative in your mind that you can translate to paper. Birds are great to watch because a lot of the time they're always on the move and engaging in a variety of behaviours that are both interesting and fun to watch, as well as perfect fodder for a dinosaur reconstruction.  

You are also working on a Masters with Phil Currie at the University of Alberta! Would you like to tell us about what you're working on?

The thought of Mesozoic birds with bonafide teeth has really interested me for a while, so the plan is to explore the evolution of tooth loss in birds by comparing the implantation and replacement rates of small theropod (like dromaeosaurids and troodontids) and bird teeth. Looking into the anatomy of the jaw and the inner structures of the teeth of these closely related groups will hopefully yield some informative results. It's not easy because the stuff I need is comparatively rare and pretty darn tiny! I'm working entirely with Alberta material at the moment, and doing so has led me in other directions in terms of understanding the province's Cretaceous avian fauna, which is most represented in terms of numbers by, you guessed it, teeth!

Pygostylia Panoply: at the bottom, the toothy Early Cretaceous enantiornthine Rapaxavis, and up top, the duck-like (and toothless) Presbyornis.

Do you have a favourite taxon to illustrate?

Birds and feathered theropods are definitely up there.The more I do ankylosaurs though the more I enjoy drawing them. [YES FOLKS, YOU HEARD IT HERE: ANKYLOSAURS > THEROPODS.] They're so unique compared to anything else around today! I also enjoy doing mammals as well, like ungulates and carnivores (fossil or modern) and primitive examples from the Mesozoic. 

I am but a young'un: a perfectly floofular dromaeosaur chick. 

What medium/media do you like to work in?

I stick almost exclusively to traditional media; mainly pencil work, both black and white and colour, although I occasionally work in acrylic or watercolour. I prefer to work with fine tooth paper so I can vary my pencil strokes, blend more easily, and just have an overall smoother surface to work on. Coloured paper is also really fun to work with, like blue or black, because it makes drawing ocean scenes with pencil pretty simple. I've also dabbled in digital art via photoshop, but most of the time I only use it to fix mistakes and touch up scanned pencil drawings. In my case I find I have much more control with pencil and paper, and the results seem to be a bit more realistic, at least to my eyes. 

Ichthyornis dispar: a classic fossil bird, brought to life!

Do you have any advice for other people who are interested in creating their own palaeoart? Any common pitfalls to avoid, or things to think about when they are recreating an extinct animal?

I think one of the most important aspects of reconstruction is attention to detail, such as the dot of light and reflections in an eye, or the wind disturbing and ruffling fur or feathers, or the bulge of a muscle as a limb is flexed, or the crumpling of skin as it moves in a certain direction or shifts under the weight of the animal. Light, movement, and substance. Those kinds of little and almost unnoticeable features can take a simple reconstruction of a fossil to something that feels tangible and alive. In terms of pitfalls to avoid, I would say there isn't too much to worry about if you're just playing around and having fun, because hey, it's just art! That being said, if you're going for a publishable, as-accurate-as-possible, realistic style of depiction, then it's a good idea to become familiar with your subject, especially anatomy. If you can read up and get as close as possible to the original source material, like scientific papers, then you're that much closer to getting your skeletal anatomy down pat. Knowing some anatomy of modern animals is extremely helpful as well, as it informs how muscle and skin attaches to the bone and changes the outline of the body.

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Thanks Sydney! You can check out more of Sydney's amazing art and photography at her website, DeviantArt gallery, and Flickr gallery.

How the ankylosaur got its tail club

Ankylosaur tail clubs are odd structures, odder than they are usually given credit for. They represent substantial modifications to two different skeletal systems – the endoskeleton, in the form of the caudal vertebrae, and the dermal skeleton, in the form of the caudal osteoderms. The centra of the caudal vertebrae lengthen but stay robust, and the neural arches undergo huge changes, such that the prezygapophyses, postzygapophyses, and neural spine become a robust, V-shaped structure on the top of the centrum, and which creates a tightly interlocking vertebral series with almost no flexibility. We call this the handle of the tail club. The osteoderms at the tip of the tail smush together and two of them become huge: although the tail club knob is small in some species, there are colossal knobs exceeding 60 cm in width. The ankylosaur tail club represents one of the most extreme modifications to the tail in terrestrial tetrapods.

Look at that thing. That is a weird thing.

(This is UALVP 47273, a really nice club that I studied for my MSc work on tail club biomechanics.)

One of the questions I became interested in during my MSc research on ankylosaur tail club biomechanics was how the tail club evolved in the first place. Most ankylosaurs with tail clubs are known from a relatively narrow slice of time right at the end of the Cretaceous, but when and where did the tail club first evolve? Did the stiffening of the tail occur before the enlargement of the tail osteoderms, or vice versa? Or did both changes happen at about the same time? This was a fun question to address during my PhD research, once I had a fairly well resolved phylogeny of ankylosaurids, and once I had looked at tons of ankylosaurid fossils.

So, how did the ankylosaur get its tail club? Well, based on what we see in the fossil record, it looks like the changes to the vertebrae predate the changes to the osteoderms – in other words, the handle comes first and the knob comes later. There is at least one ankylosaur out there that seems to have a tail club handle but not a knob: Gobisaurus!

Hello Gobisaurus! Many many thanks to my friend and colleague Sydney Mohr for preparing this awesome illustration of Gobisaurus for me.

Gobisaurus, a shamosaurine ankylosaurid, has a really nice complete tail club handle that is indistinguishable from other ankylosaurid tail club handles, but does not have a knob. And it's not just because the knob is broken off – it seems as though the last vertebrae in the tail are preserved, because they look very similar to the terminal vertebrae in a CT scan of a tail club from the University of Alberta collections. It's likely that Gobisaurus had osteoderms along the sides of the tail like we see in most other ankylosaurs, but it doesn't appear that there were osteoderms tightly enveloping the tip of the tail.

An even earlier ankylosaur seems to show some changes towards acquiring a tail club handle, as well. Liaoningosaurus, a basal ankylosaurid known only from a very small juvenile, has distal caudal vertebrae where the prezyapophyses extend about 50% the length of the adjacent vertebra. This is what we see in ankylosaurid tail clubs, but not in more basal taxa like Mymoorapelta where the prezygapophyses are much shorter. Liaoningosaurus is missing the tip of the tail and also lacks osteoderms on most of its body because it's a juvenile, so it's harder to say whether or not it had a tail club knob based just on the fossil alone.

I also did a cool and relatively simple thing with my phylogenetic tree to see if I could better understand the likelihood that some ankylosaurs without preserved tail material had a tail club handle or full tail club with a knob. Unsurprisingly, all shamosaurine and ankylosaurine ankylosaurids probably had a tail club handle. Liaoningosaurus is part of a basal polytomy of ankylosaurids, and it was a bit more equivocal whether or not any of these taxa was likely to have a tail club handle or not, partly because another basal ankylosaurid in this region of the tree, Chuanqilong, does not have modified distal caudal vertebrae.

All ankylosaurine ankylosaurids more derived than Pinacosaurus (so including things like Tsagantegia, Saichania, Euoplocephalus, etc.) almost certainly had a tail club knob, and shamosaurine ankylosaurids probably did not. Crichtonpelta, the most basal ankylosaurine, may or may not have had a tail club – we'll need more data to know for sure. There is amounted skeleton of Crichtonpelta at the Sihetun visitor center in Liaoning, and it is shown with a tail club, but it isn't clear whether or not this is sculpted or original material belonging to this specimen, and a full description of this material is necessary.

Gobisaurus and Liaoningosaurus both lived much earlier than the more familiar tail-clubbed ankylosaurs: Gobisaurus is no younger than 92 million years old, and Liaoningosaurus is about 122 million years old. The earliest ankylosaurid with a tail club in the fossil record is Pinacosaurus (from the Campanian), although there is a caveat to this: Talarurus, which is a bit older than Pinacosaurus, should have a full tail club based on its position in the phylogenetic tree, and while a tail club handle is known for this taxon, we haven't found a tail club knob for Talarurus. Talarurus is in kind of a weird spot phylogenetically, since it's from Mongolia but comes out as closely related to North American ankylosaurines, so I think it's worth keeping an eye on this taxon in the future – perhaps Talarurus is another taxon with only a handle and not a knob, which would fit a bit better with its chronologic position if not its phylogenetic position.

Regardless, the changes to the vertebrae of ankylosaurs, starting with Liaoningosaurus at least 122 million years ago and continuing on towards Gobisaurus about 92 million years ago, seem to have occurred long before ankylosaurs evolved a huge osteodermal knob at the end of the tail. Was a stiff tail as good a weapon as a full tail club with a knob? What drove the evolution of the knob so long after the evolution of a stiff handle? And why did ankylosaurs even evolve a tail club at all? Now that I've had fun investigating how ankylosaurs might have used their tails, and how the tail club evolved, the next question feels like it should be 'why'....so stay tuned for more tail club fun over the next year or so as I make an attempt at that question!


Read it for yourself! Arbour VM, Currie PJ. In press. Ankylosaurid dinosaur tail clubs evolved through stepwise acquisition of key features. Journal of Anatomy.

Know Your Ankylosaurs: Everybody's in this Together Edition

So with all of those posts about ankylosaur taxonomy over the last few weeks, what have we learned about the evolution of this group? Over the course of my PhD research, I was able to identify a bunch of new characters that seemed useful for understanding ankylosaur phylogenetic relationships, including characters related to the cranial ornamentation, pelvis, and osteoderms. Although ornamentation and osteoderms can be tricky, they can still yield useful information if you're careful about how you construct the characters.

Here's a sampling of some of the new characters from the supplementary file that goes along with the paper. Long live rainbow ankylosaur skulls.

With all the new information, here's what the results of the analyses gave us (click to embiggen):

From this, we can take away some interesting points:

1. Gondwanan ankylosaurs are probably not ankylosaurids, but they also don't form a single evolutionary group. Whatever "Minmi" is, it's a very basal kind of ankylosaur, possibly outside the split between Ankylosauridae and Nodosauridae. It's a little bit harder to say what's going on with "Antarctopelta" (previously considered an ankylosaurid), and the Argentinian ankylosaur: both came out as relatively derived nodosaurids, but my dataset isn't designed to test the interrelationships of nodosaurids. I wouldn't be surprised if future analyses incorporating more nodosaurids and more nodosaurid-based characters found that these two species were closely related. It would also be interesting to know which lineage of nodosaurids (probably a lineage from North America) dispersed into South America in the Late Cretaceous in order to give us these two ankylosaurs.

2. There are nodosaurids in the early-mid Cretaceous of Asia, but not necessarily the ones that have been proposed previously. Zhongyuansaurus, for example, was first described as a nodosaurid but is instead a junior synonym of the shamosaurine ankylosaurid Gobisaurus. However, a couple of taxa, like Taohelong, Sauroplites, and Dongyangopelta, are recovered as basal nodosaurids. At present, there doesn't seem to be much overlap between Asian nodosaurids and ankylosaurids, which is interesting! Why didn't nodosaurids hang on in Asia once ankylosaurids evolved, when the two groups seem to have coexisted pretty happily in North America later on?

3. The ankylosaurids from the Late Cretaceous of North America represent a dispersal of Asian ankylosaurines sometime during the early-mid Late Cretaceous. The earliest ankylosaurine is probably Crichtonpelta, from China, and North American ankylosaurines are a deeply nested clade within Ankylosaurinae. We propose the new name Ankylosaurini for the North American ankylosaurines (plus Talarurus, for now).

Here, have some frowny-faced rainbow ankylosaurs. Ankylosaurs are very serious dinosaurs.

4. Where do ankylosaurids first evolve? Unfortunately, that question isn't easy to answer right now: down at the base of Ankylosauridae, there's a mix of taxa from North America and Asia. The position of Gastonia as an ankylosaurid tips the scales slightly in favour of a North American origin for the clade, but some analyses recover this taxon as a nodosaurid, so I think we should be a little cautious about this result. One step up the tree, we've got a polytomy of Aletopelta and Cedarpelta (both from North America) and Liaoningosaurus and Chuanqilong (both from China). Does Ankylosauridae originate in North America with something like Cedarpelta, with a subsequent migration and diversification into Asia? Or does this group originate in Asia with something like Liaoningosaurus and Chuanqilong, and Cedarpelta represents an immigration into North America?

5. And finally, what's going on with ankylosaurids in the mid-Cretaceous of North America? Why don't we find any ankylosaurids between Cedarpelta and the later ankylosaurins? Did 'endemic' North American ankylosaurids go extinct during that time? And why does Aletopelta have such a weird basal phylogenetic position despite being from the Campanian? I don't really have answers for some of these questions, although if you come to the Society of Vertebrate Paleontology meeting in Dallas this October I'm going to try addressing some of them. For now, Aletopelta remains the biggest ankylosaurid enigma to me – it really shares very few things in common with the other Campanian ankylosaurids and I doubt it is an ankylosaurin from the Asian immigration into North America – could it represent a distinctive lineage of North American ankylosaurids stemming from things like Gastonia or Cedarpelta, for which we just don't have other representatives at the moment? Or, is it a nodosaurid masquerading as an ankylosaurid because I haven't sampled the right taxa or characters?

Darn you Aletopelta, why must you vex me so?

As usual, I wind up with more questions than answers every time I try to figure something out.

That wraps up the summaries for this paper, but stay tuned for some more cool research coming out in the next few weeks, and some summer fieldwork recaps!

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

Know Your Ankylosaurs: Mongolian Odds and Ends Edition

I'm back in civilization, so let's get back to ankylosaurs! Ready Set Go!

Gobisaurus, Zhongyuansaurus, and Shamosaurus

Shamosaurus is a really interesting ankylosaurid from the Zuunbayan Formation of Mongolia. Unlike later ankylosaurids, it still has a relatively long snout like you see in basal ankylosaurs and nodosaurids, and it lacks the distinctive tile-like skull ornamentation of ankylosaurs like Euoplocephalus or Saichania, instead just having a granular, pebbly texture on the skull surface. Gobisaurus, from the Ulansuhai Formation of China, is nearly identical in appearance, and only a few features distinguish these two taxa, namely the length of the tooth row relative to skull length and the orientation of the pterygoids. (Indeed, I think you could make an argument for subsuming Gobisaurus into Shamosaurus as Shamosaurus domoculus, but I'm generally reluctant to start making new combinations given that generic separation is pretty arbitrary anyway.)

Shamosaurus and its too-cool-for-school cervical half rings, on display in Moscow.

Gobisaurus and Shamosaurus are sister taxa; the name Shamosaurinae was proposed at one point and there's no reason to discard it at present even though it only contains two taxa. Shamosaurinae is the sister taxon to Ankylosaurinae. I also identified one new character that links Gobisaurus and Shamosaurus together which isn't present in other ankylosaurids: both taxa have a distinctive groove on each premaxilla, the purpose of which is unknown but there you go. There have been some suggestions that Cedarpelta (from North America) is also a shamosaurine ankylosaurid, and while I find the overall morphology of Cedarpelta to be pretty compelling for placing it in a clade with Gobisaurus and Shamosaurus, I didn't recover it with those taxa in my analysis (it came out more basally-positioned). However, I wouldn't be surprised if Cedarpelta winds up in Shamosaurinae at some point in the future as we find more specimens of both it and Gobisaurus and Shamosaurus.

Zhongyuansaurus was originally described as a nodosaurid ankylosaur partly because of its long snout, but it's indistinguishable from Gobisaurus (except for being smashed and flattened). The holotype is also a subadult (or at least not fully skeletally mature), since some of the cranial sutures are still visible towards the back of the skull. There are some interesting things going on with the postcrania of Zhongyuansaurus, but that's a story for a few weeks from now so STAY TUNED NO SPOILERS IF YOU'VE READ MY THESIS.

Tsagantegia

Of all of the more obscure ankylosaurs I looked at during my PhD, Tsagantegia might be my favourite for being the most surprising in person compared to what I had read about it. Tumanova included a line drawing of the specimen in her original description, which has been oft reproduced, but interestingly it doesn't really do justice to the original specimen (despite being a pretty nice drawing). The line drawing shows a long-snouted ankylosaur with amorphous cranial ornamentation, not dissimilar to Shamosaurus, but with a wider premaxillary beak more typical of later ankylosaurs. In person, however, the skull has distinct cranial caputegulae like we see in Euoplocephalus and Ankylosaurus! It's a pretty cool ankylosaur and I think it's probably really important to understanding the dispersal of ankylosaurs from Asia into North America and the diversification of ankylosaurids in the Campanian-Maastrichtian of Asia, but it's really hard to pin down the age of the Bayan Shiree Formation, and we don't have any postcrania for this taxon. I'm sure I'll be revisiting this guy in the future.

Heck yeah Tsagantegia!

Here it is again but in a more different view!

Talarurus

Way back when I originally started this blog in 2010, I had travelled to Korea to spend some time working in the Hwaseong paleo lab preparing Talarurus material and generally studying the ankylosaur material they had collected from the Gobi. Talarurus, like Tsagantegia, is also from the Bayan Shiree Formation but is clearly distinct. The holotype skull has very subtle cranial ornamentation that takes the form of small cones, rather than flat hexagonal tiles like Euoplocephalus, or bulbous pyramids like Saichania. Weirdly, this configuration is also present in the North American taxon Nodocephalosaurus – either this ornamentation style has convergently evolved, or, as I recovered in my analysis, these two taxa are closely related despite being fairly widely separated geographically and temporally. This is another ankylosaur that I'm sure we'll talk about again.

Talarurus butt in Moscow. The skeleton on display is a composite of several individuals from the same locality, and the skull is totally sculpted and a bit out of date.

Here's the holotype skull, with its weird, weird ornamentation.

Saichania

I've talked about Saichania fairly extensively here last year, but there were a few new things added in this most recent paper: Tianzhenosaurus and Shanxia (both from China) are, most likely, junior synonyms of Saichania, making this the most geographically widespread of the Asian ankylosaurids. Tianzhenosaurus has a nearly identical cranial ornamentation pattern when compared to Saichania, and I couldn't identify any differences that were outside of the usual ornamentation pattern variation we see in something like Euoplocephalus. Shanxia is known from the same formation but from a less well preserved skull, but the morphology of the squamosal horn is consistent with that of both Tianzhenosaurus and Saichania and therefore it probably represents the same taxon.

Next up: what's the big picture here, anyway?