Waking up from hibernation.

And by hibernation, I mean grad school. The last few weeks have been pretty busy here in Edmonton and I've found myself without a lot of time to blog about interesting things that have been going on. Thankfully, that busy-ness is a result of research productivity and teaching, which are both good things! So, over the next few days, as we head into the (still somewhat cold) field season here in Alberta, I'll try to cover a bit of what's been happening for the last couple months...

First up: Alberta Paleontological Society Symposium

I can't believe it's been more than a month already since the APS symposium! The Alberta Paleontological Society is a group of amateur and professional paleontologists from around Alberta, which organizes summer field trips and an annual symposium and workshop series. The symposium is always a good time, and there's usually palaeontology professors and grad students from the University of Calgary, University of Alberta, and Royal Tyrrell Museum (plus often other institutions like the T. rex Discovery Center, Canadian Fossil Discovery Centre, Tumbler Ridge Museum, and Grande Prairie Regional College). This year I gave a talk about my travels in Korea, China, and Mongolia last summer, and was also invited to host a 3-hour workshop the following day.

 

 

Because I was expecting a largely adult audience (rather than families or mostly small children), I had to come up with something a little different than previous dino-workshops I've given. What do you do that's hands-on, not super boring, and not a craft? Well, two years ago I, and my fellow grad student Mike Burns, had given a talk about the frequent (and usually hilarious) misrepresentation of ankylosaurs in the popular media, and it had been a big success. So I decided to run with that. I brought with me a bunch of ankylosaur casts (skulls, a tail club, a foot), and some real fossils (osteoderms, thin sections, teeth), and lots and lots of popular reconstructions of ankylosaurs, mostly in the form of toys (or, uh...scientific models?) and books. These were scattered around the lab, which holds about 20 people.

 

 

 

Over the course of the workshop, I talked for about 10-15 minutes at a time about the anatomy of ankylosaurs, starting with a general overview, then moving on to the skull, skin/armour, tail, and legs and feet. We finished up watching a couple of clips from various documentaries and talking about posture, movement, and behaviour. Each workshop attendee picked 2 or 3 reconstructions to evaluate during the workshop, and had a worksheet to make notes about the anatomy of their reconstructions. After I would talk about some aspect of ankylosaurs, there was 10-20 minutes for looking at specimens, discussing the pros and cons of different reconstructions, and asking questions. Then I'd call everyone back together and ask who had the worst/best reconstructions and why.

If you're ever called to do a dinosaur or paleontology workshop for adults - do this! It was lots and lots of fun. It required fairly minimal preparation, which is a plus if you're often asked to do this sort of thing. I spent a couple of hours at most putting together a powerpoint of mostly specimen images, picking out books and toys and fossils to bring with me, and making up some handouts). It's a great way to engage adult learners, and could probably easily be restructed to work for kids or families as well.

More importantly, I think this might be a really effective way of communicating a lot of information about paleontology in a way that will encourage the general public to look at popular science more critically. Since most people will generally not be going to the primary literature to answer their paleontology-related questions, their information is going to come from illustrated books, magazine

articles, computer animated documentaries, and museum exhibits. If I had just discussed the anatomy of ankylosaurs via presentation and specimens, I'm not sure a lot of the points I discussed would have sunk in as much as they did by critiquing the illustrations and toys most people are likely to encounter. This was an easy way to make a talk more hands-on, and the casual atmosphere and conversation between the workshop attendees was really great.

The next APS Symposium will be held on Saturday, March 16, 2013. The symposium is always held in the Jenkins Theatre at Mount Royal University in Calgary, Alberta.

(Thanks to Angelica for taking some photos of the workshop!)

5 Questions for Aaron Leblanc

It's been a bit quiet around here lately as I plug away at my thesis, but here's an interview with UALVP alumnus Aaron Leblanc, now a PhD student at the University of Toronto. Aaron's description of the new mosasaur Eremiasaurus was published in the January issue of the Journal of Vertebrate Paleontology.

1. What inspired you to conduct this study?

My inspiration for this study comes from my work on an undergrad honors thesis with Dr. Michael Caldwell back in 2007–2008. I remember being shown potential subjects for my project and being drawn to the skull of what is now Eremiasaurus right away. Back then only the left side of the skull was prepared and really didn’t look like much, because most of the elements of the skull had been badly crushed and displaced from their original positions (Al Lindoe did a great job preparing the other side of the University of Alberta specimen, but that didn’t happen until I had started my Master’s thesis on the same subject over a year later). Even at that point you could tell that the teeth of this mosasaur were really unusual. I think the teeth alone were what drew my attention and got this whole project rolling.

2. How does a mosasaur wind up in Morocco?

By the end of the Cretaceous, mosasaurs were everywhere, but this wasn’t the case around 90 million years ago when the first mosasaurs took to the seas. At that point in time, mosasaurs were rather small, occasionally reaching lengths of two meters or so. More importantly, these early mosasaurs were probably still capable of walking on land and may have been restricted to shallow marine habitats. Coincidentally, the early fossil record of mosasaurs is quite restricted geographically. Most of the early mosasaur fossils (a group traditionally called the aigialosaurs) have been recovered from Croatia, Slovenia, and possibly from Texas. If you fast-forward to the Maastrichtian, the time interval from which Eremiasaurus is found, mosasaurs had become incredibly large (some reaching body lengths of 15 meters) and their remains have been recovered from every continent, including Antarctica. The occurrence of mosasaurs in Morocco isn’t a new discovery, but it is certainly becoming an increasingly more valuable data point the more we learn about their diversity and paleoecology during that time and in that particular region of the globe. The Late Cretaceous seas of Morocco were home to some very unique mosasaur taxa that occupied nearly every aquatic predatory niche, from hard shell-crushers like Globidens phosphaticus to (what I believe, anyway) fish-eaters like Eremiasaurus heterodontus.

3. What is special about the tail of Eremiasaurus?

Nobody paid particular attention to the tails of mosasaurs until a recent study of Plotosaurus bennisoni from the Maastrichtian of California by Johan Lindgren and colleagues back in 2007. Their work was incredibly important, because it challenged a traditional view of mosasaurs as resembling giant sea serpents, slithering through the water using side-to-side motions of their whole bodies to move through the water. What Lindgren et al. (2007) were able to show is that there is anatomical evidence that the tails of more advanced mosasaurs were much more specialized for aquatic locomotion than this historical view. The vertebrae at the base of the tail (called pygals) did not allow much side-to-side movement and the bony supports for a tail fin were pushed further back along the body. What this means is that at some point in their evolutionary history, mosasaurs abandoned that slithering style of swimming for a more thunniform (tuna-like) body shape and swimming mode. This represents a more efficient swimming style for cruising the open waters and engaging in pursuits of food, instead of lurking and ambushing. Mosasaur researchers aren’t as lucky as those that study ichthyosaurs (an unrelated group of marine reptiles), because there are no recorded cases of soft tissue outlines of the tails of mosasaurs. While it is only speculative, the current view is that mosasaurs had a crescent-shaped tail fluke like an ichthyosaur or a Great White shark. The vertebral column in well-preserved mosasaur specimens, including Eremiasaurus, has a gentle downward bend that starts just behind the pelvic girdle, and the tall neural and haemal spines (upwards and downwads projections of the tail vertebrae respectively) form a broad sweeping fan at the back of the tail that would have supported an expanded dorsal tail fin made entirely of soft tissue.

Where Eremiasaurus differs from other mosasaurs is in how far back the bony supports of the tail fluke have been “pushed” compared to other species. While it isn’t the most ichthyosaur-like tail ever reported for a mosasaur (though it is a close second), it still suggests that it adopted a more thunniform style of swimming than most other related forms.

4. What is unusual about the teeth in Eremiasaurus?

The teeth are certainly the most conspicuous features of the skull of Eremiasaurus. I always imagined it having a fiendish grin, especially when looking at the right side of the skull of the University of Alberta specimen. The teeth are unusual in that they change shape so dramatically along the length of the jaws and along the roof of the mouth (mosasaurs had a second row of upper teeth that lined the pterygoid bones, just for good measure). The front teeth are straight and cone-shaped,

the middle ones are slender triangular blades,

and the back teeth are more bulbous and hooked.

Whatever Eremiasaurus was eating, it was very good at tearing it apart into smaller more palpable pieces. The front teeth interlock when the jaws are closed, making for an efficient trapping mechanism, while the middle and back teeth are serrated and shear past each other like a pair of scissors to cut and dismember prey. The pterygoid teeth are curved backwards and would have kept struggling prey in the mouth. From an evolutionary perspective, attributing these kinds of teeth, tail anatomy and other unusual features of the skull to anything but a new genus and species was all too difficult.

5. What does Eremiasaurus tell us about the relationships among mosasaurs?

Adding Eremiasaurus to an existing phylogeny of mosasaurs added a great deal more complexity to the story, particularly within the subfamily Mosasaurinae (the group to which Eremiasaurus belongs). In many ways Eremiasaurus is an intermediate between two well-established groups of mosasaurines, something that was clear from the first phylogenetic analysis we performed. Where these relationships became more complex was in a second analysis in which we added and updated information to the existing data set. We performed this second analysis, because many of the characters and species of mosasaurines that were originally used back in 1997 by Gordon Bell were in need of revision. New species had been described since then and had not incorporated into such an analysis, while others had since been re-examined and re-described. Using this updated data set, we were able to conclude that many of the features previously thought to be diagnostic for one of the major groups of mosasaurines were actually present in most of the members of the other group as well, or were secondarily lost. Because of this, many of the previously well-established relationships within the subfamily Mosasaurinae broke down in this analysis, suggesting that there is a need to reconsider some of our classifications, or re-evaluate the characters that we use to define them.


Thanks Aaron! You can read more about Eremiasaurus in:

Leblanc ARH, Caldwell MW, Bardet N. 2012. A new mosasaurine from the Maastrichtian (Upper Cretaceous) phosphates of Morocco and its implications for mosasaurine systematics. Journal of Vertebrate Paleontology 32:82-104.

Gobi Desert Diaries: Nemegtomaia Edition

Today I've got five questions for Federico Fanti, the lead author on a paper published a few weeks ago in PLoS ONE on a nesting oviraptorosaur. I first met Federico during the 2007 Nomadic Expeditions Dinosaurs of the Gobi expedition, in which we all had a grand time prospecting for dinosaurs and during which we celebrated a fine discovery indeed.

1. What inspired you to conduct this study?

Well, the fossil itself! I grew up with incredible pictures taken somewhere in the Gobi Desert of Mongolia, with spectacular fossil remains literally emerging from the vermillion sand. When Phil Currie and I realized that we were looking at a nesting dinosaur we were simply happy and enthusiastic: there are only 5 specimens of brooding dinosaurs known to date in the world, it is a unique find. I couldn't wait to find out more about this specimen and finally, after more than four years, I'm glad to see the paper out.

 

(MPC-D 107/15 diagram from Fanti et al. 2012, by Marco Auditore.)

2. Nemegtomaia is not exactly a household dinosaur name. Who is Nemegtomaia?

Nemegtomaia means "good mother of the Nemegt" and curiously the name was chosen long before our discovery. In the '90s, the type specimen - including a nicely preserved skull - was collected from the Nemegt Formation not far from where we found the nest: however, no trace of eggs or nest were found at the time. The discovery of MPC-D 107/15 (or Mary, as I still like to call it) definitely supports the choice of Nemegtomaia as the name for this species. Nemegtomaia is a genus of oviraptorid dinosaur that inhabited what is today southern Mongolia during the late Cretaceous period, approximately 70 million years ago. It is characterized by a well-developed crest on the skull and relatively short forelimbs with robust claws.

(Nemegtomaia skeleton reconstructions from Fanti et al. 2012, by Marco Auditore.)

3. What's so special about MPC-D 107/15?

Unlike all other nesting dinosaur that have been discovered so far, this specimen has a nicely preserved skull and therefore it was possible to reliably refer MPC-D 107/15 to the genus Nemegtomaia. Furthermore, within the context of the Nemegt area where dinosaur eggshells are frequently recovered, it was possible to refer a specific egg type to this genus. In addition, the preservation of the forelimbs allowed us to reconsider the classification of this genus within the oviraptorosaurs: unlike many other oviraptorid species, in fact, Nemegtomaia has relatively short and robust forelimbs, indicative of different adaptations and behavior.

4. In the acknowledgements section of the paper you note that MPC-D 107/15 was excavated 'under what were at times difficult circumstances'. How did you find this specimen, and what were the challenges in excavating it?

I found the specimen while prospecting in a sayr, a canyon located not far from the Camp. It was barely cropping out from a vertical cliff, about 3 feet from the valley ground. A section of the nest and of the pelvis was visible at the time, meaning that, with the exception of the tail, the skeleton was still preserved in the cliff.

(Federico did well to spot the nest, which was hardly exposed at all in the surface - just eggs and legs in cross section.)

It took a full week and the work of several people to take it out, and I must thank all the people that participated in the 2007 fieldwork (including the author of this blog! [aw, shucks - VMA]) for the help in the field.

Difficult circumstances? A mix of heavy rain, collapsing blocks of sandstone alternated with 45 degrees in the shadow are .. interesting circumstances!

(Although I didn't spend much time working on the nest excavation, I do know what Federico, Phil, and Badam are referencing when they say 'difficult circumstances'. On the second-to-last day in Nemegt, the skies opened and it poured rain ALL DAY. The nest had to come out the next day, so a team went out to the site and worked under a tarp all day in the soggy, soggy desert.)

5. What does this specimen tell us about the nesting habits of oviraptorosaurs?

Nemegtomaia has been collected in both the Baruungoyot and Nemegt formations, which are representative of aeolian/desertic and fluvial environments respectively. This indicates that Nemegtomaia was a long-living genus and adapted to different environmental and climatic conditions. The nest preserves approximately 20 eggs: we know from other spectacular specimens of oviraptorid dinosaur that they were able to laid 2 eggs at time, thus we assume that different individual laid their eggs in a single nest. As a consequence, the animals that we discover in brooding position are not necessarily the parents nor the mothers. It is possible that a male was "selected" for parental care during early development of embryos.

If you haven't yet read Fanti et al. (2012), go get it right now for free from PLoS ONE! Thanks Federico!

Fanti F, Currie PJ, Badamgarav D. 2012. New specimens of Nemegtomaia from the Baruungoyot and Nemegt Formations (Late Cretaceous) of Mongolia. PLoS ONE 7(2): e31330.

Cool Stuff

"Cool Stuff: The University of Alberta Museums Do Winter" is a winter-themed exhibit that opened last week at the U of A's Enterprise Square location. I checked it out last weekend and was pleased to see so many different types of objects on display. We have 28 different collections on campus, and most (maybe all?) were represented in the exhibit - butterflies, moss, picked parasites, textiles, and more. 

The University of Alberta Laboratory for Vertebrate Paleontology contributed fossils from our Grande Prairie and Edmonton dinosaur bonebed excavations. Although we usually collect stuff in the summer, we've had snow during our Edmonton fieldwork, even in May.

We showed off some field jackets, too. The Monoclonius and ornithomimid are pretty self-explanatory, and if I recall correctly, "Skull B" is from the Wapiti bonebed in Grande Prairie. There were also photos from our December tyrannosaur helicopter lift in Dinosaur Provincial Park, including a photo taken by me!

Last spring we purchased a cast of the Cryolophosaurus original non-reconstructed skull for both teaching and research, but it fit in perfectly with the exhibition theme! The grey slab behind the skull is the Wonder Block from the MOTH locality, which has a variety of 'jawless fish'. Phil will be giving a talk about his 2011 Antarctic expedition on March 1, as part of the exhibition's speaker series.

The exhibition also features specimens from our zoology collections, including these Arctic and sub-Arctic mammals (caribou, deer, and walrus).

"Cool Stuff" mixes natural history objects with cultural heritage objects, and in particular I was pleased to see so many Inuit and Inuvaliut art pieces. I am always astounded by whalebone sculptures like this one.

Another display had Inuit dolls, musical instruments, hunting tools, and boots. The beautiful paintings in the background are the original art from Ted Harrison's "A Northern Alphabet". Click the photo to make it bigger, and see if you can figure out what letter each painting represents.

"Cool Stuff" is open until March 4, and admission is free. It was cool, go check it out. 

Spared no expense.

Well, this weekend marked a major milestone for me: I saw Jurassic Park on the big screen for the very first time! Although I have watched it countless times, first on VHS and then on DVD, Victoria in 1993 was only 9 years old, squeamish, and easily scared by, well, scary stuff, and thus too small to see Jurassic Park during its initial theatrical release. 

The film was being shown as part of Alberta Innovates - Health Solutions film series called Science in the Cinema. They feature films with a biomedical slant, and ask health science and biomedical researchers to do a Q & A after the film. In Edmonton, the movies are shown at a cool old theatre called the Garneau Theatre, and admission is free (with free popcorn, too!).

For Jurassic Park, they also asked if some students from my lab could come out to talk about dinosaurs, and so Scott and I brought along some fossils to show off before and after the movie. We are lucky to have casts of many of the dinosaurs featured in Jurassic Park, so we brought along casts of Gallimimus, Tyrannosaurus, and of course, Velociraptor. We also brought along some sturdy, real, touchable Edmontosaurus fossils from a bonebed in Edmonton, which were also a big hit.

There were a lot of good questions about both the genetics and palaeontology sides of Jurassic Park, and apparently there were about 450 people in the audience. It was fun to see good ol' Jurassic Park with an enthusiastic crowd of people who obviously knew the film well, and lots of younger kids who were seeing it for the first time. It has held up surprisingly well, and I maintain that the computer animation in Jurassic Park, which was the first time that realistic living creatures were created using that technique, is still some of the best computer animation ever. Yes, there are some inaccuracies, and yes, the theropods should be feathered, but overall for a film that is now 19 years old, it's not too shabby. ("Cool, it's an interactive CD-ROM!" and "It's a UNIX system! I KNOW this!" both got some pretty big laughs.)

5 Questions for Phil Bell

Hot on the heels of yesterday's interview with Caleb, here's an interview with Phil Bell of the Pipestone Creek Dinosaur Initiative. Phil is a former Currie Lab member who completed his PhD last spring, focusing on the Mongolian and North American hadrosaur Saurolophus. He recently published a paper on skin impressions in Saurolophus. Thanks to David Lloyd of the Tyrrell Museum for the great photos of work at the Dragon's Tomb in 2010!


1. What inspired you to conduct this study?

Actually, it was entirely by accident. Like so many advances in science, it came from an unresolved problem: were these two species of Saurolophus (S. osborni from Alberta and S. angustirostris from Mongolia) actually different or were they the same thing? I was at the American Museum in New York in the process of looking at the bones and skeletons of Saurolophus to try and answer that question. I mean, that’s what you do if you work with dinosaurs, you look at the bones. But I immediately struck upon a load of skin impressions. Like most people before me, I thought “that’s cool” but being the first time I had worked with skin impressions, I took the time to photograph, and draw, and measure the hell out of them. When I got to Mongolia later that year, I had the chance to visit one of the great (but little known) palaeontological treasures of the world, the Dragon’s Tomb (see Q3). This site preserves a herd of ‘mummified’ Saurolophus and you can still find loads there today. It was here that I started to notice differences in the skin impressions between the two species and from there I began my search for more specimens with skin impressions that have been stashed in museums from Mongolia, Poland, to Russia.

Phil with a block of Saurolophus at the Dragon's Tomb, Gobi Desert, Mongolia, 2010.

2. Who is Saurolophus?

Saurolophus is a hadrosaur or duck-billed dinosaur. Like it’s more famous cousin, Parasaurolophus (which actually means, ‘like Saurolophus’), it had a rod-like crest sticking out of the back of its skull, but unlike Parasaurolophus, this crest was solid. There are two species: Saurolophus osborni from Alberta grew to around 10 m in length, whereas the Mongolian Saurolophus angustirostris was a giant growing to 12 m in length.

Phil at the Paleontological Institute in Moscow in 2010.

3. What is the Dragon’s Tomb?

The Dragon’s Tomb is the name given to a site discovered by Russian palaeontologists (well, actually, it was one of their drivers who found it) in 1947 in the heart of Mongolia’s Gobi Desert. When they arrived, they found not just one but six or seven Saurolophus skeletons lying on a rocky ledge, most with skin impressions. The Russian’s named it the Dragon’s Tomb for obvious reasons and since then many more Saurolophus skeletons have been found there. Unfortunately though, fossil poachers have also relocated the spot and have caused irreparable damage by using dynamite to blast out skulls and skeletons to sell on the black market. But the place is so rich you can still find great stuff there. I’m involved in a project with Michael Ryan (Cleveland Museum) and David Evans (Royal Ontario Museum) to further explore this site and to figure out why exactly tens of Saurolophus died there.

The Dragon's Tomb, Gobi Desert, Mongolia, in 2010.

4. What is special about the skin of Saurolophus?

Well, for the moment it’s the most complex scale pattern ever seen in a dinosaur. People have known of dinosaur ‘mummies’ for 100 years (actually, last year was the 100^th anniversary of the discovery of the first, and in my opinion the best ‘mummy’ ever found; that of Edmontosaurus now on display in New York) but the complexity of their scale patterns has not been really appreciated until now. The stripy pattern on the tail of S. angustirostris was a complete surprise – one that I didn’t even believe when I first saw it. I thought it was a trick of the light or something to do with how the animal was preserved. But when I started to see it on more and more specimens, there was no denying it.

Preparing latex molds of skin. Fittingly, the latex brand is called "Dragon Skin".

5. Can different scale patterns tell us anything about the colour or colour pattern of Saurolophus?

That’s always a tricky question but without the actual colour preserved (as some people have shown with fossilized proteins that produce pigment) we can never be certain. One way of testing that question is to look at modern animals with scales (crocs, lizards, snakes). If you look closely at any of these animals you will notice that not all scales are born equal – some are big, some are small, some are long, circular or hexagonal. And they all have a function of some kind. Take a snake for example; most of the scales along its back are diamond-shaped and coloured in some way. But look at its underside and the scales are really wide, spanning the entire width of the animal, which they use to grip the ground when they’re on the move. They’re also usually a different colour to the top side. So, different shape, different function, and different colour. I’m not saying this is the way it always is but it’s a pretty compelling notion wouldn’t you say?

Thanks very much, Phil!

You can read the original paper here:

Bell PR. 2012. Standardized terminology and potential taxonomic utility for hadrosaurid skin impressions: a case study for Saurolophus from Canada and Mongolia. PLoS ONE 7(2): e31295.

And also see "Saurolophus skin suggests speciation" at Superoceras, and "Judging a dinosaur by its cover" at Dinosaur Tracking, for more coverage of this paper.

5 Questions for Caleb Brown

I'm very pleased to present another UALVP-related study today, this time by Caleb Brown (formerly at the University of Calgary and now at the University of Toronto). Caleb recently published a paper in PLoS ONE featuring one of my favourite UALVP specimens, our Stegoceras partial skeleton, UALVP 2.


1. What inspired you to conduct this study?

I was initially interested in pachycephalosaur postcranial anatomy for the purpose of differentiating between isolated pachycephalosaur postcranial material and those of basal ornithopods (like Thescelosaurus and Parksosaurus), on which I was doing my Masters research at the University of Calgary with Anthony Russell. In order to get a better understanding of the postcranial anatomy of these animals I went to the source, Stegoceras - UALVP 002, one of the best (if not the best) pachycephalosaur skeletons known, and the first postcranial skeleton discovered. In addition to other things, I was initially struck by the presence of large numbers of bony elements that I could not identify and that did not match the morphology of other ornithischians.

 

 

These elements looked superficially like gastralia, and indeed that is what they were identified in Gilmore’s 1924 description. But ornithischians were not supposed to have gastralia, so my interest was peeked. Investigation into the literature revealed that others had worked on these enigmatic elements; Marya´nska and Osmólska (1974) found similar elements in the tail of Homalocephale in Mongolia, illustrating they were not gastralia, and Sues and Galton (1987) correlated the structures between Homalocephale and Stegoceras. Particularly interesting was the articulated series found in the tail of Homalocephale. These showed a distinctive pattern that matched myomeres and myosepta, the sideways “w” shaped muscles and tendons, seen in fish.

Fortunately, with funding from Lubrizol Corp. and Montessori High School (University Circle, Cleveland, OH) I was able to accompany Michael Ryan (Cleveland Museum of Natural History) and David Evans (Royal Ontario Museum) to Mongolia to do fieldwork in the summer of 2009. I was also fortunate enough to be able to examine the Homalocephale specimen while I was there. This allowed me to test my ideas about the deep homology of these interesting structures.

2. What’s so special about pachycephalosaur tails, anyway?

First off, pachycephalosaur tails, like the rest of their postcranial skeletons, are rare. Often with dinosaurs you find the rest of the skeleton but are missing the most important part, the head. This is not true for pachycephalosaurs, which we know almost everything about based on the skull. You can count on one hand the number of partial skeletons known (and these are partial skeletons). What we know about pachycephalosaur skeletons is limited to these few specimens. They are special in that when preserved they show a unique morphology of having a halo of superficial “W” shaped elements forming a cylinder around the entire circumference of the tail. This is not seen in other dinosaur groups, or any other tetrapod. That is not the only odd thing though; they lack the deep longitudinal or paraxial tendons seen in most other ornithischian groups and they have elongated and highly bowed caudal ribs.  These three things may be related, but that is not yet clear.

3. What is the difference between gastralia, ossified tendons, and ossified myorhabdoi?

This is an interesting question with a bit of a complex answer. All of these structures are similar in that they are not endochondral bones, that is they do not develop from a cartilaginous precursor, which is the case with the majority of the postcranial bones in most taxa.

Gastralia are dermal or intramembranous bones that are associated with the abdominal musculature, and can be associated with respiration. They were likely the primitive condition for tetrapods but today are restricted to Crocodylia, Sphenodon, and possible the plastron of turtles (Classens, 2004).

The term ‘ossified tendons’ describes a variety of structures including ossified myorhabdoi. Although this term would include any ossification of the connective tissues articulating muscles to bones, its usage in dinosaurs, particularly ornithischians, usually refers to longitudinal paraxial structures along the dorsal or caudal vertebral series. These tendons often have the pattern of either a trellis or longitudinal bundles, can be epaxial or hypaxial, and are usually closely associated with the vertebrae (Organ 2006). Ossified myorhabdoi are restricted to the caudal musculature, and are essentially ossified myosepta. Unlike the majority of the paraxial tendons, these are superficial, forming a halo around the circumference of the tail where the transverse skeletogenous septum intersects with the integument, and preserve a morphology reminiscent of the undifferentiated myoseptal musculature of fish. They are also different in their histological structure (Organ and Adams, 2005). We still know very little about ossified myorhabdoi and hopefully discovery of additional specimens and more research on extant taxa will reveal more regarding their significance.

4. Why don’t other dinosaurs have a caudal basket?

It is often hard to answer why some groups have a structure while other don’t, and this becomes particularly difficult when the function of the structure is not fully understood. If the function of the ‘caudal basket’ is to rigidify the tail of pachycephalosaurs, then the reason that other groups don’t have it is because many have found a different solution to the same problem.  Many other ornithischians have longitudinal or paraxial tendons (usually called “ossified tendons”) in the form of a trellis or bundles. Some theropods stiffen their tail by extending the zygapophyses across numerous vertebrae. Until the function of these structures can be better established, we may not know the full significance of their occurrence.

 

5. Does the presence of a caudal basket tell us anything about head-butting behaviour in pachycephalosaurs?

The caudal basket likely had significant implications for the posture and locomotion of pachycephalosaurs. It has been suggested by previous authors that it helped the tail to act as a tripodal prop, potentially during intraspecific behaviour. It would also have greatly stiffened the tail. Our analysis is consistent with these interpretations, and in that manner is consistent with the idea of head-butting behaviour in pachycephalosaurs. 

The presence of the caudal basket has also been used to support the idea of agonistic flank butting behaviour in pachycephalosaurs (Goodwin et al., 1998), with the caudal basket acting as armor. We suggest that the morphology of the myorhabdoi is not consistent with armor seen in other groups, and this function in pachycephalosaurs seems unlikely.

Thanks very much Caleb! You can read more about pachycephalosaur tails in:

Brown CM, Russell AP. 2012. Homology and architecture of the caudal basket of Pachycephalosauria (Dinosauria: Ornithischia): the first occurrence of myorhabdoi in tetrapoda. PLoS ONE 7(1): e30212.