Variation of tail length in dinosaurs

So I have a new paper out and inevitably I’m going to talk a bit about it on the Musings. While I’ve had a few abstracts and the odd short paper out as sole author, this is pretty much my first proper effort in a major journal where I’m the only author. Not that I didn’t have help of course (which is what the acknowledgements are for) and I do especially want to take the opportunity up front to thank various people for their contributions and help, but most especially Susie Maidment for her help in data collection.

Right, onto the actual paper. Way back in 2010 I was looking at pterosaur tails in connection to an anuroganthid that turned up with (for one of them) an unusually long tail. This got me thinking about dinosaur tails and it struck me that while we obviously had some taxa with short tails (like Caudipteryx) and some looked pretty long (like Diplodocus) that no one seemed to have looked at just what kind of variation there was. Moreover, the more I thought about it, and the more I looked through papers and collections (and then later on asked various colleagues) the more often I came across ‘complete’ specimens that were nothing but when it came to the distal caudals. And so began my investigation into the tail lengths of the non-avian dinosaurs (though admittedly Archeopteryx sneaks into the paper as do the scansoriopterigids). Obviously the paper is there to be read, but hopefully this will serve as a quick summary and discussion of the basic points for those who can’t get it or don’t want to read it.

The first thing to note is that actually we really do have very few dinosaur fossils with complete tails. Despite a good hunt through the literature, a couple of collections, and exchanges with a number of colleagues I was able to track down very few specimens where every caudal was known. Even in things from localities like the Jehol and Solnhofen where skeletons are preserved in beautiful condition and soft tissues are common, there are actually very few specimens with every caudal vertebra preserved. Sure the sauropods might expect to do badly given how incomplete they always seem to be, and we’ve got more than a few dinosaurs known from only fragmentary remains. However, on the other hand we now have thousands of dinosaur fossils, and some species are known from dozens or even hundreds of good specimens and many of these are from sites of excellent preservation. But for all my searching and asking, I found less than 20 dinosaur specimens in total that have every caudal preserved. That’s really very low. Even things like ankylosaurs and dromaeosaurs with those lovely reinforced tails don’t seem to do any better either, complete tails are really, really rare.

Now there are a good number that are probably close to being complete with only a few distal ones missing, but obviously quite how true this may be is hard to determine. Sure there tends to be a general tapering of the size of the caudals which can give you a reasonable guess as to where it likely ends, especially if they are very small when they stop, but things like Diplodocus with it’s near endless rod-like caudals or the sudden stop in Nomingia means you could easily be wrong. In short, while a specimen like Sue we can probably have a pretty good guess how long the tail was and quite how much was missing, for plenty of other species it’s not going to be so easy. And things get worse from here.

Not only are there few dinosaurs with complete tails, but in one wonderfully illustrative case we have some major intraspecific variation. Two specimens of Leptoceratops are preserved side by side and so we can be confident that these aren’t just the same species, but are even from the same population. The problem is, one has 10 more caudals than the other, and their tails are proportionally rather different in length too. There’s quite a bit of intraspecific variation there, and indeed a look across other amniotes suggests that this is quite common – caudal counts and caudal lengths can vary a lot in tetrapod species. Tail length is sexually dimorphic in some snakes for example, and can vary a lot even in mammals.

Interspecific variation can be high too, which means it may not be safe to reconstruct missing tails from even close relatives. The wonderful little Epidexipteryx has the joint shortest tail known for any dinosaur that I found, but it’s sister taxon, Epidendrosaurus, has one of the longest tail known (and that one is incomplete and would have been longer still). While this might be an unusual case, there’s a decent bit of variation seen in a couple of other clades too.

All of this means that we need to be a fair bit more careful when talking about dinosaur tails and especially when it comes to recounting their size in terms of length. The length of a dinosaur is absolutely ubiquitous in the media as a measure of size and it turns up in a few papers too. However while some taxa are of course known absolutely in terms of their length, and many are probably about right despite being not entirely complete, others would seem to be little more than a best guess – and a best guess based on not very much to be honest. The data for sauropods in particular seems to be incredibly sparse and accounting for the inter- and intraspecific variation seen, I don’t think I’d be confident in reconstructing the tail of something like Argentinosaurs to within even a 50% error – it could be really long or very short and there’s no way of picking one over the other. Even ignoring some of the outliers, there’s a fair bit of variation there and can have quite an effect on the appearance of an animal.

Scott Hartman has been good enough to make this for me – a Spinosaurus with a short, ‘normal’ and long tail. All of these kinds of lengths can be seen in various theropods and to my mind are all plausible – indeed, we’ve been quite conservative here and could easily have copped off another hatful of caudals or plugged on a good few more and the results would still be quite plausible and within the bounds seen by other theropods. Of course note that while the length of that tail in each varies enormously, and as such, so too does the total length of the animal, the mass would not change that much. A 16 m long Spinosaurus sounds massive compared to a 12 m one, but if the only difference is in tail length, then in terms of mass there might not be much in it, just a few tens of kilos in a multi-ton animal.

So, estimating the length of a dinosaur without a mostly complete tail could give you a rather inaccurate number. There does seem to have been a fair bit of inaccurate information out there in the literature in the past with people giving ranges of caudal counts for groups when individuals were known with much higher values, and clades being described as having ‘long’ tails when they didn’t (or there was no real way to tell). However, there is a little more to this, I also did an analysis where (as far as possible, which admittedly wasn’t that far) the variation in tail length was compared to snout-vent length.

When examining living species, most biologists use snout-vent length as a proxy for how large animals are. After all, the tail length can vary a lot as we’ve seen, and even weight isn’t a great measure for a lot of living animals as it can fluctuate a lot on an annual basis, and of course isn’t available for specimens in museums. So a measure from the tip of the snout to the vent / anus is a common measure of size but we don’t seem to use it much in palaeontology (and certainly not for dinosaurs). In short therefore, we’re using a measure which not only includes a lot of variation in the tail that might screw up the results (and that most of the time we don’t know for sure anyway), but it’s not compatible with other datasets on extant taxa. The question is though, would the equivalent be any better for dinosaurs?

My simple analysis suggests so – that from the available data, tails are rather more variable in dinosaurs than the body. As for the vent, well, that we obviously don’t know exactly as a decidedly soft tissue structure so I plumped for the last sacral being a point that would be close to the vent and an unambiguous point on the skeleton that would be easily identified and would likely be preserved. This measure (snout-sacrum) is one I suggest we should start using when we want to talk about dinosaurs sizes in terms of length.

So there you have it. We don’t seem to have too many dinosaur tails, those we have suggest much inter- and intraspecific variation and so estimates of total length or using total length may not be very reliable. Snout-sacrum length is probably more reliable and in any case would bring the data in line with that used by most biologists. My final note though is an appeal – despite the work I did trying to uncover dinosaurs with complete tails, I’m sure I’ve missed some. Perhaps they’ve simply not been described, or are squirreled away in obscure journals, or are only listed as paratypes etc. I have seen a couple of things published since this work was finalised that look like the tail is complete but where the paper doesn’t actually say and it’s not entirely clear from the figures. I can’t believe that some of those massed ranks of undescribed Psittacosaurus, Protoceratops and various massed ranks of hadrosaurs and iguanodontians don’t have a few more complete ones lying around that can be measured. So if you do know of any specimens out there with complete tails (and better yet, totally complete specimens in terms of the skull and vertebral column) do please let me know. I’ve exhausted all the easily available avenues to date, but I’d love to do the analysis again with much more data. One day.

Hone, D.W.E. 2012.Variation in the tail length of non-avian dinosaurs.Journal of Vertebrate Paleontology, 32: 1082-1089.

A post about a tail with no pun in the title

So onto the last major post about Bellubrunnus – the tail. The tails of rhamphorhychines are interesting as they have a somewhat unusual anatomy. Just like the dromaeosaurs (though obviously, convergently acquired) rhamphorhynchines have elongate zygopophyses and chevrons that overlap multiple vertebrae and bind the tail together so that it is a relatively stiffened structure. These extensions are basically rods of bone, and can long enough to overlap the four or five adjacent vertebrae to their origin.

However, Bellubrunnus appears to be unique among rhamphorhynchines in lacking these structures. It still has zygopophyses and chevrons (as can be just about seen in the figure) and compared to some pterosaurs at least these are long, but they are a fraction of the length of other closely related species. This begs the obvious question, of what happened – are these really reduced or is there some other factor at play? Well, there are a number of possible hypotheses to explain this and here I’ll go through them and why we have come to the conclusion that this is a genuine feature. Not all of the chevrons are there for sure, so some have been lost or remain cryptic for whatever reason, but several are quite well preserved and so the below discussions refer to the issue of the size and shape rather than presence / absence of chevrons.

First off, were these simply not properly ossified prior to preservation and were there, just as unpreserved cartilage? This seems really unlikely, even the smallest and youngest specimens of Rhamphorhycnhus (which includes specimens even smaller than Bellubrunnus) have fully ossified chevrons and zygopophyses. These seem to be present at a very early age in the pterosaurs that have them and so it would be odd if they had a different ossification pattern here. Indeed it would be doubly odd since if anything, Bellubrunnus has better ossification of its elements than is usual for small pterosaurs, with well-preserved and ossified tarsals and sternum. So it is far more likely that they are fully ossified, they are just much smaller.

Could these have been lost through decomposition or disassociation from the specimen, or may not have been preserved even if they were ossified? While a few pieces have begun to disarticulate and move (the gastralia and a few dorsal centra and the prepubes) nothing else on the specimen has really begun to move and indeed the caudal vertebrae as a whole is well articulated. It would be odd indeed if the only thing to have rotted or moved was the chevrons or parts of them, while the rest remained intact and complete. Similarly, with even tiny parts like the delicate palate and gastralia being preserved (and indeed at least a couple of chevrons) it also seems unlikely that these somehow resisted preservation when everything else is there. Certainly it’s possible, but even so, some are still there and the loss of many chevrons would not affect the shape of the ones that have survived and wouldn’t affect the zygopophyses.

Were these destroyed or modified through preparation? This is a tricky one to answer, but again, there’s no obvious reason to think so. The preparation job is superb (look at the skull!) and was done with great care and attention to detail. It’s always possible that thin and delicate structures were lost, but while the matrix has been all but scraped clean, at least some have survived (and again lots of other delicate things) and I find it hard to imagine they were modified in such a consistent manner.

Are these genuinely distinct then? That is the obvious conclusion given the problems with the other hypotheses. However, there is more than just negative arguments against these other ideas, but there are reasons to positively support the idea that these are genuinely short. Although greatly reduced in length, the anatomy of both the zygopophyses and chevrons is otherwise well consistent with the anatomy of these features in other rhamphorhychines. The zygopophyses are rather rod-like and then taper abruptly to a point, just as we see in others, only with a much shorter rod. And similarly the chevrons are long and thin and splint-like, terminating in a point at each end, just as we see in others, only again being rather shorter.

While as noted we are rather short of chevrons for whatever reason, those that remain and indeed the zygopophyses seem to have a genuinely distinct morphology to other rhamphorhynchines, including similarly small and young specimens of Rhamphorhycnhus. This then is a major anatomical difference that separates Bellubrunnus from its nearest relatives as well as providing a little more interest and intrigue in the origins of this structure since while it is present on all other rhamphorhycnhines, it’s also present in basal pterosaurs outside the group and so may well have had multiple origins and losses.

The Grant Museum of Zoology

For the last couple of months I’ve been doing some on-and-off work in the Grant Museum of Zoology in London. I had dropped into this place a few times before in the past, but recently the collection has moved (all of about 100 yards down the road) to a new and more spacious setting. The museum was started, and remains, a teaching collection for comparative anatomy and as such is devoted to zoology alone and retains a great many and varied specimens on display.

As with the traditions of older museums (like Oxford and Dublin for example), material is everywhere and there’s a lovely cluttered feel with every cabinet and shelf full of specimens. While it can be a nightmare to photograph in situ, each specimen can be see quite clearly so as a visitor it’s fine. A lot of the material is grouped taxonomically providing great opportunities for comparing details (and there are some fossils in there too), though there are small asides for relevant collections such as a case devoted to dissected heads, or one comparing different ways of preserving zoology specimens, or recently extinct taxa (featuring a quagga and thylacine skeleton, and a skin of the latter).

The vertebrates do especially well and there’s a super skeletal collection of the mammals in particular. There’s a nice line in having skeletons next to taxidermy or pickled specimens too which is great, and all manner of odd and unusual pieces that are rarely seen on displays. If you want to see a leopard seal skull, pickled baby aardvark or stuffed golden mole, this is the place for you. All in all this is a superb little museum and for those like me who do like their anatomy and simply want to see lots and lots of specimens, this really is a must.

Animals Inside Out

Friday night saw me being lucky enough to get into the superb new exhibit at the Natural History Museum. Featuring plastinated animals and anatomical dissections, this is an amazing looking inside both some well-known and unusual animals. Since I’m not really one for great anatomical details on the blog and the fact that there is a very good slide-show here online, I’ll stop here for any great analysis.

Suffice to say though, that if you are interested in anatomy or biology in general, then this really is a must. It does show things nor normally seen (even to those familiar with dissections and anatomical books and papers) and gives a greater appreciation and understanding of how things fit together. Even jaded experts seemed quite thrilled with some things like the ‘exploded’ elephant and the sectioned giraffe.

If there is a complaint it’s the almost complete lack of signs and explanation of things. I suspect too many people will look at things and say ‘cool’ but come away with very little increased knowledge and understanding of what they saw, even if they have got a much great appreciation and love of the beauty of nature.

Anyway, for those who can go, go. For those who can’t, well, at least there are some pictures.

What’s in John’s freezer?

Most people interested in dinosaurs will be aware of the research done by John Hutchinson and his group on tyrannosaurs and other Mesozoic critters with regards to their anatomy and biomechanics. But feeding into this research John does a lot of work on big extant animals (like elephants, crocs, rhinos and ostriches) both alive and in various pieces.

It’s this latter aspect of his work which has now made the leap into cyberspace and John has taken the plunge and started blogging. The title (as for that of this post) is ‘What’s in John’s freezer?’ to represent the odd and occasionally gruesome collection of animal parts he has in his walk-in freezers at the Royal Vet College in London.

Unlike most new blogs, John already has more than a token ‘here’s my new blog’ post up, so head on over there to see his first efforts looking at scanning giraffe legs. Enjoy!

 

An articulated alvarezsaur pes

Some well-know and well studied taxa can often have little bits consistently missing from their fossils and it can take many years before every part of their anatomy is known. In the case of the derived alvarezsaurs it was only very recently that a complete pes was known with the discovery of Albinykus. This was rather unfortunate for me, as it somewhat renders much of my new paper redundant.

Yes a couple of years ago while out prospecting at Bayan Mandahu I came across this delightful little alvarezsaur pes. What is nice about it though is that it has digit I intact, something unknown in the clade (well, when I found it and started work describing it). The piece was actually found within a good stones throw of the points where both Linheraptor and Linhenykus were found, so that’s clearly a good spot to be working in. In this case we refer the pes to Linhenykus given the similarity in form and that it was found in the same locality, and perhaps even the same horizon since both the holotype and this specimen were preserved in nodules.

Linhenykus pes from Hone et al. 2012. Scale bar is 10 mm

So much so not especially interesting perhaps, having been beaten to the punch. True, but we were able to expand our discussion a little in looking at the shape and structure of the metatarsus. Alvarezsaurs like a number of derived theropod groups have a ‘pinched’ metatarsal III resulting in the arctometatarsalian condition. The evolution, structure and function of this set-up has been discussed at some length in a variety of papers, but we were able to provide some extra little details which seem not to have been looked at previously.

While it recognised that despite the proximal and posterior constriction, some metatarsal IIIs have a mediolateral expansion – in other words, looking at the front of the element, it can look quite fat a way up the shaft. Our examination of this and subsequently other specimens suggests that this is a thin flange of bone that sits on top of mts II and IV. This doesn’t seem to have been mentioned before (and I was pleased to see that the referees, both of whom have looked at this extensively, agreed with this assessment) and is potentially quite odd. The structure of the arctometatarsus seems linked to running efficiency and reducing motion between elements. However, having a flange of bone here on the anterior face would presumably help stop mts II and IV moving forwards relative to III, or III posteriorly relative to the others, or even both. Only this seems a rather unlikely problem for even a highly cursorial animal. This is clearly beyond the scope of a short descriptive paper to deal with, but it is (I think) an interesting observation.

Related to this, the exact shape and structure of mt III in this specimen compared to the holotype of Linhenykus is a little different. One might expect such a derived and specialised condition to be highly conservative which suggests the selection is potentially quite weak on this specific structure. It also hints that there may be more variables and details to the arctometatarsus than previous realised.

As a final little note, this paper is out in Acta Palaeontologica Polonica so is freely accessible, though currently as a horrible uncorrected proof, as indeed is the long and detailed description of Linhenykus that follows last year’s brief work in PNAS.

Hone, D.W.E., Choiniere, J.N., Tan, Q. & Xu, X. An articulated pes from a small parvicursorine alvarezsaroid (Dinosauria: Theropoda) from Inner Mongolia, China. Acta Palaeontologica Polonica, in press.
Xu, X., Upchurch, P., Ma, Q., Pittman, M., Choiniere, J., Sullivan, C., Hone, D.W.E., Tan, Q., Tan, L., Xiao, D., & Han, F. Osteology of the alvarezsauroid Linhenykus monodactylus from the Upper Cretaceous Wulansuhai Formation of Inner Mongolia, China, and comments on alvarezsauroid biogeography. Acta Palaeontologica Polonica, in press.

Dinosaurs labelled

Inspired by the posters featured in this post of mine Mike Taylor has gone and made this picture comparing a sauropod and theropod to show the homology of all the elements.  He’s stuck it up on SV-POW, but is encouraging people to use it in their teaching so I thought I’d repost it here. Thanks Mike and great job.

Sign of the times – dinosaur anatomy

While we’re on the subject of signs and notices, this one especially caught my eye. While it’s obviously in Japanese, it’s clear than it labels all the major bones of the skeletons. This is great for several reasons. First off it shows people are interested in anatomy and that using technical terms is not going to put people off. Moreover it shows and helps people to grasp that for all the differences between these species they are at a fundamental level built to the same plan, obviously with four legs and a neck and ribs and tail etc. but with things like lacrimals and frontals too – there is a lot of things in common and the pattern of bones is effectively identical. You can really appreciate the similarities and differences and follow that while they must have much in common (a common ancestor in fact) they have also diverged from that. Finally, it does provide a frame of reference for people as a whole – I’m sure many of the people reading this sign had been told by a doctor they’d broken a humerus, or their child fractured a tibia, had a malformed metatarsal or needed to see a maxillary surgeon or whatever. These were uncommon terms to them in a hospital, but I bet they remember those words and will see the link to the signs on the wall and thus the beasts in front of them. It’s an excellent little reminder and display of how all vertebrates are linked together.

Scutes and osteoderms

The two terms of the title are often used somewhat interchangeably (and I’ve been guilty of that myself in the past) but in fact they have rather different meanings, despite their close relationship. Here is a series of fossilised crocodile osteoderms that I picked up a few years back. The term osteoderm literally means bones in the skin and that’s exactly what these are, piece of bony armour that sit in the skin.

Scutes on the other hand are the keratinous sheathes that cover the osteoderms. You can see some of these here in this croc with those bigger chunky scale-like pieces running along the back. That of course means all those fossil ‘scutes’ from things like crocs and ankylosaurs and titanosaurs should really be called osteoderms. In fact while of course claws and similar things do preserve from time to time, and there are bits of horn sheathes etc. out there, I’m not aware of any scutes being known from the Mesozoic (though not working on any of the more obvious groups, I could easily have missed them).

Plateosaurus pes

Since I seem to be writing about saurischian feet a lot at the moment, I might as well continue and bring you this Plateosaurus pes. Obviously we’ve moved some way from derived theropods and down into the ‘prosauropods’. Still, there are some interesting things here which throw up echoes of the ancestry of this group. While Plateosaurus was one of the most derived and biggest prosauropods, it’s feet are, in a few ways, quite what you would expect from something that diverged not *so* long ago from a small, cursorial predator.

First off the claws are rather big and quite sharp. While obviously even quite derived sauropods can have big claws on the feet, including especially large and pointed ones on the first toe, these are rather more narrow and curved. The fifth toe is also rather reduced with a short metatarsal and just a nub of one phalanx so there’s not much going on here. It’s hard to imagine this foot carrying a huge amount of weight as the metatarsals are relatively compact, and their disjunct lengths and positions would not do too well under major loads.

We shall call it ‘Mini-hallux’

And it didn’t even cost me one million dollars either. This post is little more than an excuse to show of Tyrannosaurus feet again but it does give me the chance to talk about the hallux a little more. This has had a bit of a mention in the past with the issue of it’s reversed (or otherwise) condition in basal birds. For those who are lost, I should really mention that the hallux is slightly unnecessarily jargon-y word for the first toe of the foot (the equivalent of the human big toe). While this has a key role in perching in birds, in the non-avian theropods it was rarely up to much. Even the earliest forms had ditched the fifth toe, and basically walked on their middle three toes, with the hallux rather reduced and lying off to the side as you can see here. In fact this is even lost entirely in a few, including the derived ornithomimosaurs.

The first metatarsal is also positioned at the midpoint of the 2nd as well. You might expect it to have shrunk such that it remained up at the top of the foot, with all the metatarsals in a line, but that’s clearly not the case. In fact, there is even a bit of an articulation on the 2nd metatarsal which is useful as in many specimens the hallux is lot but it’s original position can still be clearly marked by this.

Guest Post: trabeculae and diamond-tipped saws

Bone thickness model of trebeculae. Courtesy of M. Doube

Today’s guest post comes from Michael Doube. I found out about Michael’s work though my friend John Hutchinson at the Royal Vet College in London who regular readers might well recognise as a major dinosaur researcher, though John often dabbles in extant organisms as a basis for his palaeontological research. In this case Michael and his team were looking at trabeculae, those little supporting threads of bone. Their pattern of distribution can potentially tell us quite a lot about both extant and extinct species and Michael explains: