Posts Tagged 'dromaeosaurs'

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.

Deinonychus at the AMNH

Since the theropods are doing well this week, it’s time to wheel out another image kindly sent in by Steve Cohen. I suspect there’s a nice mount of Deinonychus in a great many museums in North America, but for all my traveling, I’ve only ever seen two of them (part shown here) and didn’t have time to study either in any detail. Here though is an excellent and indeed famous mount in the AMNH of this dinosaur and it would probably be even more impressive if Steve hadn’t sent this to Heinrich Mallison as well and he’d not put it up a couple of weeks back….

Guest Post: Yurgovuchia doellingi

Those keeping up with the scientific literature will know that a new dromaeosaur was described just the other day. One of the authors, Jim Kirkland, has been kind enough to pen a few lines about the discovery and has included some nice photos of the excavation too. Enjoy:

More on dromaeosaurs vs azhdarchids

Yesterday I covered the basic introduction to my new paper about a Velociraptor specimen with an azhdarchid element preserved in it’s gut. Today I want to move on from the basics (what is there) to what this potentially means and how this is inferred. Most of my recent research is based around theropod ecology and behaviour (like this, this and this for example) and specimens like this one can provide new information and evidence for how these animals were acting. The obvious question here is why is this inferred as scavenging and not predation? As usual with such questions going so far back in time, it’s hard to be definitive, but this is the better supported inference.

First off there is the relative sizes of the animals. While it’s not unknown for predators to tackle other predatory animals, or relatively big prey it’s certainly not normal. Lions don’t routinely hunt leopards or bears go after wolves. This is relevant here since azhdarchids were most likely active predators themselves and so a potentially dangerous animal to attempt to kill. Moreover, the azhdarchid in question was most likely 9 kg in weight with a 3 m wingspan (and could have been considerably larger), while the Velociraptor was a sub-adult of around 13 kg. In short if this was a predation it was no mean feat – perhaps the equivalent of a small coyote bringing down a big eagle. Sure it’s possible, but it’s not unreasonable to think this was really very unlikely. It’s more likely this was a young carnivore scavenging on the carcass of a dead pterosaur, as indeed was inferred for a similar previous specimen from Canada.

Even if we assume that it was a kill, other things don’t add up well to support this. Theropods don’t tend to consume large amounts of bone like this. They might consume relatively large items (like a whole small prey item) but not large chunks of bone like this. And it is a pretty big chunk of bone, probably the same length as the skull of the dromaeosaur. Moreover, we also know that theropods can be really quite delicate feeders, including other velociraptorines. The tendency seems to be to scrape meat free of the bones, now chew up and swallow whole ones (like modern birds of prey, they’ll swallow a mouse, but will pull chunks off of rabbit or sheep). Carcass consumption patterns by modern vertebrates also show that whole big bones like that don’t tend to be swallowed. Finally, the pterosaur weighted at least half and potentially more than the dromaeosaur. Given their apparent skill at stripping a carcass of meat I don’t think I dromaeosaur would be swallowing whole bones (and ones that would be pneumatic, not filled with marrow) when much of it’s own weight was sitting there in muscle and viscera.

In short, predators don’t normally predate other predators. Predators (including theropods) don’t usually seek out large prey. Predators (including theropods) don’t usually consume large bones of large prey unless they are a bone specialist or there’s nothing left. Even when there’s not much meat left, theropods tend to scape this free to eat rather than swallow bones. Sure all of these could hit the ‘least likely’ option and it’s a who-knows-what to 1 chance that a small dromaeosaur took on a big azhdarchoid, killed it and started swallowing big bones. But it’s far more reasonable to infer that it scavenged the last bit of a carcass it chanced across.

We are then left with scavenging as the most likely explanation as to why this animal was swallowing whole bones. Interestingly, we do also see shed teeth being a common feature of dromaeosaur (and indeed theropod in general) feeding yet here every tooth in the skull is intact. That is admittedly merely a soupscon of evidence for scavenging, but one might well expect a tooth or two to be lost during a fight with such a big adversary. or even biting through bones to swallow them again suggesting it just picked up and swallowed what it could find without much or any oral processing.

Uncoloured version of Velociraptor feeding. Courtesy of, and copyright to Brett Booth.

Moving on from this issue then, what does this tell us about the ecology of dromaeosaurs? Well to  degree, not much we didn’t know already. There’s already evidence for both predation and scavenging in the dromaeosaurs, and indeed already evidence they were eating pterosaurs. Even so, more evidence is always good, and it does at least reinforce the existing evidence we have. It also therefore takes us a little further away (sadly) from the idea that dromaeosaurs were some kind of hyper-carnivorous super-predator that spent their time knocking down huge prey items with all their claws and teeth. I say sadly, because it’s a great idea and a wonderfully romantic notion, but sadly these animals were every bit as opportunistic as other carnivores and clearly were not beyond taking the odd, or indeed regular, free meal through scavenging. Indeed given the number of specimens we now have supporting a scavenging interpretation, this does seem to have been a pretty common part of their behavioural repertoire as carnivores.

Dromaeosaur tails again

Since I have all these nice photos of Sinornithosaurus that show off some nice bits of anatomy, I’m trying to make the best of them. A while back I made mention of the enormous zygapophyses of dromaeosaur tails that lead to their rather rod-like appearance. The photo I had to hand to illustrate this was not bad but was far from great. This one is a bit better and shows a block of vertebral centra (lying in the middle) with the various zygapophyeses and chevrons lying wither side extending alongside.


There is a significant danger of this turning into Sinornithosaurus week since I now have a nice set of photos I took recently when the holotype went on display at the IVPP. As one of the first described feathered dinosaurs (and certainly on that has much better preserved feathers than Sinosauropteryx) it already has had a lot to say about dinosaur evolution. Although rather squashed and disarticulated the skull is also in pretty good condition and so too are the teeth.

Here’s just a general photo of the whole specimen and a couple of close-ups of the skull and the hand (the former is a bit dodgy owing to some nasty shadows so I had to play around with the image to get it this ‘good’). This animal shows off quite a few nice bits of dinosaur / theropod anatomy so I’ll be going over it in more detail in the next few days. I’m not foolish enough to waste all this material on just one big post! Not when I’m this busy and this short of good images at least.

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