Posts Tagged 'predation'

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.

Triceratops vs Tyrannosaurus

Perhaps the most obvious mainstay of dinosaurs in art and culture is the stand-off between the giant, fanged Tyrannosaurs and the horned and frilled Triceratops. It’s pretty much a cliché for dinosaurs that these two will fight each other when together and that the predator is always after his well-defended prey.

Obviously, I generally don’t think that tyrannosaurs (or most theropods for that matter) tackled healthy adult prey when there was a great selection of tasty, small, largely defenseless and naïve juveniles around. Now that makes sense at the best of times (and is borne out by what modern predators do, not to mention the inferences about dinosaurs) but a photo like this really makes you wonder.

OK, rexy is lying down (and it’s not the biggest specimen), but the difference in size is quite obvious. Triceratops is really, really big. Even in comparison to the ‘king’ it’s a huge animal with a massive skull, long horns, and a big body and a lot of weight behind it. There’s reason lions don’t generally tackle adult buffalo and won’t go near rhinos and that’s because they are too likely to get stompted, gored, or generally injured. This is a big and intimidating animal, and while eyeballing skeletons is not the best way of assessing complex behaviours, I do find it hard to believe that this was a normal prey choice for a tyrannosaur.

Theropod feeding traces

One of the most read posts on the Musings is that on my paper on theropod predatory ecology in which Oli Rauhut and I suggest that most big theropod predators probably targeted juvenile dinosaurs rather than adults. We covered a bunch of lines of evidence but one of those was the oft under reported area of dinosaur feeding traces. Quite literally bite marks left on bones by theropods.

There seem to be quite a few of these around according to some surveys, but they don’t get much attention. There’s a small (but growing) list of papers that describe individual bites or small sets of bites in some detail, and there’s a couple of review papers that talk about very large numbers of bites with little detail and not much in between. Thus while there are therefore at least several hundred documented bite marks, and probably quite a few more, only a dozen or so are actually described and illustrated in the literature. That makes it pretty hard to say much about them, since we don’t know what the distribution is like in terms of sizes, numbers, or any other obvious patterns.

I can see why this might often get overlooked, bites aren’t always obvious and nor can they always be easily distinguished from other forms of damage (though quite a few are very obvious). The obvious reason though, for not taking the time out to describe these, is that there is often only very little to say. It’s not at all clear, or even possible to tell most of the time, if the marks were made by an animal just biting another, that actually killed the other, or was merely scavenging a carcass. It’s not always even obvious which animal was bitten (if the marks are on an isolated bone) and it’s very hard to narrow down what the biting animal may have been too. Package all of this together and tooth marks are sadly, if understandably, relegated to being a ‘novelty’ rather than necessarily having anything interesting to say about predatory behaviour or trophic relationships.

However exceptions do exist, like the macerated Triceratops pelvis that’s full of deep T. rex tooth shaped holes, or the pterosaur bone with a dromaeosaur tooth wedged into it surrounded by score marks as the teeth scraped across the bones. These two examples actually demonstrate the two stereotypical bite marks with others usually falling somewhere between the two (obviously this means a dingle trace, there can be multiple kinds present on a bone or skeleton that has suffered bites). Either one will see deep punctures (or even ‘bite-through’ of the whole bone) that are tooth shaped – that is the tooth has penetrated the bone, or there will be scrape marks where the tooth has hit the bone surface and then been drawn across it. The latter is more common since few theropods probably had the power, or even desire to try and crack through big bones and of course we suggest that they probably weren’t even encountering them that often in any case.

Still, for all the problems that are associated with trying to pick out the details of feeding traces they can, on occasion, offer some illumination on the living habits of various dinosaurs. Like the example I’ll cover tomorrow with this shameless cliffhanger and obvious plug…

Szechuanosaurus strikes!

szechwanosaurs-feedingA photo from the Zigong Dinosaur Museum, this time of the excellent mount of a Szechuanosaurus with a small ornithopod its victim. I don’t normally go in for such combined mounts of specimens, but this is a great one, and with a nice balcony that overlooks it to allow you to see them form above and appreciate the set-up, something more museums could definitely benefit from. Yes you get a real appreciation for the size of a sauropod when stood next to its feet looking up, but you get a much better appreciation for the animal as a whole when you can stand above or level with it.

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