For those who are regular readers this post clears up the mystery of the Thursday post –my new paper is out. It’s time for me to be a bit self indulgent on here as I have a new paper out that I imagine will be of interest to quite a few people on the web. My interests in palaeontology have always veered strongly towards behaviour and ecology and this is my first paper that deals with it in some depth, so please allow me to expound on this at length and forgive the excesses. Right, onto the actual content!
A quick skim through the palaeontological literature (in all its manifestations) would suggest that the big carnivorous dinosaurs spent their time trying to get their teeth and claws into all manner of big adult herbivores. Tyrannosaurs tackling giant ceratopsians, allosaurs slicing into huge sauropods and carcharodontosaurs picking on the great hadrosaurs among others are commonplace. While I’ll happily admit that most of these scenarios usually come from book covers, palaeoart and the odd life reconstruction rather than detailed descriptions in technical papers, these do crop up in proper papers from time to time and certainly very few people have ever challenged them to any serious degree, so they have prospered unchallenged.
Well, I’ve taken a fair pop at them in this paper, and I rather suspect there will be a backlash by those who misread, misunderstand, over interpret, misinterpret or simply have not read my words, so I’m going to try and get my revenge in first. This is a new and as yet untested hypothesis and there are lots of caveats that I detail and lots of ifs, buts and maybes, so if you want to ask a question do please read this carefully first (and the paper too, obviously there’s loads more in there that I don’t cover here), it covers quite a lot of ground. The long and short of it is that I think that the concept of big theropods tackling comparably sized large adult prey would have been a rare event – the exception not the rule. There are a fair number of reasons to support this contention and some more to suggest possible problems and I’ll go through them at tedious length. So hang onto your hats, it’s a thrill a minute here! Possibly.
The thesis set out in the paper (written with Oli Rauhut [L], my boss from my time in Munich I should mention at this point) is that in the main theropods stuck to predating on juveniles and largely left the adults alone. I’m only really referring here to big theropods on the grounds that small one would probably not have been able to trouble big herbivores (though many of the concepts would probably apply to small theropods tackling comparatively small prey too). The real concern here is the idea of bringing down multi-ton plant eating giants, so we’ll stick to the bigger theropods (themselves ton and multi-ton animals). There is also a blurred distinction between the adults of small species and the juveniles of big ones as prey, though in addition to their size as a whole juvenile animals suffer several penalties in the face of defence towards predators.
When we look at the fossil record for theropod predation we first off see that there is only a very limited amount of it. That is, while there are famous incidents of theropods leaving bite marks on bones or having wounded or eaten other animals, the record is really pretty limited. This is odd in a sense, as given that there were a great many theropods out there, and lots of prey, they have to eat and really should be leaving more of a mark (so to speak) on the bones of their prey species. While one might think that of course any animal that was killed for food would simply be eaten and this could not be fossilised this can’t be the whole truth of the matter – it’s hard to see any theropod crunching its way through a brachiosaur femur and actually eating it for example – but you would expect to at least see some tooth marks when they stripped the flesh from it. Even in an open environment like the African savannah with plenty of predators and scavengers you will often see half bones and skulls sitting in the open which of course at least have the potential to be buried and preserved. Certainly the fossil record has all kinds of biases and a dead and half-eaten animal is not necessarily going to be buried or found (and see below) but the record is still remarkably low considering the numbers of specimens we have and the relative rates of bone damage seen in mammalian faunas, both extant and recently extinct. In short, if theropods are going after big animals, we should see the damage wrought on big bones, and that is quite rare.
This suggests that perhaps we are missing such traces because theropds were exceptionally good at destroying bones in the fossil record either through exceptional bone consumption or digestion or both. This doesn’t really stand up though. While the big tyrannosaurs certainly seem to have some adaptations towards attacking and breaking up bones the others do not, and obviously some bones would be too big and too robust for any theropod to tackle effectively (brachiosaur femora again). In terms of digestion, both birds and mammals tend to pass out pretty significant amounts of bone that they consumed, whereas at least some crocodiles can pretty much eliminate all bone in their digestive system, or at least turn it into a liquid and not leave any recognisable fragments. Theropods would seem to be closer to the former than the latter however. Despite evidence of acid etching on bones and fish scales in theropod stomachs we can still identify these fragments (they have not been destroyed) and there are also coprolites containing identifiable bits of bone (in the sense that we can tell femurs from vertebrae and what clade they came from, even if crunched up and acid worn). If even the biggest, most bone-crunching, best adapted bone feeder like T.rex can eat a small juvenile hadrosaur and still pass out bits that we can identify then one would expect that if they were chowing down on whole vertebrae or humeri or just bits of adult bones we’d find these too as stomach contents, but we don’t. They just don’t seem to be eating big bits of bone, or for that matter big bones. In fact on the rare occasions we find anything what we do find are bits of juvenile dinosaurs as stomach contents.
Turning to living animals we see two intertwining patterns that are common throughout not just the amniota, but really across metazoans as a whole (at least as far as they have been studied and as far as I can track them through the literature). First we see that in general predators prefer to take juvenile prey over adult prey (and indeed juveniles are, for a variety of reasons more vulnerable to predators than adults, and not just the most obvious ones like being small and lacking big horns or whatever amking thme safer for predators to attack) and indeed predators still prefer a juvenile of a big taxon over an adult of a small one. Secondly, (and unsurprisingly to anyone even vaguely familiar with the basic principles of natural selection) there are generally large numbers of juveniles in any given animal population and they tend not to survive to reproduce. Indeed the classic animal population structure is crucially one of large numbers of juveniles, few intermediates (‘subadults’ or adolescents), and large numbers of adults. Each year many new juveniles are born, yet the subadult population remains stable and low because most juveniles don’t make it past the early part of their lives. They are being killed, and ecological studies show that the vast majority of this loss is through predation. Overall, predators take juvenile prey first and foremost and there is no reason to think theropods were any different. In fact the records of bite marks, stomach contents and coprolites rather suggest that they fitted this pattern (even if that evidence is sparse).
Finally turning back to the fossil record, what we see of dinosaur populations would also support this hypothesis. Despite their size, dinosaurs produced very large number of offspring – compare the ability of an elephant to give birth once every few years with a sauropod or hadrosaur that could potentially lay dozens of eggs at a time, perhaps several times a year. Dinosaurs were producing juveniles at a fantastic rate and they were not all making it to adulthood, and nor were they all dying from falling into lakes or under rockslides (and thus turning up in the fossil record) – they were being eaten. There is a clear and obvious bias against finding juvenile animals in the fossil record (see below) but given the extraordinary reproductive ability of dinosaurs can we really attribute the strong absence of juvenile dinosaurs in the fossil record to this alone? We do actually have some good collections of juvenile animals, but perhaps pertinently these actually are often the result of a catastrophic death assemblages. We only find them when they died in a manner unlikely to make them vulnerable to scavenging (like mud traps). Of course unlike big adult skeletons with bigger (and better mineralised) bones and all those awkward horns and plates and spikes, juveniles were small, easy to consume whole, with bones that could be crunched and digested easily and leave little or no trace to ever be found.
It all adds up to a fairly coherent picture of population structures, reproductive rates, digestion, feeding, hunting strategy and even the raw ability of theropods to even bite bone (or otherwise) and combines data from the fossil record and living animals. It certainly fits the current data better than the idea that theropods targeted big adult animals while somehow juveniles avoided getting preserved and theropods fed without leaving bite marks or having stomachs full of big bones. However, as ever this is obviously not the whole story.
So now come the inevitable exceptions and cop-out clauses which are exactly the kind of thing the media don’t report on and then people assume you never thought of or ignored or bypassed. I don’t pretend for a moment that this is a watertight case (the data is too limited in quite a few ways) but based on the available evidence, I do think that as a general rule and based on the currently available evidence, it’s plausible and, yes, even likely. That is, in most ecosystems large theropods preferentially targeted juvenile prey and as such played a significant role in eliminating juvenile dinosaurs from entering the fossil record.
OK first off, as I say on here repeatedly, the biological world in incredibly diverse and varied and there is no way at all that *every* big theropod only ever hunted juveniles or made them its primary target. Even casting aside possible dedicated scavengers, or those eating other things like fish, there were doubtless a few theropods that had very different behavioural / ecological patterns and were dedicated adult hunters. Equally, even the ‘dedicated’ juvy killers would take the opportunity to attack an unwary or sick adult. Plenty of adults would still get dispatched by theropods and indeed those that were very old or ill were more likely to die at the hands of a carnivore than ‘natural causes’. This is a rule of thumb I’m presenting here, not a law of absolutes.
Secondly, yes the data is a bit thin, very thin actually. I am essentially advocating an ‘absence of evidence’ hypothesis. While there are those who stick to “‘absence of evidence is not evidence of absence” as a mantra, there are limits. If you search for a needle in a haystack for a few days and check as many pieces of hay as possible, sift it, run a magnet through it, and burn what’s left and still don’t find a needle, there’s probably none there. No, you can’t say it for sure, but you can have a pretty good idea, and you can’t look forever: we must be practical and pragmatic as scientists, especially as palaeontologists! The old canard ‘more research is needed’ is genuinely true in this case – there is no good record / catalogue in the literature of feeding traces, or stomach contents, or a record of how many juvenile specimens we have, and many have probably been missed. There might be sufficient evidence already out there to turn this concept over, or it might yet be found in the future, but we do have some pretty good collections and some studies of these incidences and they do seem to support these contentions.
Similarly, it is of course very hard, if not impossible, to separate out the possible distinction between an absence of juveniles via theropods or from taphonomic biases. Juvenile skeletons are smaller, less ossified and at greater risk of loss in the fossil record (and harder to find) so maybe we are just missing them rather than their being eaten. Again, further research might alleviate this, but it may never be possible to tell the difference. I think that the supporting evidence (especially from extant organisms’ behaviour) tips the balance in favour of predation but this is certainly the biggest drawback here. Certainly I would not rule this aspect out – taphonomy is certainly going to be important and I am not suggesting that *all* juveniles are killed and eaten, nor that theropods were the only factor at play – merely that they were an important one.
Finally, there is of course the possibility that theropod were just doing something weird and really didn’t act like any other terrestrial predators, or that the dinosaurs themselves as a whole had some odd physiological or ecological quirks that we have not yet spotted or thought of which would switch things around. However, this does seem pretty unlikely and would require special pleading as an argument and if anything is one of the most intensely researched areas of dinosaur palaeontology right now, so if this is the case we’ll probably see something turn up quite quickly.
There are however new things that might support this work, or at least help to provide auxiliary evidence that may help sort out the varying possibilities that are discussed above. As I have lamented before on here, we don’t know that much about many extant animals, and this makes it hard to extrapolate to extinct ones. More research on predatory habits of living animals, how they damage bones (and which bones, and how much leaving what kinds of traces) and when and where they strike what kinds of prey would flesh out the story. Similarly, a better understanding of how bones are buried and preserved would assist the interpretation of the picture in the fossil record – are juveniles really biased against being preserved, and by how much? Can we create some kind of correlation to correct for this bias? Turning to the dinosaurs themselves if we can get a better handle on their reproductive rates and growth we might see patterns that support this hypothesis (like lots of eggs being laid and very fast growth towards adulthood, some of which is starting to appear in the literature) and a better understanding of how their populations were divided (adults, juveniles and intermediates and predators vs prey) would greatly enhance our understanding of their ecological pressures and interactions.
That largely wraps it up, (and well done for making it this far, this is nearly as long as the original paper) so I’ll summarise. Giant theropods probably took juvenile prey for preference over adults, and healthy adults would have been generally avoided. This matches what (admittedly little) we know of dinosaurian reproductive rates and population ecology and can be reflected in both the fossil record for dinosaur feeding traces (bites, gut contents and coprolites) and for the manner in which extant organisms of all kinds hunt their prey. This is collectively hard to separate out from taphonomic effects and baises but now we have a hypothesis we can try to test it. Doubtless on occasion a Tyrannosaurs would face off with a Triceratops bull in a fantastic one-on-one combat the like of which films are based on, but it would be a once in a lifetime experience for at least one on them, and that’s why a T.rex would avoid the possibility and simply snap up something a fraction of the size and lacking the horns and spikes and most importantly, nous, to know better.
HONE, D., & RAUHUT, O. (2009). Feeding behaviour and bone utilization by theropod dinosaurs Lethaia DOI: 10.1111/j.1502-3931.2009.00187.x
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