Social behaviour in the dinosaurs

So yesterday I looked at the groups of Protoceratops specimens and the inference that at least one population of P. andrewsi tended to form groups throughout ontogeny. I also commented on how this was put in really conservative terms – I carefully avoided using the term ‘social’ and didn’t extrapolate up to other populations, species or genera, let alone entire clades. This is an area I’ve commented on before, but in this paper take a more detailed look at social behaviour and what we can and cannot say about extinct dinosaurs.

The first point to make is about the terms themselves. Look through the literature and discussions of dinosaur behaviour and you will see the term ‘social’ especially thrown around but often without a specific definition or context. Unfortunately this is really unhelpful as, although there is no strict definition out there, it does cover a multitude of different magnitudes of behaviour and seems often to be used to mean little more than ‘in a group’. This really needs cleaning up, and we need to be much more careful and specific – you can find a whole group of grizzly bears together fishing out salmon, but I’d not call them social (if anything they are antisocial the vast majority of the time) and this is a far cry from the social groups formed by say chimps or meerkats which are almost always together and have constant interactions. A group of dinosaurs together does not inherently mean some form of social group with say hierarchies, social bonding, shared responsibilities etc. and could be a simple as asocial animals coming together to breed, migrate, avoid some natural disaster or other effect. Separating out say truly eusocial animals like molerats from bears or some crocs which will tolerate each other under some circumstances is going to be hard given the limitations of the fossil record, and is probably impossible most of the time.

On top of that, individuals can form groups for part of their lives, switch between solitary and group living at different stages (ontogenetic or annual), and can be wildly different between populations of a single species, let alone other members of the genus or family. Groups can be all male, all female, equal ratios, harems, mixed adults and juveniles, or all of single cohorts. The net result of course is that conservatism I mentioned before. Taking a trackway or a mass mortality event or set of nests and saying “hadrosaurs were social” is a terrible idea, and I think most of the time the best we can and should say is “this species has some gregarious tendencies”.

Now I should make clear two things. First off, I don’t think that this means we have no evidence for sociality in dinosaurs or that many were not social, merely that (as with a great many behaviours) the evidence is profoundly limited in the fossil record. Given how diverse dinosaurs were and the sheer number of mass mortality sites etc. many species I am sure were social or at least tended to aggregate into groups, but picking an individual genus and saying “this is the social one” based off one or two mass mortalities that probably span different species, times, places etc. is probably a poor inference. Secondly I also think we can make good inferences for some species – multiple mortalities that are from different seasons, evidence of strong social interactions like display structures or intraspecific combat, inferences from other very close relatives showing similar patterns can probably build up to make a pretty strong pattern, but this would still not rule out some individuals being solitary or complex switches between different systems.

So, if we are at least seeing some degree of gregariousness within some populations (and as before, I think we can make a decent case for Protoceratops) why might this be happening? Another interesting aspect of this is that when we do have mass mortalities of dinosaurs they are very often exclusively of juveniles. Given how rare juvies are generally, it should be a bit odd that a rare event of a mass mortality should trap juveniles. There are adult only groups and mixed groups for various dinosaurs, but there are plenty that are of only subadults, or younger animals, and these may have multiple mixed age groups, while still all being juveniles.

Now both juveniles and adults would come together for some reasons like feeding, migration, natural disasters like drought, or perhaps long-term parental care. We would also expect to see adults come together to breed and nest, but that won’t apply to the little ones, so what effect might drive juveniles together but not adults? One obvious factor is predation. Yes, again this is an area I have heavily trodden before but juveniles of almost all species are much more vulnerable to predators that are mature animals. Adults are better at recognising threats, forage in better areas and for less time, and are typically either faster or better equipped to fend off attacks too.

One thing that can really benefit juveniles however is vigilance. Their long foraging times in poor areas means they are often not spending much time looking out for threats. Hanging around in a group though means that at least someone is generally keeping an eye out, (and as a bonus if you are found, at least the predator may eat the guy next to you, rather than attacking you). Adults may even keep juveniles away from them since as well as competing for food, but actually drawing in predators and so creating danger, so we might expect juvies to bunch up, when the adults may be less fussed. I would expect juvenile ankylosaurs for example to hang around in groups when their armour is little protection against a big tyrannosaur, but the adults might be largely immune and so would not need this effect to help them. Plenty of studies on extant species show that groups form, or increase in size, when there are more predators around and so this would fit the patterns we see here – juveniles are likely to stick together at times when adults may not because they want to avoid being eaten.

So overall we suggest that juveniles of dinosaurs might have formed aggregations, (and in some species where the adults were largely solitary) as a defence against predation (or at least as a major driver of it) but that this does not necessarily imply strong social interactions, merely the formation of groups. We need to separate out much more carefully what we mean by the term ‘social’ and start being much more specific about what that word means and degrees of social interactions, group formation, gregariousness and the like. Conflating multiple different terms (or leaving them so broad and undefined as to cover almost anything) does no one any favours – we can’t compare and contrast different specimens or make meaningful statement about what they might have been doing. We can call migration, group hunting, group formation, nesting together, and parental care social behaviours if we want to, but it’s worth separating them out and we need to do just that if we want to have meaningful discussions about what these animals did and did not do.

 

 

A block of baby Protoceratops

My new paper is out today and it describes a wonderful new specimen of four baby Protoceratops together in a single block. Unlike many other groups of exceptionally preserved specimens from the Mongolian Gobi, the animals are effectively stacked on top of one another and all facing in different directions and importantly, their inferred age is different to other Proto specimens.

This specimen was actually collected in the early 1990s, something I hadn’t realised when I saw it in 2011 in the Hayashibara museum in Japan. This was my second trip to the museum after having been in 2009 (that led to the Tarbosaurus bite marks paper) and this was the specimen that really grabbed me and I am obviously most grateful to co-author Mahito Watabe for allowing me to lead the paper on this.

The preservation is superb, and although there’s been some erosion and damage (especially to the uppermost animal) at least one of them is brilliantly exposed and almost immaculate in condition. At this point I must praise the preparator for his incredible work here, this is a huge block (close to a metre cubed), the matrix is exceptionally soft and brittle and the organisation of the specimens must have made the whole process extremely difficult and the result is both beautiful and impressive.

There are two major aspects to the paper (which is in PLOS ONE so for all the details and tons of pics so you can read it all there) and I’ll deal with them in separate posts. The first one is the block itself and the implications for Protoceratops generally. There are a number of groups of this dinosaur known already – several sets of adults, a pair of subadults (also briefly covered in the paper – and shown below) and a set of very young animals that were described a few years ago as something close to hatchlings in a nest. In the paper we actually suggest that these were not in a nest, but free living, but the wider point is that we have similar sized animals (that are probably of a similar or the same cohort) together at multiple different life stages, and we don’t seem to see mixed cohorts as with many other dinosaurs.

The block here slots into this pattern beautifully, the animals are about twice the length of the smallest ones, and about half the size of the subadults. That means we can put together a sequence of specimens at four pretty distinct life stages where we have groups of animals together at different times of their lives. That is something we have not been able to do for any extinct dinosaurs before – we do often have groups together and often of adults or juveniles or the two mixed together, but we are not aware of a so many obviously different cohorts of a single species showing this. Wonderfully, these are not all just Protoceratops, but all P. andrewsi and even better all of these are from a relatively narrow time and space window.

As non-avian dinosaurs go, that’s about as close to a single population as you are really going to be able to find, so collectively we are inferring that this was a pretty normal behaviour for this population. That sounds like a pretty conservative approach (can we not apply it to the genus or species as a whole?), but I think it’s something we really need to do a lot more of in palaeontology. The sheer variety and plasticity of many behaviours, especially when it comes to forming groups, means that is probably dangerous to extrapolate without some good supporting evidence and that sets things up quite nicely for the second post which will follow tomorrow.

A population of Shantungosaurus, the largest ornithischian

Sadly I have to report that after many years working on various diapsids and having published plenty of papers on dinosaurs generally and theropods specifically, and yes even sauropods, I’ve gone and published two papers on ornithischians. I hang my head in shame, obviously, and I hope too many readers won’t think too little of me (though I doubt Tom Holtz will ever return my calls now). The first is on the wonderful Protoceratops and delves deep into dinosaur behaviour (and should be out on Wednesday), but this time it’s the monstrous hadrosaur Shantungosaurus, which has not really had anything like enough attention given just how much material is floating around.

The paper is a chapter in the new ‘Hadrosaurs’ volume that has been long in the making (and indeed publishing, since it as basically done a year ago) and if at this point effectively out. Actually I’m not sure quite how available things are, but the volume has appeared on Google Books (with the incorrect date of 2015 on it) and copies are apparently in mail, plus at least some coverage of various chapters is already out. As a result, I don’t think I’m jumping any particular embargo. though I appreciate not everyone may be able to read it in the next few days. Anyway, onto colossal hadrosaurs.

After the initial excavations of the 1960s, not much happened in the quarries where the remains of Shantungosaurus were first found. It was identified as a giant hadrosaur, plenty of isolated remains were collected and distributed to various collections and then, well, not much. The new digs over the last decade or so have seen a raft of new finds, but all the attention has really been on the other things coming out of the quarries, namely the new tyrannosaurs, ceratopsians and other beasties. That’s a shame as there are literally thousands of elements available to study and these are coming out in multiple quarries.

Over several visits, my good friend and longtime collaborator Corwin Sullivan and I went over the largest of the three main sites at Zhucheng, the Kugou Quarry, and took note of every bone that we could find and identify. The quarry maxes out at some 300 by 30 m, so it’s truly giant, and both ends are missing thanks to the erosion of the hill and it’s not clear how deep it might be. We also could not access every part of it safely and thus although we noted some 3000 elements, we estimate there are closer to 5000 exposed, and there could be huge numbers still to find. Out of these, barely a handful belonged to anything other than Shantungosaurus – a tyrannosaur tooth, a couple of tyrannosaur bones, a croc osteoderm and a bit of turtle. (And, oddly the near complete and articulated Zhuchengceratops, though I suspect it is from a different horizon). In short, this entire area and material essentially represents just one genus and probably a single aggregation.

All the material is essentially disarticulated and while basically every part of the skeleton is there, it is horribly jumbled. There’s no evidence of scavenging or trampling, and little sorting either, so this looks like a pretty major event that led to a rapid burial of the remains. We don’t dwell on what might have done this, but bearing in mind the size of these animals and how many there were and this is clearly something big, and also probably quick (this is not a long term accumulation of material).

Already 5000 elements is quite a bit, but the bones are also big. Shantungosaurus is well known as being a really large hadrosaur, but more than that, it’s absolutely colossal. While femur length is not the best size proxy out there, neither is it that bad, and was the only thing we could reliably measure for large numbers of the elements preserved that would give a decent size estimate. The largest femur we could accurately measure was 172 cm long – bigger than the largest specimens of Diplodocus and comparable to many big sauropods like Apatosaurus and Antarctosaurus. While they do have very different builds as animals, don’t forget that hadrosaurs were not pneumatic, so it’s quite reasonable that these animals had similar masses to those huge sauropods. Similarly that also means that  perhaps many sauropods were not as heavy as the largest hadrosaurs which does have implications for how we look at things like the reasons sauropods did get so large. Mass estimates that are available or can be calculated for Shantungosaurs are extremely varied and this is perhaps due to it being so much larger than anything else known when it comes to hadrosaurs or even other ornithischians. Is is basically off the charts (few ornithischians have femora that exceed 1 m in length, and the smallest specimens we measured were bigger than this) and it probably needs to be tackled with a specific rigorous analysis to get a good estimate. Still, I’d be very surprised if the larger individuals were under 10 tons, and it is probably the heaviest ornithischian known and by extension, probably the heaviest terrestrial biped, since I didn’t see anything in the available material to suggest it could not walk bipedally.

Femora were also measured as they are large elements that are relatively easy to identify correctly and were in relatively decent condition, and so go some way to determining a minimum number of animals in the quarry. We counted 110 and so there is a minimum of 55 animals here, and I would be stunned if there were not very considerably more than that in reality (or indeed many more femora in there that are simply not exposed). But any measure then, this is a lot of animal – over 50 individuals, the smallest of which had a femur over 1 m long, and many of which were large sauropod sized. Indeed, the distribution of the femora actually tells us something too.

The range of sizes seen is actually really narrow: almost 85% of them fall between 135 and 175 cm and aside from three small ones that were little more than a meter, the rest form an almost perfect normal distribution. In short, this looks like a natural population of adult animals and we can infer they are adult both on the general size and the fact that all the elements of things like sacra in the quarry were fully fused. It has been suggested before that hadrosaurs form separate groups and that adults may have aggregated without juveniles, and with juvies and /or subadults forming separate groups, and that fits well with what we see here (and this also fits with the ideas covered in the forthcoming Protoceratops paper).

Collectively then the remains from this quarry do look something close to a natural aggregation, representing a pretty massive accumulation of biomass (over 50 animals and likely closer to 100, and probably over 10 tons each). It’s hard not to think about just what this means for a Mesozoic landscape, even a big Zhuchengtyrannus would be pretty much outclassed by one of these, let alone dozens together, and they would presumably have been able to strip huge swathes of vegetation clear as they foraged. For me at least it’s a nice evocative image, though perhaps not a long lasting one given that something massive rather dismembered and buried them shortly afterwards. Happily for palaeontologists we have now found this graveyard and there’s a massive amount of material available on these massive dinosaurs, and I hope that there is much more to come now that it is becoming available for study.

 

Crocodiles of the World

Morelet’s Crocodile

Recently I took a trip up to this unusual establishment in Oxfordshire on something of a whim. I’d been planning to go for quite a while but the opportunity came up and I wanted to make the most of it so headed over (so apologies to various people who I’d been muttering to about arranging a trip up there). I did not actually know what to expect really, but did know that it was a small operation and that they had lots of the smaller, and very much lesser seen, croc species. I’ll enjoy any good zoo, but there are generally only so many Celebes macaques or Asian short-clawed otters you can see, and filling in on a raft of the crocs not yet present in the crocodilian panoply made it a likely hit.

Cuvier’s Dwarf Caiman

It is indeed a pretty small outfit, but what there is available, is very well presented (the signs are numerous and excellent) the enclosures are great and spacious, and the animals in great condition and clearly breeding well and behaving naturally. It is not going to be a full day for anyone and even a reptile obsessive is unlikely to be able to spend more than a few hours there, but it is reasonably priced and thanks to numerous and well placed viewing areas, it’s almost impossible not to see every animal pretty well. Best of all, there are numerous small talks and feeding sessions scheduled for every day, so no matter when you go, there’s going to be some extra information and a chance to grill the knowledgeable and engaging staff.

Yacare Caiman

There are a pair of macaws and several large tortoises and a few terrapins knocking around, in addition to some nice lizards (including the biggest varanids that are not Komodo’s I’ve ever seen) and a monstrous python, but obviously we are really here for the crocs. A total of 14 species are on show and most of them are not commonly kept in zoos and are hard to see at the best of times. Sure there’s a couple of American alligators, and Nile crocs and some not uncommon ones in the spectacled and black caiman, and West African dwarf croc and the endangered-but-often-in-zoos Chinese alligator. There were also more unusual ones like both Siam and Cuban crocs and a group of three salties. Then we get into the real rarities – Cuvier’s dwarf caiman, Morelets’ crocodile, blunt-snouted caiman, Scheneider’s dwarf caiman and finally the stunning Yacare caiman.

Schneider’s Dwarf Caiman (or smooth fronted caiman)

Almost all of these were in at least pairs, and generally there were more than that. In the case of the Niles, they were in a huge pool since there were more than 30 of them (though all were only a meter or so long). Obviously most of these are small species even when they max out, but the biggest Siam and big alligator were at the 3 m mark and every big the major carnivore you expect at that scale and were very impressive. Despite the usual level of activity in crocs (especially with winter coming, even in a heated environment) plenty were moving around at least a little, and the feeding times stimulated plenty of activity, and I was able to see crocs high-walking, belly crawling, juveniles calling to their parents, some low-level aggression between individuals, and best of all, some of the Niles rocketing up out of the water to take food.

Broad Snouted Camian

I do think people going expecting a full on zoo, or anything like a normal reptile house might be, if not disappointed, then at least surprised. This really is 90% croc, but that’s in no way a criticism, and the excellent set ups and the animals were a real joy. As someone who does like to target species I’ve not seen before, it was a real revelation, a good half dozen that were new to me, and plenty more I’d seen only occasionally (I’d not seen a Cuban croc before this year, only seen a Siam once before). Moreover with the good signs and all the animals in one place, it was really easy to compare them to one another and get a real feel for some of the differences and how they line up to one another. If reptiles are in any way your thing, this really is something that should be on a to-do list and it’s a great addition to the UK collections.

Black Caiman