Wednesday, 20 July 2011

Why IS it so Hard to Believe in Gregarious Tyrannosaurids?


The recent documentary Dino Gangs has met with the usual mixed reaction. The general opinion appears to be that the program (in the UK) was too long by at least an hour and suffered the usual mix of repetitiveness and also that the amount of CGI used was excessive. Still, any new programme on dinosaurs is always welcome and Phil Currie is very watchable.

When discussing the programme with Mark Graham, a colleague of mine, and the newly appointed preparator at the Natural History Museum in London, Mark made a comment which made me take a step back and do a little thinking. He said “How is it that pack hunting in dromaeosaurids seems to be generally accepted but whenever pack hunting is suggested for tyrannosaurids there is always a huge outcry declaring it unlikely?” Good point.

The focus in Dino Gangs is too concentrated on pack hunting tyrannosaurs – something akin to pack hunting in today’s extant mammal populations and yet it is wrong to compare. Don Henderson remarked that the dinosaurian brain was small and relatively underdeveloped when compared to mammals. His observation that "I doubt the thought would cross their tiny reptilian brains" when discussing the possibility of co-ordinated pack hunting tyrannosaurs is probably correct. And yet we should not be too dismissive about the possibility.

It was interesting, however, that throughout the programme, there was not one dissenting voice arguing against tyrannosaurs displaying ANY gregarious behaviour – not one. I accept that the onus was on pack hunting but never the less, perhaps we can take this that there has been a slow acceptance that tyrannosaurs (and other theropods) did gather together for an unspecified amount of time even if it was only briefly, to mate or perhaps to sort out territorial disputes.

Looking at the fossil evidence, there are now many examples of bone beds that contain the associated remains of multiple theropods. Multiple, it should be mentioned, may actually be only two animals but generally speaking when recovering predatory dinosaurs, two is indeed a bone bed. The most famous in recent years is the Albertosaurus bonebed in Alberta, which we now know contains the remains of at least 12 individuals although there are revised figures suggesting more than 20 individuals are represented . There are, however, other examples of tyrannosaurid bonebeds that include the taxa Tyrannosaurus, Tarbosaurus, and Daspletosaurus.

Other theropods found in bonebeds ranging throughout the Mesozoic include Coelophysis in the Triassic, Saurophaganax in the Jurassic and Mapusaurus in the Cretaceous (for an extensive list see Currie 2010). Other evidence for theropods perhaps living in groups includes what I consider to be some of the most compelling evidence and that is the fossilised trackways that are found worldwide. Some of these display tracks of multiple individuals moving together in a uniform direction and which, significantly, do not overlap thus indicating the likelihood that the animals were walking or running together. You also have to bear in mind that other related dinosaurs such as hadrosaurs and ceratopsians have also been found in mass bonebeds containing hundreds, even thousands of individuals. So there is significant fossil evidence that is suggestive that tyrannosaurs did cohabit for an unspecified amount of time.

Moving on to phylogenetic inferance using the extant phylogenetic bracket. Dinosaurs fall between crocodiles and birds and, by inferance alone, can suggest something about the behaviour in theropods, indeed dinosaurs as a whole.

Crocodiles often live together in large numbers but this is not strictly a communal group and is more often than not just an area where they happen to be together usually due to environmental conditions. But at certain times of the year they do gather together in vast numbers to intercept migrating herds of wildebeest and zebra. However they do not pack hunt and prey animals are taken by individuals but the dismembering is very much a joint effort. The seething mass of animals that bite, hold and twist the carcass apart looks as primitive as it gets and yet it is a very efficient way for a number of animals to share the spoils around a large number of animals.

Crocodiles also display amazing courtship, parental care and even a deftness of touch that can be almost regarded as genteel. Courtship is surprisingly complex and includes bubble blowing, slapping the water and vocalising before coupling takes place. The nest of eggs, once laid, is protected by the female and when the hatchlings appear the young are actually carried to nursery pools in the jaws of the female and they benefit from the protection of the mother for a further period of time.

Crocodiles are often imagined to be slow moving, dim witted, cold blooded animals that are throwbacks to the time of the dinosaurs. Well, they outlived the dinosaurs and continue to thrive, often in extreme conditions and are actually a very successful and complex animal. This brings us to the birds.

Birds (which are, of course, surviving avian theropods) also live and breed together in sometimes enormous numbers. In fact the birds often play the numbers game with great success since the more of your fellow birds you breed with, live with or fly with the greater your chances of survival for you and your young are. Birds also display high degrees of courtship and parental care and include some of the most amazing displays of ritual, nest building and care on the planet.

Birds are also known to hunt in groups and, again, complex behaviour is often displayed. Harris hawks hunt in small groups of two to six birds to maximise their chances of a successful hunt in desert terrain. Boobies and gannets also display both visual and oral communication combined with superb diving techniques in often large numbers, again to maximise their efficiency in catching fish. Vultures are very adept in spotting carcasses on the ground and also use their excellent eyesight to spot when one of their number has found such a carcass and join in on the feast almost immediately.

So what about dinosaurs other than theropods? I’ve already highlighted that many species of dinosaur have been found together in large numbers in vast bonebeds. Although these too are studied intensively, there is more acceptance that animals such as ceratopsians and hadrosaurs moved around in large numbers and herds. The numbers of animals in some sites defies belief and that in itself is a huge pointer to gregarious animals moving together.

Of course just why they were moving together cannot be wholly explained because extinct animals and their behaviour cannot be observed as you would observe in extant animals today. It’s easy to imagine migration, breeding and safety in numbers as possibilities for these dinosaurs moving together but it has to remain a point of conjecture that remains impossible to prove.

Parental care in dinosaurs has long been established – to a certain degree. Large nesting sites are known all over the world and in such numbers that almost certainly points to large numbers of animals nesting communally. The Maiasaurs of Egg Mountain are amongst the most famous and fossils here suggest complex parental care. Additionally, and more relevant in this case, are the egg brooding oviraptorids found in the Jiangxi Province of China , which are not that distantly related from tyrannosaurs.


So using the available evidence in combination with phylogenetic inference, it appears to me that the chances of tyrannosaurs living gregariously are very high, indeed maybe should even be expected. For how long a period of time they may of have been together is a matter of conjecture and is impossible to say without direct observation and, until a time machine is invented, that is how it will remain. Quite possibly it was only for short periods such as a mating and breeding season but, on the other hand, there may have been permanent groups of animals living together throughout the year – there is just no way of knowing.

Personally, I do see some tyrannosaurs living in groups for an indeterminate amount of time – but not all. I would also expect a degree of parental care, maybe not as advanced as some birds but nest protection seems likely and maybe feeding and protecting the young as well to some degree.

But what about pack hunting? Impossible to prove and it would appear to be a level of behaviour and coordination above what could be expected of dinosaurs. As things are right now, I see the possibility of communal hunting and feeding which is quite different – more akin to the crocodiles of today. Maybe one tyrannosaur would attack a prey animal and others would join in or, if the prey was injured or cornered, then more tyrannosaurs may have joined the attack and help despatch the unfortunate beast. And again, just like crocodiles, multiple tyrannosaurs ripping in to a carcass would make short work of it and a lot of meat and bone would get shared out real quick. Perhaps the face biting pathologies found in tyrannosaur skulls are actually indicative of this (Tanke 200), as intraspecific squabbling broke out during feeding.

So for me, it’s time to accept that tyrannosaurs and other theropods probably lived in groups for at least part of the time and all the available evidence, albeit circumstantial, suggests there may have been a degree of group hunting but I find it hard to accept that they could hunt in a coordinated, complex manner similar to a pride of lions although, of course, even that cannot be ruled out. Perhaps its time to turn the argument on its head and ask what evidence is there that tyrannosaurs were solitary hunters?

References

Currie, P.J. & David A. Eberth. On gregarious behaviour in Albertosaurus. Canadian Journal of Earth Sciences, 2010, 47:1277-1289, 10.1139/E10-072


Tanke, D.H. & Philip J. Currie. Head-biting behaviour in theropod dinosaurs: paleopathological evidence. Gaia 15: 167-184

13 comments:

Taylor Reints said...

Some find pack-hunting in most theropods unlikely due to Roach and Brinkman, et al. 2007. This paper, overall, suggests that dromaeosaurids (particularly Deinonychus, but this paper stated that it applies to many non-avian theropods)*

* Roach, B. T.; D. L. Brinkman (2007). "A reevaluation of cooperative pack hunting and gregariousness in Deinonychus antirrhopus and other nonavian theropod dinosaurs". Bulletin of the Peabody Museum of Natural History 48 (1): 103–138.

http://www.bioone.org/doi/abs/10.3374/0079-032X%282007%2948%5B103%3AAROCPH%5D2.0.CO%3B2

Larry Witmer said...

Nice post, Mark. As I see it, the major task is to document that these predators were actually hanging out together. If you can show that they were communal (living together), then the likelihood that they were hunting communally goes way up...but is still difficult to demonstrate. That said, I have a hard time believing that theropods *wouldn't* be hunting communally if they were living together. Try and stop 'em! On the other hand, true cooperative hunting (like wolves) is probably impossible to demonstrate for extinct animals. In my opinion, some theropods might have had the cognitive abilities to engage in some crude level of cooperative hunting, but it will always remain just that...my opinion. For anyone reading this that may not know, I blogged about "Dino Gangs" myself: http://bit.ly/mYBRHA.

Mark Wildman said...

Thanks for the comments Larry - it would appear we think along similar lines and, again, as we all keep saying - behvioural inferrance can only be that - inferred but the circumstantial evidence appears compelling.

And to Taylor - thank you too for your input. In fairness to the Roach & Brinkman paper, they also focus on cooperative hunting and find it unlikely but they do not rule out the possibility of some form of loose gathering that may have been group beneficial.

Gorgosaurus said...
This comment has been removed by the author.
Gorgosaurus said...

(Punctuation and spacing corrected)The collective noun "pack" infers a "mammalian" behaviourial bias. Never heard of a "swarm", "host", "chattering" or a "clamoiur" of tyrannosaurids. "Flock" or "gang" would seem more appropriate terminology than "pack" considering that theropods lacked the cerebral development of wolves. A "murder" (as used of ravens) of tyrannosaurids conjures up quite different associations.
Spike.

Gorgosaurus said...

Hmmm, there´s also an "unkindness" of ravens for less violent tyrannosaurs. >;~} >

Mark Wildman said...

Ha! And they were probably very adept at "mobbing" hadrosaurs! Thanks for the comments Spike.

Herman Diaz said...

"When discussing the programme with Mark Graham, a colleague of mine, and the newly appointed preparator at the Natural History Museum in London, Mark made a comment which made me take a step back and do a little thinking. He said “How is it that pack hunting in dromaeosaurids seems to be generally accepted but whenever pack hunting is suggested for tyrannosaurids there is always a huge outcry declaring it unlikely?” Good point."

If I were you, I would've answered Mark's question in the following way: 1stly, unlike tyrannosaurids, there's good evidence for eudromaeosaurs being group hunters (Off the top of my head, adult-only clumps, multiple shed teeth, & trackways); 2ndly, unlike tyrannosaurids, eudromaeosaurs had the brains of pack hunters (See the 1st Gardom & Milner quote). Otherwise, good blog post. The 2nd Gardom & Milner quote sums up my opinion of large theropod gregariousness.

Quoting Gardom & Milner ( http://www.amazon.com/gp/product/184442183X?tag=533643275-20 ): "When the first Deinonychus fossils were found in Montana in 1964, scientists noticed that there were the remains of several of the predators near the body of a much larger herbivore, Tenontosaurus. Did they hunt in groups? Certainly Deinonychus' brain was relatively large and probably well developed in the areas vital to a pack hunter, namely those of sight and sound. Bringing down a big dinosaur would have provided several days' food supply, even if it had to be shared."

Quoting Gardom & Milner ( http://www.amazon.com/gp/product/184442183X?tag=533643275-20 ): "Except in very rare cases, no skeletons of big hunting meat-eaters have been found near each other. This is in contrast to many 'mass grave' finds of herds of plant- eaters, and suggests that some, at least, of the large hunters operated alone. This is good survival logic, since there is no point in cutting down your food supply by sharing a hunting ground with another large stomach. The carnosaurs would have had little to gain from co-operative hunting as they were so much bigger than many of their potential victims and would have needed no help to kill them. For a single hunter the best places are forests or dense scrub, since this gives plenty of opportunity for ambushing animals which would certainly spot you coming on a flat plain. The modern leopard uses this lone hunting technique.
There are some areas where the number of meat-eating dinosaur trackways, both large and small, is surprisingly high. These may have been favourite hunting spots, perhaps along the shores of rivers and lakes, where the hunters constantly patrolled to pick up carrion that drifted ashore, or attack plant-eaters as they came to drink.
However lonely a life the big hunting dinosaurs may have led, they must have got together to produce baby dinosaurs. Again, we have very little evidence of their mating habits, although eggs attributed to meat-eaters have recently come to light in the Gobi Desert. Did they nest in groups or alone? One clue could be the discovery of different meat- eaters' skeletons with the bones of their own young inside their stomachs. Among today's animals, male bears and lions will occasionally try to kill off the young of other males, and some reptiles will actually eat their own hatchlings if put under stress. If cannibalism was part of the hunting dinosaurs' behaviour they would have tended to nest alone. They might also have been female dominated with the male being kept well away from the nest by the aggressive mother, and even being seen off completely after successful mating. The hatchlings would probably have stayed with the parent, or parents, during their most vulnerable early months, eventually leaving to find their own territory. A tantalising recent fossil find seems to show an adult Tyrannosaurus in association with a smaller adult, a juvenile and a baby. This may prove to be the first evidence of a carnosaur family group."

Mark Wildman said...

Hi Herman and thanks for your input. As is always the case, it is how the available evidence is interpreted that dictates any one side of an argument.
Dromaeosaurids, fascinating as they are, are not an animal I've spent a great deal of time studying. But I will say that the finding of several specimens of Deinonychus together is a somewhat isolated example. You would expect to find similar sites in Asian deposits where dromaeosaurids are omni-present but they are, in fact, strangely absent.

Brinkman, in an earlier paper (1998), suggested that the multiple shed teeth at the Tenontosaurus site may simply be evidence of an opportunistic feeding frenzy.

As for tyrannosaurids, and the quotes above, there is nothing there that I would not disagree with in principle as well. But there are large groups of theropods found together now and the Albertosaurus bonebed is the classic example of this. It seems likely that tyrannosaurs did spend time together, but for how long and for what reasons is impossible to say.

Herman Diaz said...

Many thanks for getting back to me, Mark.

"But I will say that the finding of several specimens of Deinonychus together is a somewhat isolated example. You would expect to find similar sites in Asian deposits where dromaeosaurids are omni-present but they are, in fact, strangely absent."

While there aren't any Asian adult-only clumps AFAIK, there are Asian trackways (E.g. See Figure 1.5: http://books.google.com/books?id=GzrCV2BLcyQC&pg=PA8&dq=pack+velociraptor&lr=&cd=34#v=onepage&q=pack%20velociraptor&f=false ).

"Brinkman, in an earlier paper (1998), suggested that the multiple shed teeth at the Tenontosaurus site may simply be evidence of an opportunistic feeding frenzy."

Which Tenontosaurus site (See the 1st Maxwell quote)? In any case, opportunistic feeding frenzies might explain some of them, but definitely not all of them (especially not MOR 682, given the 2nd Maxwell quote). Also, it's hard to take Brinkman seriously as a good (I.e. Reliable/up-to-date) source, given his & Roach's 2007 paper, which (AFAIK) was poorly-received by good sources (especially after Li et al. 2007). This makes sense, given said paper's many problems, which I'll list in another comment if you want (I don't want this comment to get too long).

"But there are large groups of theropods found together now and the Albertosaurus bonebed is the classic example of this."

Family groups, probably, but not necessarily hunting groups. That's what I was getting at when I said "adult-only clumps", which (as indicated by the blog post in this link) are better evidence for group hunting than are multi-age clumps (like those of large theropods): http://blog.hmns.org/?p=6371

P.S. I forgot to mention in my previous comment that I liked your blog post as much as I did partly b/c it showed how well informed you are. Specifically, you seem to know the difference btwn true cooperative (I.e. Pack) hunters & pseudo-cooperative (I.e. Communal) hunters. I tried to make that difference clear when I said "group hunters" in reference to adult-only clumps/mulitple shed teeth/trackways & "pack hunters" in reference to brains. Specifically, I was trying to say that while the former evidences could be interpreted as being for either pack hunting eudromaeosaurs or communal hunting eudromaeosaurs, combining them w/the latter evidence makes pack hunting eudromaeosaurs much more likely.

Quoting Maxwell ( http://findarticles.com/p/articles/mi_m1134/is_10_108/ai_58360822/?tag=mantle_skin;content ): "Nobody knows for certain what took place at the Shrine site. We do know, however, that whether hunted down and killed by a pack or simply scavenged after death, Tenontosaurus was the preferred food of Deinonychus. Approximately eighty occurrences of Tenontosaurus remains have been discovered in the Cloverly formation to date, and thirty-five include Deinonychus teeth. While Deinonychus fossils are rarely found with other possible prey animals, three or four Deinonychus teeth typically turn up wherever there are Tenontosaurus remains."

Quoting Maxwell ( http://findarticles.com/p/articles/mi_m1134/is_10_108/ai_58360822/pg_2/ ): "We know that this Tenontosaurus was not yet an adult, so it didn't die of old age. Of course, this doesn't rule out death from disease or injury and doesn't confirm that it was cut down by a pack, but it's a start. Next, we have a concentration of teeth around the abdomen and pelvis. This may indicate that the pack fed on the abdominal contents while they were still warm and moist. If, after the viscera had been consumed, the remainder of the carcass was scavenged over time by many individuals, we would expect a much more disturbed carcass and a wider scattering of teeth."

Herman Diaz said...

Part 1

Thanks for getting back to me so soon & sorry for taking so long to get back to you. I tried to on Wednesday, but for some reason my comment didn't show up & I only just realized that.

"But I will say that the finding of several specimens of Deinonychus together is a somewhat isolated example. You would expect to find similar sites in Asian deposits where dromaeosaurids are omni-present but they are, in fact, strangely absent."

Based on what I've read (E.g. See the Holtz quote), YPM 64-75 isn't the only example (assuming that's what you meant by "somewhat isolated"). Also, while there aren't any Asian adult-only clumps AFAIK, there are Asian trackways (E.g. See the Fastovsky & Weishampel quote).

"Brinkman, in an earlier paper (1998), suggested that the multiple shed teeth at the Tenontosaurus site may simply be evidence of an opportunistic feeding frenzy."

Which Tenontosaurus site (See the 1st Maxwell quote)? In any case, opportunistic feeding frenzies might explain some of them, but definitely not all of them (especially not MOR 682, given the 2nd Maxwell quote). Also, it's hard to take Brinkman seriously as a good (I.e. Reliable/up-to-date) source, given his & Roach's 2007 paper, which (AFAIK) was poorly received by good sources (especially after Li et al. 2007). This makes sense, given said paper's many problems, which I'll list in another comment if you want (I don't want this comment to get too long, which is why I'm replying in 2 parts).

Quoting Holtz ( http://www.amazon.com/Dinosaurs-Complete-Up---Date-Encyclopedia/dp/0375824197/ref=sr_1_1?s=books&ie=UTF8&qid=1312642999&sr=1-1 ): "Did dromaeosaurids hunt in packs? That seems to have been the case with Deinonychus at least. There are several examples of fossil quarries where many individuals of this species were found buried with a single skeleton of the large iguanodontian herbivore Tenontosaurus."

Quoting Fastovsky & Weishampel ( http://www.amazon.com/Dinosaurs-Concise-Natural-David-Fastovsky/dp/052171902X/ref=ntt_at_ep_dpt_1 ): "Figure 1.5. Photograph from Shar-tsav, Gobi Desert, Mongolia, showing the tracks of a medium-sized theropod dinosaur among those of a pack of smaller theropods. Our drawing suggests one interpretation, consistent with the evidence: the trackway could record a pack of Velociraptor hunting down a single Gallimimus."

Quoting Maxwell ( http://findarticles.com/p/articles/mi_m1134/is_10_108/ai_58360822/?tag=mantle_skin;content ): "Nobody knows for certain what took place at the Shrine site. We do know, however, that whether hunted down and killed by a pack or simply scavenged after death, Tenontosaurus was the preferred food of Deinonychus. Approximately eighty occurrences of Tenontosaurus remains have been discovered in the Cloverly formation to date, and thirty-five include Deinonychus teeth. While Deinonychus fossils are rarely found with other possible prey animals, three or four Deinonychus teeth typically turn up wherever there are Tenontosaurus remains."

Quoting Maxwell ( http://findarticles.com/p/articles/mi_m1134/is_10_108/ai_58360822/pg_2/ ): "We know that this Tenontosaurus was not yet an adult, so it didn't die of old age. Of course, this doesn't rule out death from disease or injury and doesn't confirm that it was cut down by a pack, but it's a start. Next, we have a concentration of teeth around the abdomen and pelvis. This may indicate that the pack fed on the abdominal contents while they were still warm and moist. If, after the viscera had been consumed, the remainder of the carcass was scavenged over time by many individuals, we would expect a much more disturbed carcass and a wider scattering of teeth."

Herman Diaz said...

Part 2

"But there are large groups of theropods found together now and the Albertosaurus bonebed is the classic example of this."

Family groups, probably, but not necessarily hunting groups. That's what I was getting at when I said "adult-only clumps", which (as indicated by the blog post in the following link) are better evidence for group hunting than are multi-age clumps (like those of large theropods): http://blog.hmns.org/?p=6371

P.S. I forgot to mention in my 1st comment that I liked your blog post as much as I did partly b/c it shows how well informed you are. Specifically, you seem to know the difference btwn true cooperative (I.e. Pack) hunting & pseudo-cooperative (I.e. Communal) hunting. I tried to make that difference clear when I said "group hunters" in reference to adult-only clumps/multiple shed teeth/trackways & "pack hunters" in reference to brains. Specifically, I was trying to say that while the former evidences could be interpreted as being for either pack hunting eudromaeosaurs or communal hunting eudromaeosaurs, combining them w/the latter evidence makes pack hunting eudromaeosaurs much more likely.

Mark Wildman said...

Hi Herman – there’s enough material here for several more posts I reckon and thanks again for your input. I’ll address a couple of points though.

I agree that trackways are compelling evidence for animals moving in groups – the recent confirmed multiple didactyl dromaeosaurid tracks in China, as well as others demonstrating several animals moving together, have provided further data.

Referring to the Albertosaurus bone bed again, this post was really to point out that tyrannosaurs probably did get together in groups for a specific reason for an unspecified amount of time. Maybe group hunting occurred, maybe not. It is the word “pack” that needs to be taken out of the equation and then we can all have constructive discussion, whether tyrannosaurs or dromaeosaurs are involved.

As stated, there are multiple Tenontosaurus sites with shed Deinonychus teeth amongst the remains and there is good evidence for multiple animals feeding on carcasses but as Maxwell stipulates:

“For the moment, the discoveries in Oklahoma present no evidence that Deinonychus was doing anything there other than scavenging carcasses of Tenontosaurus that were washed into the area, but it does indicate that Deinonychus roamed--and fed on Tenontosaurus--across an enormous expanse of what is now the western United States.“

However, the very good circumstantial evidence that you have highlighted does point to “group” hunting in deinonychosaurs but, as is the norm, we cannot assume “pack” hunting. And, as we all know, herein lies the problem, regardless of which taxon is involved, and despite the earlier comments that you highlighted. Thanks again for the comments Herman – they really are very much appreciated.

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