The number one rule of the Taphonomy Club is don’t talk about marks on bones … without placing them in context. Many marks on bones could have multiple causes, such as putative cut marks caused by stone tools on animal bones found on early hominid sites. In that case, hard sharp stony objects in the ground can cause marks that are hard to tell apart from stone tool marks. But when you find almost all the possible stone tool marks in the exact locations they would be if a hominid was butchering or defleshing the animal, then you can assert that that butchery or defleshing with stone tools was highly likely to have happened.
A similar logic has been applied by paleontologists DWE Hone and DH Tanke in their study of the fossil remains of a dinosaur from Dinosaur Provincial Park in Alberta, Canada. The dinosaur exhibits numerous bite marks, and apparently (unlike stone tool cut marks) identification of these marks as caused by carnivorous dinosaur teeth is not in question. But the location of the marks and other features allowed these scientists to argue that some sort of combat regularly occurred between members of members of the same species, or similar species, during the animal’s life. Given what is known about animal behavior and the kinds of dinosaurs around at the time, they claim that it is most likely combat between members of the same species.
The dinosaur in question is a juvenile Daspletosaurus. This is a genus of dinosaur extant in western North America between 77 and 74 million years ago (Late Cretaceous).
Since everyone knows all about Tyrannosaurus, it is helpful to compare Daspletosaurus to Tyrannosaurus. Daspletosaurus was smaller and older. Daspletosaurus ranged around 8 or 9 meters long and 2.5 tonnes, while Tyrannosaurus could be over 12 meters long and 10 tonnes. Tyrannosaurus also lived later (68 million years ago up to about the time of the great extinction). Both had short arms but Daspletosaurus’s arms were longer. Note that this kind of dinosaur, suborder Theropoda, gave rise to birds.
This particular juvenile Daspletosaurus was well preserved. Many of the bones are present, and their position in the matrix that bore them is not too far off from anatomical location. A good number of the missing bones may have actually eroded away after this part of the bone bed was exposed by erosion. There are marks on some of the bones that indicate post-death scavenging. But, most of the tooth marks are of the kind one would expect if a theropod dinosaur was biting it, and most interestingly, most of these marks show evidence of healing, and all but one mark indicating damage is on the head. Normally, theropod inflicted bite marks are found on various different bones of their prey. It appears that this individual was engaged in combat with other individuals of the same sort … other theropods. And, since this is probably the only theropod of this size at the time in the area, it is reasonable to conclude that this is evidence of infraspecific combat or competition.
From the study’s abstract:
Trace marks on the bones of non-avian dinosaurs may relate to feeding by large carnivores or as a result of combat. Here the cranium and mandible of a specimen of Daspletosaurus are described that show numerous premortem injuries with evidence of healing and these are inferred to relate primarily to intraspecific combat. In addition, postmortem damage to the mandible is indicative of late stage carcass consumption and the taphonomic context suggests that this was scavenging. These postmortem bites were delivered by a large bodied tyrannosaurid theropod and may have been a second Daspletosaurus, and thus this would be an additional record of tyrannosaurid cannibalism.
I contacted lead study author Dave Hone with a few questions and he was kind enough to give me answers.
I asked him if he had any guess as to the sex of this individual. While it is possible to sex some dinosaurs, he told me that this was not possible in this case.
I asked Dr. Hone to comment further on the suggestions that the most likely species to have inflicted the pre-mortum wounds was another Daspletosaurus, even though another similar dinosaur, Gorgosaurus, was around at the time. He told me, “We favour Daspleto for the premortem as we think (and based on previous papers) this is a more likely case with more intra than interspecifc aggression leading to these kinds of interactions,” similar to what we see in modern animals that exhibit this behavior. I also wondered if the size of the teeth could indicate the size of the offending beast, and thus confirm the species. He told me they did not look at this too closely because there are various problems with that approach. “We did look at the patterns of tooth distribution briefly but between different sizes of animals (juveniles vs adults) different sizes of teeth within the jaws (front vs back) and then things like missing teeth etc. there’s no way of separating them out. There’s just way too many variables and they are only leaving limited marks. It’s mostly hard to tell even very different animals apart from bite marks let alone two similar and close relatives like this.”
I asked how common Daspletosaurus is in the fossil record and if this was one of the more common tyrannosaurids. He told me that “Actually it’s not that common. The Albertan Tyrannosaurs are generally pretty common but we do for example have more Gorgosaurus and Albertosaurus than Daspleto,” though Daspletosaurus is well represented.
Daspletosaurus is distinct in part because of various extra bony bits in the face and around the eyes, which could be for any of a number of functions. I asked if it is possible that Daspletosaurus was more involved with usually-but-not-always non-lethal infraspecific combat than other tyrannosaurids, if these features are related to what might have been extra protection (or signaling features that might arise from sexual selection). If so, would this indicate something about social structure? He told me, “I’m very wary of making these kinds of extrapolations as some things that look like certain classic signals turn out not to be. My personal opinion is that these hornlets in various Tyrannosaurs likely did function in sociosexual signaling (at the very least I suspect they wouldn’t do much to protect the eyes since that would be tricky place to bite) but it’s hard to say much. Sociality is misleading here as some things can be very social and fight lots and others almost never and vice versa for solitary animals.”
I also wondered about how infraspecific combat square with the individual being relatively young. Would this imply it was fighting off adults intent on cannibalism? Or, were juveniles fighting it out like hyenas do (new born hyena males from the same litter engage in deadly combat)? Or fighting over food? Or engaged in ritual fighting behavior that precedes, as preparation/practice, adult fighting behavior? I wondered if this would say anything about life history development of behaviors in this dinosaur. Dr Hone told me that “it is really hard to say. This isn’t an adult, but then nor is it really a juvenile. We know that some dinosaurs at least can reproduce before they are fully grown (so they are sexually mature when they are not osteologically mature – actually rather like humans, though obviously rather unlike most mammals, and certainly birds). So things get complex fast. This animals was certainly old enough to have been fully independent (though of course they may or may not have been gregarious / social etc.). I doubt cannibalism was normal, I’m sure there were the odd fights that resulted in deaths or adults killed the odd small juvenile (just like crocs do) but it’s a rare behaviour to go after other big carnivores for food – they are rare and dangerous, so stick to baby herbivores. After that it gets even harder so I’d prefer not to speculate too much, though I’d guess that IF solitary, smaller individuals would probably not be holding territories, since they are not big enough to defend them, and obviously immature animals would not be competing for mates or breeding sites or IF in a group to be an alpha of some kind (though that’s not to rule out some aggression to maintain even a lower rank), but it’s not much to go on – just too many unknowns.
What we need, obviously, is some way to bring these creatures back to life so we can observe them alive!
Caption for the figure at the top of the post: Figure 1: Skull in right lateral view showing numerous injuries indicated with black arrows and the relevant code letter (see the text for details).