So tyrannosaurs were scavengers then…

The argument over ‘was Tyrannosaurus (or big tyrannosaurs) a predator or scavenger’ has been going on for far too long and is, I think, not only dull, but largely pointless. It rather assumes that they were one or the other, when pretty much no carnivore is, what they are, is both. Jackals might favour scavenging and lions predation, but jackals hunt and lions eat random carcasses. Where a given carnivore (or even omnivore) lies on the scavenger-predator spectrum is of interest, but still not worthy of the coverage Tyrannosaurus has had on this subject.

Interesting though a great many papers have been on the subject, and clever and intriguing that many analyses have been, getting the ‘answer of predator or scavenger was never likely to be satisfactory *because* the answer was always very likely to be ‘both’. All of this has recently been reviewed and covered by Tom Holtz and thus the book might appear to be closed on the subject, since both are in evidence. But there is always room for more data, especially when it comes from a new fossil find.

Predation has long been in evidence for tyrannosaurs what with various healed bites and even teeth wedged into the bones of surviving meals. Certainly they tried to kill and eat living herbivores on occasion. But scavenging is by definition rather harder to generate. You might find a totally mutilated carcass where every bit of possible nutrition has been extracted such that even the bones have been eaten (or at least ‘gnawed’ at) but was this a scavenging event or did the predator have a very leisurely time over the meal and was able to effectively destroy the prey item. Hyena are indeed predators but a pack will leave little but horns after having killed an antelope just as they would if they found a dead one on the plain. So while there are several incidences of bodies having been seriously got-at by tyrannosaurs it’s been difficult to *really* be totally confident that they represent scavenging. For the record, I think they do, but there has naturally been some doubt about it. What you really need is a body which could not have been killed by a tyrannosaur, but was still fed on by one.

What you need is something like the specimen presented in my new paper. This is a humerus of the hadrosaur Saurolophus from the Late Cretaceous of Mongolia that is covered by bites from a Tarbosaurus. Tarbosuaurs is a giant tyrannosaur that rivals Tyrannosaurus in size (though is generally a bit smaller with a narrower skull) and indeed the two are very close relatives. We can match up the spacing of the tooth marks with the tooth spacing of the jaws of Tarbosaurus, some marks are deep implying a large carnivore was at work (and it is the only thing around at that time that could deliver such a bite) and other specimens are known in the Formation. It’s the only reasonable candidate and thus we have a good idea of both feeder and feed-e.

Saurolophus humerus with tyrannosaur bite marks. From Hone & Watabe, in press.

We know the humerus is from a Saurolophus because, critical bit, nearly the whole skeleton is preserved. And it’s in superb condition, one of the best I have ever seen, even from a place that often yields superbly preserved material. To be more specific, the humerus is the only thing with any marks at all on it and there are dozens of bites and scrapes. How could this have happened?

Well that hadrosaur was found it what was, when it was buried, a river channel. And that humerus was uppermost on the skeleton and had suffered not only tyrannosaur-based damage, but also a little erosion. We hypothesizes therefore that the animal died in the channel, or more likely died elsewhere and came to rest at that point. It was then covered by sand such that only the arm (which may or may not have lost the lower arm) was on the surface. Along comes a large Tarbosaurus who makes as much as a meal as is possible and leaves his teeth marks behind but not much else.

There’s a little more evidence to come too since it seems unlikely that a large tyrannosaur would pick on one part of the body to this degree and leave the rest alone (no other tooth marks). Furthermore, living carnivores usually go for the legs and belly first and the arms close to the end of their feeding cycle (since there’s less meat there). The only obvious explanation for such dedicated feeding on one, relatively small, and atypical part of the skeleton is that this was the only option. In short, the animal was already dead and buried with only the humerus exposed (suffered weathering when the rest did not) and this is therefore scavenging.

The paper is open source so you can download and read it here for even more details. I’ll be posting more tomorrow too on the fascinating selective feeding strategies of tyrannosaurs. One last comment in passing, in the reviews for this paper one referee got to the point where we claimed that this was pretty definitive evidence of scavenging and left the note “Finally!”.We all assumed that tyrannosaurs would and did scavenge and we already had pretty good evidence that they did, but this paper hopefully does provide a fair shove in this direction to show it really did happen. Tyrannosaurs were indeed scavengers, *and* predators.

Hone, D.W.E., and Watabe, M. 2010. New information on scavenging and
selective feeding behaviour of tyrannosaurs. Acta Palaeontologica Polonica.

More to come tomorrow, with good coverage also at New Scientist and Live Science. Fare less good coverage alos present on the web if you look.