Tooth marks on a teenage Albertosaurus point to a small raptor scavenging a tyrannosaur

The bone shows no sign of healing, so the marks were made around the time of death or afterward ^[9]. That rules out a tyrannosaur that survived an attack and carried the scars ^[9]. On its own, the timing cannot separate a kill from a meal taken off a body. Doyon's preferred account is that the animal died of unknown causes and was eaten shortly afterward ^[1]. The paper's title calls the behaviour scavenging ^[2].

The second suspect is an unusual one. Doyon puts the baby Albertosaurus at about 2 metres long, feeding on a subadult estimated at about 6 metres and 530 kilograms ^[7]. That is an animal a third the length of the one it was eating ^[12]. "If it is a young Albertosaurus, this would be an event of cannibalism," Doyon said ^[6]. The published summary does not explain why the raptor ranks as more likely than the baby ^[1].

Albertosaurus was the apex predator in what is now Alberta between about 73 and 67 million years ago ^[8]. "This turns everything on its head, because you're dealing with a very small dinosaur eating a very big, mean, nasty dinosaur like a tyrannosaur," said Philip Currie, the University of Alberta paleontologist who supervised Doyon ^[4]. The tyrannosaur in this case was a subadult of about 530 kilograms ^[7]. On Doyon's preferred account, it was also already dead ^[1].

The find cannot show how often this happened. The study says the find points to a more complex food web than previously thought ^[3]. The evidence for that is one scattered collection of skull fragments ^[10]. One collection can show what was possible. Imagine two versions of Cretaceous Alberta: in one, small raptors routinely fed on tyrannosaur carcasses; in the other, a single raptor once found a dead teenager. Both would leave the same marks on this jaw.

I think the evidence supports a narrower claim than the food-web one, and supports it reasonably well. The broader version, that dromaeosaurids regularly scavenged tyrannosaurs, would need bitten tyrannosaur bones counted across collections. What this jaw supports is that a dromaeosaurid probably fed on one teenage Albertosaurus shortly after it died ^[1].