Tooth marks indicate that a small carnivorous dinosaur may have fed on a tyrannosaur, much larger than itself.
The scene dates back to the end of the Cretaceous, in what is now Canada. Researchers examined a fossilized Albertosaurus skull discovered in Alberta. This animal belonged to the same broad family as Tyrannosaurus rex.
Albertosaurus skull on display at the National Museum of Natural History in Washington.
Credit: via Wikimedia Commons.
The fossil comes from an individual that was still young, but already imposing. Study of its bones indicates that it was at least eight years old when it died. Its length is estimated at around 6 m. Yet two sets of marks found on its jaw appear to have been left by much smaller teeth.
To identify their origin, the researchers studied the spacing of the marks and the fine grooves produced by the teeth. They also corrected their measurements according to the angle at which the teeth had slid against the bone. Without this precaution, oblique traces can give a misleading impression of the size of the jaw responsible.
The results best match a dromaeosaurid, the group of carnivorous dinosaurs that includes raptors. An animal related to Atrociraptor may have left these marks. It was much smaller than the Albertosaurus, part of which it may have consumed.
Reconstructions of Albertosaurus at the Royal Tyrrell Museum.
Photo: D’Arcy Norman, via Wikimedia Commons.
The marks are located on parts of the jaw that would have been accessible when an animal was already dead or severely emaciated. They therefore most likely testify to a meal taken from a carcass, a scavenging behavior.
Another explanation remains possible. A very young Albertosaurus could also have left some of these traces. In that case, it would be cannibalism, a behavior already documented in several tyrannosaurids. The researchers nevertheless consider the raptor the most likely candidate.
This discovery changes our view of the relationships between these predators. Large tyrannosaurs could themselves hunt smaller animals, but their carcasses also represented a food reserve. An opportunistic raptor did not need to defeat a living giant to benefit from its flesh.
If this identification is confirmed, it would be the first direct evidence of a dromaeosaurid feeding on a tyrannosaur. The researchers now propose applying their measurement method to other bones bearing bite marks, in order to better reconstruct the dietary relationships between dinosaurs.