This study has already made a few headlines, and why wouldn’t it—first evidence that Tyrannosaurus rex, arguably the most familiar dinosaur and one of the biggest terrestrial predators ever, was a cannibal.
The conclusion is based on examination of bones from Tyrannosaurus and its known prey species that bear what appear to be Tyrannosaurus tooth marks. I’m actually surprised a study like this hasn’t been done before. Perhaps it’s been a long time in the making because of the time it would take to get access to and examine all the specimens from multiple locations.
The process of determining what caused the marks is intriguing. Large crocodiles are ruled out, because their teeth are conical, not serrated (I assume the lack of serration also rules out claw marks). Large lizards have serrated teeth, but not as big as the teeth that bit into these tyrannosaurs. Likewise for dromaeosaurids (relatives of Velociraptor). Scavenging insects leave meandering channels in bone rather than straight gouges. Only a large theropod could have made the bite marks, and T. rex is the only known large theropod from that place and time.
Cannibalism is by no means uncommon; if you’re a large predator or scavenger, you’re not likely to be picky about what you eat, especially if it’s conveniently dead already. The authors note that, given the low probability of any given dead thing becoming a fossil, the fact that they found any cannibalized specimens at all indicates that it may not have been unusual in this species.
Some aspects of the taphonomy suggest that these dinosaurs were scavenged, not hunted. However, given that two of the specimens are isolated bones (i.e. who knows what the cause of death was), and given that some T. rex skeletons show signs of healed wounds that could well have been inflicted in intraspecific conflicts, I’m not sure that we can absolutely rule this out. It is clear, though, that the bite marks were made after the victim was dead, and furthermore, three of the four specimens seem to have been gnawed on by juvenile or subadult T. rexes. It’s not likely that a juvenile could bring down an adult T. rex, and, as the authors say (in my favourite sentence of the paper), “[i]t seems unlikely that a small Tyrannosaurus would be allowed to repeatedly bite a much larger individual several times on a single toe.” So scavenging does seem like the best explanation. Interestingly, the opposite is true of many extant predators: when cannibalism occurs, it’s often a case of outright hunting and killing rather than scavenging. There is a surprising amount of controversy that this article is glossing over regarding how often T. rex actually hunted versus merely scavenged. (See Horner’s book, The Complete T. rex; the Wikipedia article on T. rex sums up the arguments.)
One other large theropod, Majungatholus, is known to have eaten its own kind, and there’s a possibility that Daspletosaurus and/or Gorgosaurus did too. I’m intrigued as to how common this might have been amongst theropods. There must be at least one or two other species for which we have many specimens and few other potential predators (it would be unusual to find more than one species of gigantic predator in a given ecosystem, although it’s not unheard of). Allosaurus might be a good one to try because it’s abundant, and especially interesting since some have suggested that it was a cooperative hunter; however, it co-occurs with two other large theropods that might be hard to rule out.