By Ian Sample
Threats to the fearsome reputation of Tyrannosaurus rex appeared to have been seen off on Monday by fresh evidence unearthed in the US.
The dinosaur's feeding habits have long been debated by academics, with some claiming that T rex was less a ferocious hunter and more a lumbering slowcoach that scavenged the carcasses of beasts that had died at the claws of others.
The latest evidence comes from palaeontologists who found remnants of a prehistoric skirmish in a slab of rock at the Hell Creek Formation in South Dakota. The clash, which occurred around 66m years ago, involved a T rex and a large, plant-eating hadrosaur, and ended with the tooth of the former lodged firmly in the spine of the latter.
Scans of the tooth and two surrounding tail vertebrae showed clear signs of bone healing around the wound, taken as proof that the hadrosaur was alive at the time of the attack and survived for several months or even years afterwards.
"This is unambiguous evidence that T rex was an active predator," the authors write in the journal Proceedings of the National Academy of Sciences. "Such evidence is rare in the fossil record for good reason – prey rarely escapes."
Tyrannosaurs shed their teeth frequently as fresh sets came through. In this case a weaker rear tooth broke free as the T rex, which was not fully-grown, chomped on the hadrosaur's tail. The hadrosaur is believed to have been an adult Edmontosaur, which grew to around 10 meters in length.
"We not only have a broken-off tooth embedded in the bone of another animal, but the bone has healed over the wound, and a nasty wound it was too," said David Burnham at Palm Beach Museum of Natural History in Florida.
The remains join a large collection of fossils that tell their own partial stories about the dining habits of T rex. Previous discoveries reveal rake, puncture and chew marks on bones, while one specimen – an impressive half-meter of fossilized faeces – contained partly digested dinosaur bones. In all of these cases, it is hard to differentiate between predation and scavenging.
Palaeontologists expressed mixed reactions to the latest findings. Jack Horner at the Museum of the Rockies in Montana, who served as technical adviser on the Jurassic Park movies, said: "This one piece of evidence does seem to suggest that a tyrannosaur bit a hadrosaur, but certainly doesn't provide any indication of the sort of carnivore the rex actually was."
In 2011 Horner and his team reported that T rex was probably an opportunistic carnivore like hyena, which take carrion and occasional live prey. "This paper certainly offers no evidence to refute that hypothesis," Horner added.
Paul Barrett, a dinosaur researcher at the Natural History Museum in London, expressed exasperation that the debate was still ongoing. "The whole T rex scavenger or predator debate is pretty intractable and not particularly enlightening. Work on living carnivores, like big cats and wolves, clearly show they use both strategies depending on what's available to them. They'll generally make do with a meal from either source if it satisfies their dietary needs. Any other extinct carnivore, including T rex, is likely to have been the same," he said.
"This paper shows without question that a T rex bit a living hadrosaur, but it can't show if this was a regular behavior or not, or even if this was hunting behavior rather than some other kind of interaction," he added.
But Sam Turvey, a senior research fellow at the Institute of Zoology in London, called it "important and convincing" new evidence. "Even though T rex may have fed on carcasses when the opportunity arose – a behavior also seen in modern-day carnivorous large mammals such as lions – the new findings provide strong evidence that these iconic dinosaurs were fully capable of being active predators, and help to dismiss the ecologically unrealistic hypothesis that they were restricted to a scavenging lifestyle," he said.