Tyrannosaurs were pretty capable killers, and it’s easy to assume they didn’t need much help bringing down their prey. But contrary to the popular image of these fearsome predators being lone hunters, new analysis of a mass grave site adds to evidence tyrannosaurs may have lived and hunted in packs.
As depicted in the Jurassic Park movies, the T-Rex was thought to be a solitary creature – and it’s hard to argue that one would have been terrifying enough. Pack-hunting behavior was more thought to be limited to smaller predators like the Velociraptors in the movie.
But the question has been raised about whether tyrannosaurs really were loners, or social carnivores. After all, when you’re hunting dangerous prey like Triceratops or Ankylosaurus, a tag team might be the surest way to a meal.
Evidence for the hypothesis has included fossilized tracks that suggest several tyrannosaurs moving together and, most importantly, sites containing the bones of multiple individuals, who apparently died together.
The new study is based on one of these boneyards, known as the Rainbows and Unicorns Quarry in Utah. The site was discovered in 2014 by paleontologist Dr. Alan Titus, and contains the remains of four or five individual tyrannosaurs from a species called Teratophoneus.
The problem with these sites is that just finding a few skeletons together doesn’t necessarily mean that they all lived and died together. They could have died years or centuries apart, and had their bones washed into the same spot by currents.
“We realized right away this site could potentially be used to test the social tyrannosaur idea,” says Titus. “Unfortunately, the site’s ancient history is complicated. With bones appearing to have been exhumed and reburied by the action of a river, the original context within which they lay has been destroyed. However, all has not been lost.”
So the team conducted geochemical analyses of the bones and the rocks surrounding them. By studying the concentrations of stable carbon and oxygen isotopes, as well as the concentrations of rare earth elements within, they found that the animals had indeed all died together.
The tyrannosaurs most likely died during a seasonal flooding event, the team says, which washed their bodies into a lake. They would have sat there for a while, before the lake eventually dried up and, later still, a river carved its way through the rock and disturbed the bones.
“The new Utah site adds to the growing body of evidence showing that tyrannosaurs were complex, large predators capable of social behaviors common in many of their living relatives, the birds,” says Dr. Joe Sertich, an author of the study. “This discovery should be the tipping point for reconsidering how these top carnivores behaved and hunted across the Northern Hemisphere during the Cretaceous.”
The research was published in the journal PeerJ.
Source: Bureau of Land Management