Dino-mite Utah discovery bolsters theory that T. rexes, like lions, hunted in packs

Did the dinosaur age’s most fearsome predators hunt by themselves, or did tyrannosaurs live cooperatively in groups?

A surprising discovery in Utah’s Grand Staircase-Escalante National Monument provides compelling evidence of the latter.

Scientists recovered several tyrannosaur specimens, representing animals of varying ages, that apparently died together in the Kaiparowits Plateau, suggesting complex social behavior akin to that seen today in lions and wolves, according to Alan Titus, a paleontologist with the Bureau of Land Management.

“Most predators are solitary. The reason for that is you have to have a very special purpose as a predator to want to get together and cooperate with what were originally your competitors for prey,” Titus said Monday in a news conference announcing the discovery. “And that involves guaranteeing success of taking down larger animals.”

Group hunting would reflect complex behavior that requires a level of intelligence not often associated with reptiles that lived millions of years ago.

“If you’re going to actually share in a hunt and have roles in a hunt and have a hierarchical structure with an alpha who’s in charge, that takes a fair amount of brainpower,” Titus said. “So the idea that large predators like T. rex could have actually been socially complex hunters with role playing and division of the hunt, with ambushers and chasers, and then sharing in the kills is somewhat controversial because a lot of researchers feel like these animals simply didn’t have the brainpower to engage in such complex behavior.”

Joined by collaborators from the University of Arkansas and the Denver Museum Nature & Science, BLM scientists published the tyrannosaur findings Monday in the open-access science journal PeerJ.

“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,” said contributor Joe Sertich, the Denver Museum’s curator of dinosaurs. “This discovery should be the tipping point for reconsidering how these top carnivores behaved and hunted across the Northern Hemisphere during the Cretaceous [Period].”

Major Utah discovery bolsters theory

This research builds on a theory first developed two decades by paleontologist Philip Currie, then a curator at Canada’s Royal Tyrrell Museum. Digging at a site called Dry Island Buffalo Jump in Alberta, he and his team found fossils of some 12 tyrannosaurs that perished at varying stages of their lives. The evidence suggested these individuals died in a single event, offering evidence that the predator was a social animal, Titus said.

The Utah discovery dates to 2014, when Titus and two colleagues, MJ Knell and Katja Knoll, were exploring a site on the Kaiparowits for turtle fossils. Titus noticed what turned out to be a tyrannosaur ankle bone protruding from the ground. It didn’t take long before they realized they had a major find underfoot.

“Within minutes of brushing around the ankle bone, we uncovered dozens of other tyrannosaur bones, including large hind foot toes,” he said, displaying one of the fossils. “We recovered so much bone from so many different kinds of animals over such a wide area, about an acre, that we nicknamed it the Rainbows and Unicorns site. I consider this a once-in-a-lifetime sort of discovery for myself.”

The individual tyrannosaurs, members of the Teratophoneus species, ranged from very young to fully mature. They estimated the dinosaurs ages at 4, 7, 10, 15 and 22 years at the time of death.

The quarry’s seemingly silly name started as a dig at Titus’ tendency toward hyperbole when describing promising paleontological places on the Kaiparowits, which has become one of the world’s most productive locations for new dinosaur discoveries. In the case of the new tyrannosaur quarry, the site truly did live up to the hype, so the Rainbows and Unicorns name stuck.

There is nothing silly about the resources there.

“The BLM is protecting these fossils as a national treasure,” Titus said. “They are our ancient heritage and part of the story of how North America came to be and how we came to be.”

Solving more dinosaur mysteries

In 2017, then-President Donald Trump slashed the 1.9 million-acre Staircase monument by half and broke it into three units — a move that President Joe Biden is expected to reverse. Located on the north end of the plateau a few miles east of Grosvenor Arch, the quarry is in a part of the monument unaffected by these politically wrenching decisions.

These 76-million-year-old fossils were found with bones of varying species of fish, turtle and alligators, raising yet another set of questions, Titus said. Why would a terrestrial creature like the tyrannosaur be found with the remains of so many aquatic animals?

“That becomes the first mystery we had to solve,” Titus said. The researchers ultimately concluded the tyrannosaurs died and their bones fossilized elsewhere, then were later washed away and deposited where he found them, mixed with aquatic creatures, perhaps in a river channel.

“You’ve got these sandbars and gravel bars in which all the bone is now residing, and so it’d be easy to conclude these animals just died upstream somewhere and were washed down the river during some major flood event,” Titus said. “But it got more complicated when we started to look at the details of the bones.”

Packed inside the bones were fine-grained muds and calcium carbonate, material that was of a different nature than the coarse sands they were found in. Geochemical analysis of the fossils and their embedded sediments helped prove the dinosaurs roamed the same environment and were fossilized in the same place, according to co-author Celina Suarez, a paleontologist with the University of Arkansas.

But adding to the mystery was the presence of ancient charcoal at the site.

“Did a fire rage through here and sweep all these tyrannosaurs in terror out in front of it and lead them down to a lake to get out of the fire and then they died in the flames?” Titus asked. Their analysis concluded the trees that burned were mostly redwoods, and the fire occurred after the dinosaurs died.

“It turned out it was a very low-temperature fire — that kind of creeping, slow-moving, smoldering fire that’s almost never able to kill large animals,” Titus said. “This is the kind of fire that a tyrannosaur could have walked away from.”