New Fossils Suggest Tyrannosaurs May Have Hunted in Packs

Researchers say the trove of four or five specimens found in southern Utah challenges the assumption that these predators were solitary

Teratophoneus curriei
A mounted specimen of the type of tyrannosaur at the heart of new research that suggests these predators may have lived in groups. These skeletons are from a species named Teratophoneus curriei, and show an adult (left) and juvenile (right) at the Natural History Museum of Utah. Jens Lallensack via Wikimedia Commons under CC BY-SA 4.0

Paleontologists studying a group of four or five tyrannosaurs at a single fossil site in southern Utah say the find suggests the imposing predators may have lived and even hunted in packs, reports Sophia Eppolito for the Associated Press.

These fossils, described in a study published this week in the journal PeerJ – Life & Environment, make up the third mass fossil site of tyrannosaurs found in North America, according to a statement. Around 20 years ago, those prior discoveries prompted paleontologists to hypothesize that the extinct carnivores might have been more social than initially imagined.

“A lot of researchers feel like these animals simply didn’t have the brain power to engage in such complex behavior,” Alan Titus, a paleontologist with the Bureau of Land Management and lead author of the study, said in an online briefing Monday. But the three mass graves of tyrannosaurs found preserved together are beginning to tell a different story. “This must be reflecting some sort of behavior and not just a freak event happening over and over again,” said Titus.

The newly described fossils were found in 2014 in Grand Staircase-Escalante National Monument at a site that paleontologists have nicknamed the Rainbows and Unicorns Quarry because of the splendid array of fossils it has produced.

The tyrannosaurs in question are close relatives of Tyrannosaurus rex from the genus Teratophoneus, which comes from the Greek words for "monster” and “murderer," that lived in the Late Cretaceous between 77 and 76.5 million years ago.

The size of the bones suggest the group was made up of one adult around 22 years old, one subadult and two or three juveniles that appear to be roughly four years old, reports Cameron Duke for the New Scientist. Per the statement, the adult skeleton measured just shy of 30 feet long.

To prove that the dinosaurs died together and weren’t just thrown together by chance, the team combined the standard physical study of the fossils with chemical analyses using rare earth elements, stable carbon and oxygen isotopes, and charcoal concentrations, reports Juliet Eilperin for the Washington Post.

“None of the physical evidence conclusively suggested that these organisms came to be fossilized together, so we turned to geochemistry to see if that could help us,” says Celina Suarez, a paleontologist at the University of Arkansas and co-author of the research, in the statement. “The similarity of rare earth element patterns is highly suggestive that these organisms died and were fossilized together.” Apparently, the group all drowned in a flood that subsequently washed them into a lake bed.

In the statement, tyrannosaur expert Philip Currie, a paleontologist at the University of Alberta who first proposed the notion that these dinosaurs might have been social animals, says the results convincingly argue that the group died together, “which adds to a growing body of evidence that tyrannosaurids were capable of interacting as gregarious packs.”

Speaking with the AP, Kristi Curry Rogers, a biology professor at Macalester College who wasn’t involved in the research, says she’s not so sure about interpreting the mass grave as a sign of group living or pack hunting under normal circumstances. “It’s possible that these animals may have lived in the same vicinity as one another without traveling together in a social group, and just came together around dwindling resources as times got tougher,” Rogers tells the AP.

The public lands that birthed these intriguing fossils are located in one of two national monuments in Utah that former President Donald Trump dramatically reduced, the other being Bears Ears. David Polly, a paleontologist in Indiana University who wasn’t involved with the study, tells the Post that the new paper shows the importance of these national monuments for scientific research at a time when the Biden administration is mulling whether to expand Grand Staircase-Escalante and Bears Ears to their former size.