This week, researchers in southern Utah published a scientific paper suggesting that tyrannosaurs—a large, three-toed, carnivorous genus of dinosaurs—were gregarious animals, meaning they lived and hunted in packs. The idea challenges the prevailing belief that tyrannosaurs were lone predators. The suggestion that they actually had complex social relationships is controversial because it implies that tyrannosaurs had more sophisticated intellects than scientists have so far given them credit for. The newest evidence supporting the dino-pack theory comes from a fossil quarry in Grand Staircase-Escalante National Monument.
Dr. Alan Titus, Paria River District paleontologist for the Bureau of Land Management, discovered the quarry in 2014. Always enthusiastic about his work, he described the site to his employees at the time in glowing terms; one of those employees asked for a reality check from a co-worker.
“We all know with Alan Titus it’s always ‘rainbows and unicorns’ all the time,” the employee said, according to Titus’s retelling. The employee asked for someone else’s assessment of the site, without the rose-colored glasses.
However, the reply was that this time, the site really was as fantastic and magical as rainbows and unicorns. The name stuck.
“I consider this a once-in-a-lifetime discovery,” Titus said of the Rainbows and Unicorns Quarry at an April 19 press conference. “I probably won't find another site this exciting and scientifically significant during my career.”
Fossils from the quarry tell the story of a tyrannosaur mass death: five of the animals appear to have died in the same location, at the same time. The specimens found represent a range of ages: The youngest may have been only 4 years old, with the oldest being a full adult at 22 years old (scientists estimate the tyrannosaur lifespan to have been about 30 years.) Juveniles, subadults and adults gathering is an indicator of pack behavior.
“You don't see these sort of multi-generational groups among alligators or crocodiles, which would have been the previous analogy made for these tyrannosaurs,” Titus said. He noted that a modern example of dinosaur descendants behaving gregariously exists in the Harris Hawk, which can be traced back to a “second cousin” of the tyrannosaurs at the Rainbows and Unicorns site. The Harris Hawk lives in the Sonoran Desert and hunts in cooperative groups, dividing tasks and sharing kills.
“This behavior is possible among dinosaurs because here is a modern example of a dinosaur doing just what we're saying the tyrannosaurs would have done,” said Titus.
Scientists conducted a variety of tests to try to piece together the story of the fossils at the Rainbows and Unicorns Quarry. While uncovering the first bone found there—an ankle bone from an adult tyrannosaur—researchers quickly found dozens of other tyrannosaur bones. Along with the tyrannosaur bones, they found fossils from a variety of fish and turtles, as well as a giant alligator, all suggesting an aquatic setting. Since tyrannosaurs were terrestrial, scientists looked for an explanation for why their bones were in what appeared to be a lake or river bed. Analyzing the rock layers at the site and the rocks and minerals that had hardened inside the bones convinced researchers that the animals had all been fossilized in the same environment, and that the tyrannosaurs had died in the same event.
“You've got various isotopes of oxygen and carbon and they can be used to fingerprint both the environment that an animal lived in, and the environment that it was fossilized in,” Titus explained. An analysis of rare earth elements found in the bones also revealed links and context.
“We used a truly multi-disciplinary approach (physical and chemical evidence) to piece the history of the site together, with the end result being that the tyrannosaurs died together during a seasonal flooding event,” said Dr. Celina Suarez of the University of Arkansas, who led the geochemical research, in the press release.
This isn’t the first time scientists have theorized that carnivorous dinosaurs may have been gregarious. About 20 years ago, Canadian paleontologist Dr. Phil Currie pointed to a mass death site of Albertosaurus at a bone bed in Dry Island Buffalo Jump Park in Alberta, Canada as evidence of pack behavior in predatory dinosaurs. The Dry Island site yielded bones from 12 animals of the same species but different ages that appear to have all perished at the same time, suggesting that a group of varying-aged dinosaurs were “hanging out” together.
Currie commented on the significance of the Rainbows and Unicorns site in analyzing dinosaur behavior.
“Localities that produce insights into the possible behavior of extinct animals are especially rare, and difficult to interpret,” he said in an April 19 press release from the BLM, but he expressed conviction that the research done on the Rainbows and Unicorns fossils indeed indicate a mass death event.
“Undoubtedly, this group died together, which adds to a growing body of evidence that tyrannosaurids were capable of interacting as gregarious packs,” Currie said.
Researchers say excavation and study at the extensive Rainbows and Unicorns site will continue into the foreseeable future. Titus noted that the site is within the Kaiparowits Unit of the national monument, which is not affected by the ongoing dispute over the monument’s boundaries. [See “Haaland concludes Utah visit,” April 15 edition. -ed.] He said the controversy has not impacted his research.
“I haven’t noticed any disruption to my work; the fossils are all still there,” he said at the press conference when asked about the uncertain boundaries and political pressure on the monument.
At the April 19 press conference, Titus displayed a few of the impressive fossils found at the Rainbows and Unicorns site, like a reconstructed foot bone as long as his arm and a 7-inch long serrated tooth, “just like a steak knife, for sawing through flesh,” Titus said.
Some bone beds, like the La Brea Tarpits in California and the Cleveland Lloyd Dinosaur Quarry in Utah have yielded a large number of fossils, but they are not as valuable in providing clues to dinosaur behavior because they are “trap sites,” or locations where various individuals died at separate times by the same mechanism, such as being sucked into a mire. Rainbows and Unicorns offers more insight into animal behavior, a rare find in paleontology. According to an April 19 press release, future research plans for the Rainbows and Unicorns Quarry fossils include additional trace element and isotopic analysis of the tyrannosaur bones that paleontologists hope will determine with a greater degree of certainty the mystery of Tyrannosaur’ social behavior.
BLM Utah invites the public to learn more about the research by following it’s social media platforms on Twitter, Facebook, Flickr: @blmutah, and Instagram: @utahpubliclands throughout the week of April 19, 2021, and by viewing the research at: https://peerj.com/articles/11013/.