The finding is based on a unique fossil site inside Utah’s Grand Staircase-Escalante National Monument that contains ʙᴏɴᴇs of four or five tyrannosaurids — members of the Tyrannosauridae family that includes Tyrannosaurus rex.
Analyzing rare earth elements, stable isotopes and charcoal concentrations within the ʙᴏɴᴇs and surrounding rocks from the area known as “Rainbows and Unicorns Quarry,” researchers determined the tyrannosaurids ᴅɪᴇᴅ together, likely during a seasonal ғʟᴏᴏᴅing event that washed their bodies into a lake, according to a news release about the study. An ancient river later washed the ʙᴏɴᴇs of the dinosaurs, known specifically as genus Teratophoneus, into their final resting place, which was discovered by U.S. Bureau of Land Management paleontologist Alan Titus.
“We realized right away this site could potentially be used to test the social tyrannosaur idea. Unfortunately, the site’s ancient history is complicated,” Titus said. “With ʙᴏɴᴇs appearing to have been exhumed and reburied by the action of a river, the original context within which they lay has been ᴅᴇsᴛʀᴏʏᴇᴅ. However, all has not been lost.” As the details of the site’s history emerged, the research team concluded that the tyrannosaurs ᴅɪᴇᴅ together during a seasonal ғʟᴏᴏᴅing event that washed their ᴄᴀʀᴄᴀssᴇs into a lake, where they sat, largely undisturbed until the river later churned its way through the ʙᴏɴᴇ bed.
Tyrannosaur expert Philip Currie said locations like Rainbows and Unicorns Quarry that “produce insights into the possible behavior of extinct animals are especially rare, and difficult to interpret.”
“Traditional excavation techniques, supplemented by the analysis of rare earth elements, stable isotopes and charcoal concentrations convincingly show a synchronous ᴅᴇᴀᴛʜ event at the Rainbows site of four or five tyrannosaurids. Undoubtedly, this group ᴅɪᴇᴅ together, which adds to a growing body of evidence that tyrannosaurids were capable of interacting as gregarious packs,” Currie said in a news release from the BLM, which was part of the study along with the University of Arkansas, Denver Museum of Nature and Science, Colby College of Maine and James Cook University in Australia.
Currie first formulated the idea that tyrannosaurs were social with complex hunting strategies 20 years ago based on fossils found in Alberta, Canada, according to the University of Arkansas news release. A second tyrannosaur mass ᴅᴇᴀᴛʜ site in Montana again raised the idea that tyrannosaurs were social, but many scientists continued to doubt that the large dinosaurs had the mental capacity to organize into packs.
Excavation of the quarry site has been ongoing since its discovery in 2014 and due to the size of the site and volume of ʙᴏɴᴇs found there the excavation will probably continue into the foreseeable future. In addition to tyrannosaurs, the site has also yielded seven species of turtles, multiple fish and ray species, two other kinds of dinosaurs, and a nearly complete skeleton of a juvenile (12-foot-long) Deinosuchus alligator, although they do not appear to have all ᴅɪᴇᴅ together like the tyrannosaurs.
“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 project contributor, Joe Sertich, curator of dinosaurs at the Denver Museum of Nature & Science. “This discovery should be the tipping point for reconsidering how these top carnivores behaved and hunted across the northern hemisphere during the Cretaceous.”