Evidence indicates tyrannosaurs may not have been a solitary species after all

New scientific paper published after seven years of research and excavation in Grand Staircase-Escalante National Monument


Bureau Of Land Management

Media Contact:

David Hercher

KANAB, Utah — The Bureau of Land Management (BLM) Utah today announced publication of peer-reviewed research based on a fossil site inside Grand Staircase-Escalante National Monument. The paper provides new evidence that fearsome tyrannosaur dinosaurs may not have been solitary predators as popularly envisioned, but social carnivores, similar to wolves.

The findings about Grand Staircase-Escalante National Monument’s very own “dinocidal” tyrannosaur (Teratophoneus-pronounced Ter-at-oe-foh-nee-us) were released through publication in the open-access scientific journal PeerJ on April 19.  

Written by a team of researchers from the BLM, Denver Museum of Nature and Science, University of Arkansas, Colby College of Maine, and James Cook University in Australia, the study examines a unique fossil bone site inside Grand Staircase-Escalante National Monument called the “Rainbows and Unicorns Quarry” that they say exceeded the expectations raised even from the site’s lofty nickname.  

"Localities [like Rainbows and Unicorns Quarry] that produce insights into the possible behaviour of extinct animals are especially rare, and difficult to interpret,” said world-renowned tyrannosaur expert Dr. Philip Currie. “Traditional excavation techniques, supplemented by the analysis of rare earth elements, stable isotopes and charcoal concentrations convincingly show a synchronous death event at the Rainbows site of four or five tyrannosaurids. Undoubtedly, this group died together, which adds to a growing body of evidence that tyrannosaurids were capable of interacting as gregarious packs." 

In 2014, BLM Paleontologist Dr. Alan Titus discovered the Rainbows and Unicorns Quarry site in Grand Staircase-Escalante National Monument and led the subsequent research on the site, which is the first tyrannosaur mass death site found in the southern United States. Researchers ran a battery of tests and analyses on the vestiges of the original site, now preserved as small rock fragments and fossils in their final resting place, and sandbar deposits from the ancient river.

“We realized right away this site could potentially be used to test the social tyrannosaur idea. Unfortunately, the site’s ancient history is complicated,” said BLM Paria River District Paleontologist Dr. Alan Titus. “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.” As the details of the site’s history emerged, the research team concluded that the tyrannosaurs died together during a seasonal flooding event that washed their carcasses into a lake, where they sat, largely undisturbed until the river later churned its way through the bone bed.  

“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. Using analysis of stable carbon and oxygen isotopes and concentrations of rare earth elements within the bones and rock, Dr. Suarez and her former PhD student, Dr. Daigo Yamamura, were able to provide a chemical fingerprint of the site. Based on the geochemical work, they were able to conclusively determine that the remains from the site all fossilized in the same environment and were not the result of an attritional assemblage of fossils washed in from a variety of areas. “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. The similarity of rare earth element patterns is highly suggestive that these organisms died and were fossilized together,” said Dr. Suarez. 

Excavation of the Quarry site has been ongoing since its discovery in 2014 and due to the size of the site and volume of bones 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 died 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, Dr. 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.” 

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 Teratophoneus’ social behavior. 

BLM Utah invites the public to learn more about the research by following our 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/.  

In stark contrast to the social interaction between humans and among many species of animals, paleontologists have long debated whether tyrannosaurs, the undisputed rulers of the Northern Hemisphere during the Late Cretaceous Period (66-100 million years ago), lived and hunted alone or in groups.  

Based on findings at a site in Alberta, Canada with over 12 individuals, the idea that tyrannosaurs were social with complex hunting strategies was first formulated by Dr. Philip Currie over 20 years ago. This idea has been widely debated, with many scientists doubting the giant killing machines had the brainpower to organize into anything more complex than what is observed in modern crocodiles. Because the Canadian site appeared to be an isolated case, skeptics claimed it represented unusual circumstances that did not reflect normal tyrannosaur behavior. Discovery of a second tyrannosaur mass death site in Montana again raised the possibility of social tyrannosaurs, but this site was still not widely accepted by the scientific community as evidence for social behavior. The researcher’s findings at the Unicorns and Rainbows Quarry provides even more compelling evidence that tyrannosaurs may have habitually lived in groups.” 

This research is part of cooperative partnership funded in large part by the BLM, as well as individual donations through Experiment.com (“Death of a Tyrant Project”), Grand Staircase Escalante Partners, and the National Science Foundation. High resolution imagery of the “Rainbows and Unicorns Quarry” discovery and excavation are available at: https://bit.ly/2Q1RWmF.  

The BLM manages more than 245 million acres of public land located primarily in 12 western states, including Alaska, on behalf of the American people. The BLM also administers 700 million acres of sub-surface mineral estate throughout the nation. Our mission is to sustain the health, diversity, and productivity of America’s public lands for the use and enjoyment of present and future generations.