Discovery of Mass Death Site Bolsters Theory That Tyrannosaurs Hunted in Packs

Discovery of Mass Death Site Bolsters Theory That Tyrannosaurs Hunted in Packs
Artist's impression of the tyrannosaurs shortly after being killed in a flood and washed into a nearby lake. A Deinosuchus alligator is shown n in the background. (Image: Victor Leshyk)

A remarkable fossil site in Utah, in which several tyrannosaurs were found buried together, strengthens a burgeoning theory that these fearsome creatures hunted in packs, similar to wolves.

That tyrannosaurs were social hunters is a possibility palaeontologists have been considering for more than 20 years. Back in 1910, palaeontologists working in Alberta, Canada, discovered the remains of 12 tyrannosaurs that appeared to have died together. This discovery was largely forgotten until Canadian palaeontologist Philip Currie, now with the University of Alberta, revisited the old finding in 1998, arguing that it was evidence for “gregarious behaviour” in tyrannosaurs and that these animals were pack hunters.

Seven years later, Currie, along with several colleagues, reported on a similar discovery made in Montana, in which the remains of three tyrannosaurs, belonging to the genus Daspletosaurus, were likewise found together. And in 2014, palaeontologists described fossilised dino footprints found in British Columbia, Canada, which appeared to show three tyrannosaurs moving in the same direction at the same time.

Despite this evidence, scientists have been reluctant to ascribe gregarious behaviour to tyrannosaurs, claiming that the limited cognitive capacities of dinosaurs couldn’t have possibly allowed for it. Critics of this theory will now have to consider a third mass death site, as described in a new paper published in PeerJ.

The fossil site is situated within the Grand Staircase-Escalante National Monument, and it yielded the remains of four, possibly five, tyrannosaurs, all of whom appeared to have died at the same time. The fossils were buried at the site of a former river, with the authors of the new paper saying their deaths were likely the result of seasonal flooding.

“The new Utah site adds to the growing body of evidence showing that tyrannosaurs were complex, large predators capable of social behaviours common in many of their living relatives, the birds,” Joe Sertich, a co-author of the paper and curator of dinosaurs at the Denver Museum of Nature & Science, explained in a press release. “This discovery should be the tipping point for reconsidering how these top carnivores behaved and hunted across the northern hemisphere during the Cretaceous.”

Currie, who wasn’t involved in the new study, said the finding “adds to a growing body of evidence that tyrannosaurids were capable of interacting as gregarious packs,” as he was quoted in the press release, prepared by Utah’s Bureau of Land Management.

Team members mapping bones at the site.  (Image: Bureau of Land Management) Team members mapping bones at the site. (Image: Bureau of Land Management)

The bones of these dinosaurs were found buried in the late Campanian age Kaiparowits Formation of southern Utah, which goes by the wonderful nickname, “Rainbows and Unicorns Quarry.” Study co-author Alan Titus from the Bureau of Land Management discovered the site in 2014, and it represents the first tyrannosaur mass death site to be found in the southern United States.

Titus and his colleagues uncovered the remains of several Teratophoneus, a genus of tyrannosaur that lived in the Cretaceous approximately 77 million to 76 million years ago. This genus is known by a single species, Teratophoneus curriei, the largest members of which measured somewhere between 21 and 26 feet (6.4 and 7.9 meters) in length. Tyrannosaurs, or tyrannosaurids, describe a family of oversized carnivorous dinosaurs that stood on two legs, the most famous examples being Tyrannosaurus rex, Albertosaurus, Daspletosaurus, and Tarbosaurus.

In addition to the Teratophoneus fossils, Titus and his colleagues uncovered several turtles, various fish and ray species, a nearly complete skeleton of a 3.7-metre long Deinosuchus alligator, and two other dinosaur species (none of these animals are believed to have died in the event that killed the Teratophoneus specimens). In addition to these bones, the scientists gathered fragments of tiny rocks and sandbar deposits from the former Cretaceous river.

“We realised right away this site could potentially be used to test the social tyrannosaur idea. Unfortunately, the site’s ancient history is complicated,” said 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.”

Indeed, both chemical and physical evidence recovered from the site allowed the team to make sense of this ancient scene, despite the aforementioned geological disruptions. Analysis of stable carbon and oxygen isotopes, along with concentrations of rare earth elements, yielded “a relatively homogeneous signature,” as the paleontologists wrote in their paper. This strongly suggests that the fossils were all derived from the same source population and that the animals died and became fossilised together. It also suggests that no other animals were introduced to the burial site at a later date.

The scientists suspect that a seasonal flood killed the tyrannosaurs, washing their bodies into a nearby lake, where they eventually became buried. The team entertained several other possibilities to explain the mass deaths, including poisoning (e.g. drinking water contaminated with cyanobacteria), drought, fire, and even drowning in quicksand. Of these scenarios, the flood is considered to be the most plausible explanation, according to the scientists.

The find at the Rainbows and Unicorns Quarry is obviously a big deal, as it’s potential evidence for not just cooperative hunting among tyrannosaurs but sociality in general, which could apply to other domains as well, namely extended parental care. That said, not everyone is persuaded by the new evidence.

“It is a little tougher to be so sure that these data mean that these tyrannosaurs lived together in the good times,” Kristi Curry Rogers, a biology professor at Macalester College, told the Associated Press. “It’s possible that these animals may have lived in the same vicinity as one another without travelling together in a social group, and just came together around dwindling resources as times got tougher.”

Fair enough. Just because these dino bodies were all buried together doesn’t automatically mean they actually partook in pack hunting. As Rogers suggests, the Teratophoneus dinosaurs may have gathered together to feast on a fallen animal, which may or may not have been typical behaviour for these theropods. Vultures, for example, descend upon a common meal, but these birds can hardly be described as pack hunters.

Consequently, other lines of evidence will be required to bolster this hypothesis, specifically evidence to show that these animals willingly hung out with each other and that they did so in a cooperative manner. That won’t be a simple thing to prove, but nobody said paleontology was easy.