Within the theropod dinosaurs existed tyrannosaurs, bipedal carnivores with lots of teeth. They first emerged as small dinosaurs but grew in size in the Late Cretaceous, giving rise to some of the largest land predators ever to stomp about on Earth (including the 2.5 billion Tyrannosaurus rex that once existed). A new study has proven that, though one of the better-studied groups of dinosaurs, we still have much to learn about tyrannosaurs as a treasure trove of fossilized specimens provides new evidence for prosocial living among these predators. In short, tyrannosaurs may not have been the solitary predators we thought, and instead hunted in packs.

The announcement, from the Bureau of Land Management (BLM), presents research on a collection of fossils discovered in the Grand Staircase-Escalante National Monument in Utah. It centers around the tyrannosaur Teratophoneus whose name roughly translates to “monstrous murderer”. The cluster of Teratophoneus remains in the Rainbows and Unicorns quarry was discovered by BLM paleontologist Dr Alan Titus in 2014 and contains individuals of different ages ranging from juvenile to large adult. 

"Localities [like Rainbows and Unicorns Quarry] that produce insights into the possible behavior of extinct animals are especially rare, and difficult to interpret,” said world-renowned tyrannosaur expert Dr Philip Currie (for whom Teratophoneus species T. curriei is named) in a statement. However, according to the study published in the journal PeerJ, researchers led by Titus have used the final resting place of this group of tyrannosaurs to argue the social tyrannosaur theory.

“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,” continued Currie. “Undoubtedly, this group died together, which adds to a growing body of evidence that tyrannosaurids were capable of interacting as gregarious packs."

The discovery flips the commonly held belief that tyrannosaurs were solitary predators, instead indicating they were social dinosaurs that may have hunted as social carnivores do, in packs like wolves.

Reaching this conclusion was a fraught path owing to the site’s somewhat muddled fossil record. Bones found in the area have shown signs of having been brought up and reburied likely due to a river, meaning their final resting place may not represent the context in which they died. This meant the researchers had to go deeper if they wanted to establish whether this group of Teratophoneus remains were brought together in life or after death.

“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. “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.”

As Currie mentioned though, interpreting behaviors from fossils is difficult. These dinosaurs may have died together, they may have even lived in the same vicinity, but that doesn't necessarily mean they lived and traveled as a social group together. They may have been forced into proximity due to dwindling resources. Although that could also be an argument for prosocial behavior – a social behavior that benefits the whole group, like hunting in numbers.

This is an exciting discovery no doubt, but the team isn't finished with the Rainbows and Unicorns quarry tyrannosaurs just yet. Their next step is to run further trace-element and isotopic analyses on the Teratophoneus remains, hopefully adding further clout to the concept that these dinosaurs lived in groups.