What is scarier than a tyrannosour? Try a herd of tyrannosaurs. But that would never happen because they were solitary hunters, right? All the movies tell us so. Turns out, tyrannosaurs may have been a whole lot more social than we thought.

In 2011, guide outfitter Aaron Fredlund was traveling near Tumbler Ridge, northeastern British Columbia. Deciding not to “cross the river for the millionth time” Fredlund crossed a ridge instead and came across two giant footprints. His wife persuaded Fredlund to pass on the photographs and Richard McCrea, curator of the Peace Region Palaeontology Research Center considered them important enough to mount an expedition within the month – a rare event in a slow-moving field.

McCrea’s team found five more prints from tyrannosaurs, as well as 30-40 of other species. The most significant feature, besides the outstanding preservation of the prints, is that the seven tyrannosaur prints come from three individuals, one of which was missing a claw.

^{Schematic of print locations}

The clay of the time was “pretty much the consistency of Play-Doh,” according to McCrea, before being covered by a thick layer of volcanic ash that preserved the prints so well McCrea can make out traces of the dinosaurs rough skin like a fingerprint.

In PLoS One, McCrea notes, “The skeletal record of tyrannosaurids is well-documented, whereas their footprint record is surprisingly sparse. There are only a few isolated footprints attributed to tyrannosaurids and, hitherto, no reported trackways.”

In this case however, the prints show the three animals heading in the same south-easterly direction in an 8.5m-wide corridor. The depth and preservation of the tracks suggest the dinosaurs were traveling very close to concurrently.

The distance between the prints will help work out these giant creatures’ gait, estimated at 8.5km an hour when not chasing prey, a boon for future Walking With Dinosaur films. However, for most people the bigger news is that tyrannosaurs were gregarious.

The idea is not entirely new to palaeontologists, indeed some people have gone so far as to coin a collective noun, a terror of tyrannosaurs. The paper refers to an existing hypothesis along these lines based on related species and three fossils being found near each other, but this is by far the most powerful evidence yet produced.

The three dinosaurs in question were thought to be in their mid-20s, standing 2.35m at the hip and weighing around 3 tonnes, so this was not a case of young following a parent. Being almost fully grown, the species in question were substantially smaller than Tyrannosaurus rex, thought to reach 6 tonnes.

^{Coloring the prints by depth helps reveal the forces applied to make them}

Such a find would normally promise further revelations, but McCrae fears the prints only survived while they were covered in earth, and may not make it through an icy winter now they have been exposed. While the prints have been covered for protection, the Center lacks the money to cut them from the rocks and fly them out. British Columbian paleontological sites are not protected by law, so the researchers are relying on secrecy of location to prevent this one being despoiled by trophy hunters.