Did tyrannosaurs - such as Gorgosaurus - live in social groups?
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Photo by Brian Switek.
Did tyrannosaurs - such as Gorgosaurus - live in social groups?

Tracks Hint at the Social Life of Tyrant Dinosaurs

What’s scarier than a tyrannosaur? Three tyrannosaurs. That’s simple, undeniable math. The question is whether or not the tyrant dinosaurs ever prowled together in real life. Up until now, the evidence has been equivocal. But a trackway found in British Columbia finally provides firmer ground for speculation on the social lives of these celebrated carnivores.

The idea that large tyrannosaurs – like Albertosaurus and the mighty Tyrannosaurus itself – worked together to bring down prey isn’t new. University of Alberta paleontologist Philip Currie has been championing the hypothesis for over a decade, citing bonebeds that contain multiple individuals of the Late Cretaceous tyrannosaurs Albertosaurus and Daspletosaurus. And three years ago Currie went to the public with his idea in the form of a book and documentary called Dino Gangs. “If dinosaurs hadn’t become extinct, would gangs of killer tyrannosaurs now rule the world?”, the hyperbolic documentary asked.

At the time, I wasn’t sold on tyrannosaurs being communal carnivores. Just because a whole bunch of dinosaurs ended up buried together doesn’t mean that they actually lived and hunted together. A mass of dinosaur skeletons – such as an Albertosaurus bonebed containing 12 individuals – can contain elements from animals that died at different times or were brought into a small area by unusual circumstances. Accumulations of dinosaur  bones represent the circumstances of death and burial more than clues about life.

If evidence for social tyrannosaurs exists, it has to be in a less ambiguous form. Tracks hold the most potential.

Tracks and other traces are signs of prehistoric behavior and biology. For example, we know tyrannosaurs fought by biting each other on the face from healed wounds on their skulls. Whether tyrannosaurs lived in groups, though, requires something more. While an Albertosaurus bonebed is ambiguous evidence for social behavior, a trackway showing that tyrannosaurs walked together would be a much clearer sign of social tyrants.

This is what happened with “raptors.” The pack-hunting idea played out in Jurassic Park was based on a Deinonychus quarry that contained multiple predators as well as the apparent prey, but the site only really shows that several of the sickle-clawed predators died at the site. It has only been more recently, with the discovery of distinctive, two-toed tracks that paleontologists have been able to confirm that raptors at least sometimes walked together.

The problem for tyrannosaurs is that their footprint record is very poor. Some supposed tyrannosaur tracks – such as a big, three-toed prints found in a Utah coal mine – turned out to be prints left by shovel-beaked hadrosaurs. Other, authentic tyrannosaur tracks are isolated specimens, recording a single footfall on the Cretaceous ground and nothing more. This historic dearth of trace evidence is what makes the tyrannosaur trackway discovered in Canada so important.

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A diagram of the tyrannosaur trackways found in British Columbia. From McCrea et al., 2014.

In October 2011, a few months after Dino Gangs debuted, local outfitter Aaron Fredlund discovered two dinosaur tracks in the roughly 75 million year old rock of northeastern British Columbia’s Wapiti Formation. An excavation by the Peace Region Palaeontology Research Centere turned up a third track in the sequence, but the best find didn’t come until the following summer. In August 2012 paleontologists and volunteers found two more sets of tracks right next to each other.

Unfortunately for paleontologists, the dinosaurs who left the footprints did not die in their tracks. But, as reported by Peace Region Palaeontology Research Centre researcher Richard McCrea and colleagues in a new PLoS One paper, there was only one type of dinosaur alive in the area that could have left such prints. All of the tracks are big – with a length of over 19 and a half inches – and were made by a dinosaur with three forward-pointed toes that ended in sharp claws. The tracks had to be made by a tyrannosaur.

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Detail of Trackway A. From McCrea et al., 2014.

Which species of tyrannosaur created the tracks is unknown. Three large tyrannosaurs – Albertosaurus, Gorgosaurus, and Daspletosaurus – all lived in western Canada at the time the tracks were made. But the tracks are distinctive enough that, following trace fossil convention, McCrea and coauthors have given them their own name. The tracks have hence been labeled Bellatoripes fredlundi, an homage to the “warlike foot” shapes of the tracks found by Fredlund.

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The hypothesized step cycle for the tyrannosaurs that made the trackways. From McCrea et al., 2014.

While they might superficially look like trident-shaped potholes, the tracks provide a wealth of information about the animals that made them. The animal that made the first-discovered trackway, for example, was apparently missing the end part of the second toe of the left foot, leaving impressions of the nub as it walked. And the details of the footprints showed that these tyrannosaurs walked in a different way than some other carnivorous dinosaurs. Trackways made by predatory dinosaurs typically show that the dinosaur put their foot down and lifted their foot out in a forward motion, while the tyrannosaur tracks suggest that these animals lifted their feet out in a backwards motion.

But the arrangement of the tracks is what has gained the most attention and plays into speculations of tyrannosaur packs. All three trackways face the same direction and were made in close proximity, with 18 feet between Trackway A and B, and eight feet between B and C. While there’s a chance that the trackways were made by three individuals that traveled the same area at different times, McCrea and colleagues consider this unlikely. Tyrannosaurs were a rare part of the fauna, there is no sign of an obstacle that would create a bottleneck requiring tyrannosaurs to walk the same path, and the detail of the tracks – down to foot scales on some – hint that they were all made around the same time, while substrate conditions were consistent. That two, if not three, tyrannosaurs were walking together is the simplest explanation of the pattern.

So did tyrannosaurs hunt in packs? Maybe. The tracks, stunning as they are, can only take us so far.

There remains a shred of doubt that these footprints were made by a single social group. If the tracks showed some sign of interaction between the animals – like raptor footprints that show one adjusting its course to move out of the way of another – then we could be sure. In fact, I’ve encountered this problem out in the field while looking for tyrannosaurs and their Mesozoic kin. Sometimes I’ll find myself closely following the tracks of another fossil hunter, even though they passed by minutes or hours before I did, and I’ll leave my own trail right alongside theirs.

But let’s say the site really does record a social group, as it seems to. We don’t know why the tyrannosaurs were walking together. Was this a family? A hunting party? Suitors trying to follow a mate? Nor do we know how long the band remained together. The tracks only record a few brief moments in Late Cretaceous time when, if even for a moment, tyrants flocked together.

Reference:

McCrea, R., Buckley, L., Farlow, J., Lockley, M., Currie, P., Matthews, N., Pemberton, S. 2014. A ‘terror of tyrannosaurs’: The first trackways of tyrannosaurids and evidence of gregariousness and pathology in tyrannosauridae. PLoS ONE. 9, 7: e103613. doi:10.1371/journal.pone.0103613