The fierce T.rex hunted in female-led packs

It is hard to imagine a more terrifying creature than Tyrannosaurus rex, a predator with jaws bristling with banana-sized teeth serrated like steak knives.

But a dinosaur fossil deposit in Alberta is revealing an even scarier picture: this huge and fearsome animal probably hunted in packs.

“It would have been a horrifying sight,” says Philip Currie, professor of dinosaur paleobiology at the University of Alberta. Currie is one of the world-famous dinosaur scientists appearing in the Nature of Things documentary The Real T. rex. The show explores the very latest science to bring the ancient animal to life as accurately as possible.

A bonebed is a concentrated layer of fossilized bones from many animals. Currie has been excavating the fossil bonebed, found in Alberta’s Dry Island Buffalo Jump Provincial Park, since 1997. Usually, bonebeds are dominated by remains of plant-eating dinosaurs, but this one is unique because it contains fossils from as many as 26 individual meat-eaters, young and old.  They are all from a single species, Albertosaurus sarcophagus, a close relative of T. rex that is classified as a “tyrannosaur” family member.

“It just struck me as being so crazy that tyrannosaurs, which we consider being incredibly rare in places like Dinosaur Provincial Park or Drumheller [Alberta] was suddenly very common,” says Currie. “The indications are very clear in this bonebed that the tyrannosaurs were here because they died together at the same time and almost certainly were living together at the time of their death.”

Currie has several theories on what might have killed the animals all at once. They might have died in a forest fire. Or perhaps they were killed by a lightning strike from a powerful storm.

We used to think that T. rex was a solo hunter that came together only to mate. But in the 1970s paleontologists uncovered strong evidence that plant-eating dinosaurs probably travelled in massive herds. Currie says that changed our picture of the animals that preyed on them: “Plant-eating dinosaurs protected themselves by grouping together when they were migrating. The only way the carnivores could counter that was to move in packs to group-hunt with some chance of breaking up these bigger herds of plant-eating animals.”

Tracking the location of a 100-year-old dig

The bonebed was actually discovered in 1910 by pioneering palaeontologist Barnum Brown. Currie says Brown realized that his find was strong evidence that Albertosaurus lived in packs, but he did not explore the idea any further. In 1996, Currie examined Brown’s Albertosaurus fossils at the American Museum of Natural History in New York. He wondered if Brown’s original dig site held any more secrets.

The exact location was unknown, but they did have a photograph from the 1910 dig. Currie and his team ultimately found the site by matching the topography in the background of the photo.  “We started working on it, and it was pretty incredible to find that it wasn’t just adult animals. There were juveniles in there, including some very small animals.”

The biggest animal was 11 metres nose-to-tail and weighed as much as an elephant. Most of the juveniles were seven to eight metres long. “To me, that is very suggestive that the juveniles had to be part of whatever social system they had,” says Currie. “It wasn’t just the adults feeding the juveniles. The juveniles were probably taking care of themselves to a large extent.”

A female-led pack of dinosaur predators, young and old

Currie thinks the juveniles would charge huge herds of migrating plant-eaters, driving vulnerable animals back toward the slower, larger adults who would make the kill. “Up front of this whole group you’d have the small but agile young tyrannosaurs that are moving incredibly fast,” says Currie. “Behind that, you would have these giants walking that do so much damage with their incredible strength.”

Today the Albertosaurus bonebed is famous among dinosaur researchers as the best evidence tyrannosaurs lived in social groups.

Currie believes that the leader of the pack was probably the biggest animal, so it is likely tyrannosaur packs were matriarchies. “The bulk of the evidence these days seems to be suggesting that the biggest tyrannosaurs were the females,” he says. Researchers can sometimes determine the gender of a dinosaur by looking for extra bone tissue growth in hollow spaces inside big bones such as femurs. Like modern birds, female dinosaurs stored the extra bone for a supply of calcium to build eggshells.

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Currie says his team has only excavated an area that’s 50 metres across, and the bone bed extends for at least a half a kilometre. “I’d say that it still holds a lot more secrets.”

Watch The Real T. rex on The Nature of Things