Predator or Putz?

New evidence suggests that the famous T. Rex wasn’t so scary after all

Predator or Putz?With its bone-crushing teeth and monstrous proportions, there’s no creature more fearsome in the public imagination than the mighty Tyrannosaurus rex. Sixty-five million years after it went extinct, though, T. Rex is having an identity crisis: in recent months, much of what we know about this iconic monster has been “flipped on its ear,” says Stephen Brusatte, a vertebrate paleontologist at the American Museum of Natural History in New York. Indeed, the tyrannosaur’s size, speed and eating habits have all been thrown into question, leaving dino lovers wondering if the T. Rex really was so fearsome after all.

The North American T. Rex and its Asian cousin, the Tarbosaurus, are two of about 10 dinosaurs that fall into the tyrannosaur family, Brusatte says. Because tyrannosaurs were at the top of the prehistoric food chain, paleontologists long believed different members of this family couldn’t coexist. But a newly described fossil has challenged that assumption. With its thin teeth, horns, and hollow bones, Alioramus altai was a “ballerina” compared to T. Rex, says Brusatte, lead author of the study. Unearthed in Mongolia’s Gobi Desert, Alioramus lived side by side with the Asian Tarbosaurus, suggesting not all tyrannosaurs were “top chiefdom brutes,” he notes. Perhaps because Alioramus wasn’t viewed as competition, Tarbosaurus didn’t seem to mind this smaller, daintier relative.

But Alioramus isn’t the only recent addition to the tyrannosaur family tree. In September, researchers unveiled a 125-million-year-old fossil with many of T. Rex’s defining features, at only one per cent of its weight. This tiny T. Rex, dubbed Raptorex kriegsteini, lived 60 million years before its larger cousin; unlike T. Rex, which weighed up to six tons, Raptorex was only about three metres long, and weighed just 140 lb.

Smaller tyrannosaurs have been discovered before, but there’s something special about Raptorex: its puny forearms, which are much like T. Rex’s. The latter’s own tiny arms were thought to have evolved as compensation for its gargantuan size, notes Paul Sereno, a paleontologist at the University of Chicago, but Raptorex suggests otherwise since it has disproportionately small arms too, despite its smaller stature. T. Rex’s tiny arms could have evolved out of some behaviour, then, or might be a compensation for its massive head, a feature Raptorex shares. Either way, Sereno says, both dinosaurs have a similar body plan—one he describes as “jaws on legs”—that helped T. Rex and Tarbosaurus dominate the food chain, across the northern hemisphere, for millions of years.

Jaws on legs, indeed—who could forget that scene in Jurassic Park, when the bloodthirsty dinosaur runs down a Jeep? In real life, though, T. Rex might not have been so spry. Karl Bates is a Ph.D. candidate at the University of Manchester, and studies the movements of dinosaurs. Using a CT scan, Bates takes an image of a dino skeleton, then “puts the scans in software they use to make animations, like Shrek,” he says. Adding muscle and bone—making educated guesses on muscle strength, tendon length, and so on—Bates then tries to figure out in what order these muscles moved, to reveal the dino’s gait. The computer runs through millions of possibilities, much like cracking a code. “Usually, the model falls straight over, because the pattern is rubbish,” he says. When it finally doesn’t, though, Bates has a better idea of how the dinosaur might have walked.

Working with the skeleton of an Acrocanthosaurus, a predatory dinosaur similar in size to a T. Rex, Bates has determined it would have had an average running speed of about 24 km/h, slightly slower than a typical human. (The margin of error is “huge,” he admits, since the dino’s true muscle size isn’t known.) Bates’s Ph.D. supervisor, zoologist Bill Sellers, has calculated a tyrannosaur’s running speed at 29 km/h, a bit faster than a human but slower than an emu. Still, it’s quite a difference from some early estimates that pegged a T. Rex’s top speed at 70 km/h, according to Tim Tokaryk, acting head of paleontology for the Royal Saskatchewan Museum—and nowhere near as fast as the T. Rex that ran down the Jeep, a scene Bates calls “rubbish.”

Fleet-footed or not, most paleontologists agree T. Rex was a predator; but one of the world’s most prominent says otherwise. Jack Horner, rumoured to be the real-life inspiration for the character of Alan Grant in the Jurassic Park movies, insists T. Rex was a scavenger, feeding off carcasses like a vulture instead of hunting and killing its own prey. The dino’s massive size would be a disadvantage while hunting, Horner believes, pointing out that T. Rex “couldn’t run, and if it fell from that distance, it has no hands to catch itself.” As a scavenger, though, size would be a boon, scaring off smaller predators so it could feed on their kill. And T. Rex had bone-crushing teeth, he notes, not tearing teeth like a velociraptor.

Horner admits he’s a “loner” in his views. Other experts think so, too. “There’s no reason to believe T. Rex was a scavenger,” Brusatte says, noting that its strong bite force, binocular vision (which provides depth perception) and keen sense of smell all suggest otherwise. “There’s no analogue today of a five-ton collector of dead carcasses,” he says. Adds Sereno, “I don’t think [T. Rex], as the largest meat eater out there, was waiting around for something to keel over.” Horner stands by his opinion. Other paleontologists “want T. Rex to be a predator,” he says. “They think it’s cooler.”

The debate on T. Rex’s eating habits was further complicated recently when University of Alberta researcher Phil Bell announced he’d discovered the first known evidence of tyrannosaur cannibalism: the tooth of one Gorgosaurus, a 10-m long T. Rex cousin, lodged in the jawbone of another. (Dino cannibalism has only been demonstrated once before, he notes, in a Majungasaurus from Madagascar.) The new find doesn’t help clarify whether tyrannosaurs were scavengers or not, Bell says, as one dino might have killed the other, or gorged on the victim after it was already dead. Even so, “if they’re brave enough to go up against a member of their own species, I don’t think they’re going to be cringing from a young hadrosaur,” he says.

As if all that weren’t enough, a final T. Rex shocker emerged in September: Sue, the world’s most famous dinosaur, wasn’t killed in a vicious dino brawl, as was previously believed. It seems she succumbed to a lowly parasite. The most complete T. Rex skeleton in the world, Sue has a series of holes in her jaw once thought to be battle scars; experts have concluded they were probably caused by an infection instead. But that might have been the least of her worries. “Sue had all sorts of problems,” including arthritis, another infection in her leg, and a shoulder injury, says Bill Simpson of the Field Museum in Chicago, where Sue is on display. At 28 years old, she’s the world’s oldest known T. Rex, and “that’s what happens when we get older,” he says.

And so, dino buffs shouldn’t take it too hard. Whether it can run down a Jeep or not, T. Rex will always be king.

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