The max speed of a Tyrannosaurus rex was about 12 mph, according to scientists at University of Manchester. (Reuters)

Fifty years ago, if you'd asked the dinosaur experts, they would have told you that Tyrannosaurus rex was a speed freak — a giant predator that could outrun racehorses. The paleontologists would have pointed to its 3-meter hind limbs, leggy for even a big dinosaur, and described the creature as sprinting after prey at speeds of 40 mph.

But a better understanding of physics kneecapped the swift T. rex concept. Scientists, rather than looking at the bones alone, began to gauge dinosaur locomotion via models of skeletons and muscles. The picture that came into focus was a slower beast: T. rex, although no less of a predator, was certainly less fleet of foot.

A new report, published Monday in the journal PeerJ, refined just how fast the dino could go. T. rex wasn't much of a runner, the study authors say. In fact, it couldn't run at all. Instead, the animal speed-walked.

Thanks to its wide stride, T. rex would reach about 12 mph at its quickest hustle. “Most of us would have difficulty jogging to keep up,” said paleontologist Phillip L. Manning, director of the Mace Brown Museum of Natural History at the College of Charleston, S.C. (Not that you'd want a dinosaur with 8-inch-long teeth as a walking buddy.)

Previous estimates of T. rex's maximum pace ignored weight loads on the animal's bones. The new study took those into account, Manning said, and the result was “a bit like putting a governor on an engine.” The scientists asked the model T. rex to move as fast as it could. At a run, the model indicated that T. rex's bones would snap because its legs would buckle under its weight.

Unable to break into a sprint, it must have stepped its way through the Cretaceous Period.

But that doesn't mean a 7-ton T. rex, at a full-bore walk, was anything less than terrifying. Elephants also don't run, which is to say they never have all four feet off the ground, and have been reliably clocked at about 10 mph. That's fast enough to leave a lasting impression. “The joy of an elephant charging you, trust me,” Manning said, “it's one of those trouser-filling moments.”

John Hutchinson, an expert on evolutionary biomechanics at the University of London's Royal Veterinary College who has also estimated T. rex speeds, called this work a “very sophisticated computer simulation.” The max estimate of 12 mph was in line with the low end of previous tyrannosaurus models.

But like all computer simulations, this one came with certain assumptions baked in, Hutchinson noted. The study authors assumed that the animal's muscles were optimized to be as strong as they could, figuring that bone strength was the T. rex's weak point. Hutchinson said he was uneasy about that assumption, noting that “we don't understand the principles that govern speed limits in living animals.”

Manning acknowledged that the model could be improved: It gave a conservative estimate and did not account for features such as elastic tendons, which could bump up the T. rex's top speed.

At this point, Hutchinson said, such a consensus has built around T. rex behavior that the dinosaur “is quickly running out of questions” for scientists to answer. The locomotion of other dinosaurs presents more interesting scenarios, in his view. He is investigating how the earliest dinosaurs moved — animals the size of house cats that would have had to scamper away from giant land crocodiles or end up as lunch.

Manning is also curious about other dinosaur species, particularly the biggest of the bunch. “It's modeling a house walking across the plain,” he said. “It is quite exciting.”

Read more:

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How T. rex’s powerful bite crushed dino bones to a pulp

Scientists believe they’ve found a ‘pregnant’ T. rex