Assuming you could pick any time to wander along the Rocky Mountain Front, would you rather be hunted by a grizzly bear or a Tyrannosaurus rex?

As top-of-the-food-chain predators go, Montana has always provided charismatic megafauna. More than half of the T-rex fossils on display worldwide came from the Treasure State’s 65-million-year-old Cretaceous seashore. Today, the largest concentration of grizzlies in the Continental United States prowls the Bob Marshall Wilderness Complex, numbering around 1,000.

In between, we had Velociraptors, saber-tooth cats, dire wolves, rattlesnakes and the occasional letter bomb-mailing hermit.

A new computer model released last week pegs T-rex’s top speed at 12 mph: a devaluation from the often-assumed 30 mph other researchers surmised from its body shape. What were those two lanky back legs for if not outrunning anything on the lunch menu? It certainly wasn’t making a living with those two dinky forearms.

“(D)espite a century of research since Osborn’s (1916) work on tyrannosaur limb anatomy there remains no consensus on the most accurate maximum speeds for T. rex, or indeed whether or not its gigantic body size prohibited running completely,” University of Manchester lead author William Sellers and colleagues wrote in the journal PeerJ. Estimates ranged from 5 to 15 meters per second, or 11 to 33 mph.

The authors estimated a T-rex weighed 7,206 kilograms, or almost 8 tons. Then they calculated the stress levels its leg bones could absorb at different speeds and gaits. At that size, their model found the predator would suffer something between shin splints and thigh fractures at speeds above 7.7 meters per second, or 17 mph.

They also concluded that T-rex was a race-walker, not a real runner. It probably never got both feet in the air at the same time — the definition of a running stride.

The statue of a sprinting Daspletosaurus (a predecessor of T-rex) shades the parking lot of Bynum’s Two Medicine Dinosaur Center. Director Cory Coverdell said he regularly gets asked about the running speed of the various fossil species on display.

“We just don’t have enough information about what they were doing when they were alive to evaluate a lot of these things,” Coverdell said. “None of our dig sites have footprints, and of the ones that are known, most aren’t complete enough to give stride length. It’s a more complex question than you’d think.”

We don’t know if T-rex out-ran its prey like a modern cheetah or ambushed victims with quick bursts like a crocodile. We do know from its dagger-like teeth and organ structure that it ate only meat, so it probably prowled alongside herds of prey dinosaurs like a wolf follows an elk herd. While it may not have needed speed, it did require stamina. 

The all-meat diet meant T-rex wouldn’t settle into some Cretaceous huckleberry patch like a grizzly bear. But it probably would scavenge anything already dead, just as grizzlies frequently do. As Coverdell noted, "Anything will take a free meal if it can get it."

Grizzlies hold the present-day keystone predator status in Montana, with adult males weighing 500 pounds or more. They can nevertheless get all that bulk moving in a hurry.

“They can run faster than horses for short distances,” said Montana Fish, Wildlife and Parks spokesman Bruce Auchly in Great Falls. “A bear will catch you at any distance. Think of the finishers at the Missoula Marathon — what was their time? If you run a 12-minute mile, that’s five miles an hour. At that rate, you’re gonna die.”