03/23/2006 01:00 AM EST

BY BRYAN ROURKE
Journal staff writer

PROVIDENCE -- Tyrannosaurus rex was not a runner, or even a jogger.

T. rex was a walker, mingling among other dinosaurs, occasionally meeting up with a slower one for a meal.

"It's not that Jurassic Park was right or King Kong was wrong," says Stephen Gatesy, an associate professor of ecology and evolutionary biology at Brown.

It's that our understanding of dinosaurs is lacking. And it always will be, according to Gatesy. We don't know how these creatures lived, he says, or how they behaved. That is, in part, because we don't know how they moved.

"We'll never know," Gatesy says. "It's not like you can go out and film one. The evidence is very limited."

Basically, it's bones. Muscles are missing. Movement is imagined.

This is the thrust of "Beyond the bones," an article in last week's Nature magazine, written by Gatesy, who specializes in the study of animal movement, and John Hutchinson, a paleontologist at The Royal Veterinary College in London who specializes in the study of physical forces on animals.

"We can make general inferences about individual species, as well as about large-scale patterns of locomotor evolution," the authors write. "But specific hypotheses about dinosaur motion, mechanics and performance [such as speed, acceleration and agility] remain controversial."

That makes Hutchinson and Gatesy controversial. They've made a specific hypothesis about the motion, mechanics and performance of a T. rex.

"There are thousands of possibilities," Gatesy says. "We were forced to pick one."

THE AMERICAN Museum of Natural History, in New York City, asked the men to make a choice. Last year the museum created a new exhibit, "Dinosaurs: Ancient Fossils, New Discoveries." Would Gatesy and Hutchinson be interested, the exhibit organizers wondered, in serving as animation consultants?

Frankly, they weren't.

"John and I are more conservative than some people who are willing to go on TV and say, 'It had to go like this,' " Gatesy says. "We don't think that's particularly useful."

While Gatesy can't conclusively say how T. rex moved, he can say how it didn't move. That's what he told the people at the American Museum of Natural History when he received its animator's initial rendering of a moving T. rex.

"Animators are trying to be inspired by the science," Gatesy says. "But that's not the same as doing the science."

The animator's rendering, Gatesy says, showed the dinosaur moving forward but in a slight side-to-side manner. Walking straight forward, Gatesy says, would be more likely, more consistent with the rest of the animal world, and better supported by science.

For their recommendation on T. rex movement, Gatesy and Hutchinson chose by process of elimination, discounting the least likely options based on physics and physiology.

What remained was what they recommended to the American Museum of Natural History.

"To be honest, we don't even know if that's viable," Gatesy says.

GATESY CAN'T describe dinosaur motion with certainty because there are too many variables and unknowns: multiple joints, possible angles, range of motion, body posture, tail position and, additionally problematic, no notion of where the dinosaur's center of gravity was.

"If it had large air sacs in its chest, that would affect everything," he says.

One thing Gatesy and Hutchinson can say with relative certainty is that T. rex didn't run.

It was too heavy -- about six tons -- so its bones might break.

"Let's assume the skeleton is strong enough," Gatesy says. "Then the soft tissue is the weak link. How much muscle do you need?"

As an animal runs, Gatesy says, it's not merely moving its weight, but up to three times its weight. The animal's legs must counter their own energy, the body's weight and the force of gravity.

"The weight is on one leg in a collapsed posture," Gatesy says. "How much muscle do you need to not continue falling into the ground?"

The answer is found in a six-ton chicken. You'll be relieved to know no such thing exists. But for hypothetical purposes, Hutchinson took a normal chicken, with its disproportionate leg muscles (20 percent of its body mass) for running. Then he extrapolated how it would manage at the size of a T. rex.

The short answer is it wouldn't: 120 percent of the animal's weight would have to be in its legs to withstand the trauma of running.

"As you get larger and larger, your muscles become less and less effective," Gatesy says.

Larger animals tend to move on relatively straight legs to better support their weight, and, consequently, are not fast runners. Smaller animals tend to move on bent legs, which requires lots of energy but allows for faster running.

T. rex walked, Gatesy says. And evidence suggests that it walked relatively quickly: 10 meters per second, about the speed of a running human (10 mph).

HOWEVER, everything regarding dinosaurs is debatable to some degree.

"The problems of missing data [such as muscle size] will always remain and cannot be overlooked," the authors write.

Yet every year, Gatesy says, dinosaur documentary-makers ask him to say how a T. rex moved.

"I can't," he says. "I say, 'Try something and send it to me and I'll tell you why it's wrong.' They always find someone who's willing to tell them how they moved, and maybe not mislead them horribly, but give them a firmer conclusion than is justifiable."

So if Gatesy can't say how T. rex moved, why did he? He wants discourse on dinosaurs. His article in Nature is essentially a disclaimer. He talks about how there are hundreds of plausible presentations, albeit with small differences, for T. rex movement. Did the dinosaur stand erect or crouch? How much bend was in its knee? How much in its hip?

Gatsey and Hutchinson, with their theory of straight-ahead walking, present one model.

THE PALEONTOLOGY world has one footprint that's believed to be that of a T. rex, according to Gatesy. It doesn't have a series of tracks, which would be helpful. It has bones, but no muscles, without which there's no surety of the animal's movement and, ultimately, its behavior.

"That's what everyone wants to know: Did it scavenge or did it chase after Triceratopses and bite their heads off?" Gatesy says. "If your first level of knowledge is weak, you've created a house of cards. John and I are at bone-muscle function. We may never get any higher."

Still, their study continues, as do those of other palentologists, fueled by the public's fascination for dinosaurs. There's something about them, Gatesy says. They engage people's imaginations in ways that other ancient creatures, living or dead, don't.

"As scientists, our job is to explain the history of life, whichever aspect that is," he says. "I don't think I get equally excited about snails, but maybe that's just me."

To learn more about Tyrannosaurus rex and see a model and an animated presentation of one moving, visit www.amnh.org/exhibitions/dinosaurs/theropod/problem.php.

brourke@projo.com / (401) 277-7267