With their massive jaws lined with dagger-like teeth, they could stand flat-footed and crush the limbs of five-ton foes like matchsticks.

Whatever poetic license the makers of the Jurassic Park films took in make exciting pictures, they pretty much nailed it in portraying the fiercesome chomping-power of the once mighty Tyrannosaurus rex.

Though extinct for 65 million years, T. rex is hot stuff these days, both in the box office and in the once-dusty warrens of paleobiology, the scientific study of ancient life.

Thanks to a revival in dinosaur research that began in the mid-1970s, a fresh new breed of young scientists is finding amazing things in the still skimpy fossil record. The discoveries are rapidly putting flesh on some very old bones. For the first time, scientists are now beginning to get beyond the digging, cleaning, sorting and cataloging of fossilized dinosaur parts-still critically important—to a revolutionary appreciation of how the beasts actually lived and died.

Greg Erickson, 39, typifies this eager corps of young researchers armed with an array of powerful new tools for probing ancient life forms. A recent (2000) arrival at Florida State's department of biological science, Erickson is earning a reputation as the world's leading expert on T. rex feeding behavior.

Log onto the Web site of Scientific American and for all dinosaur questions you'll find a direct “Ask The Expert” link to Erickson's e-mail. The distinction followed a lengthy cover story he wrote, “Breathing Life into Tyrannosaurus Rex,” for the magazine's September 1999 issue. Last March, National Geographic weighed in with its own cover story on how dinosaurs lived. The article led with a description of Erickson's latest work—which surprisingly has nothing directly to do with dinosaurs at all.

Croc Jock

These days, Erickson's fieldwork is a long way from where it began in 1986 when he was a geology undergrad at the University of Washington. In walking through the rugged terrain of Hell's Creek, a site in eastern Montana that once was a favorite haunt of T. rex, Erickson discovered a passion for learning about how the fabled monsters might really have lived.

For a beast so obviously equipped for taking big bites of life, few clues existed about how it did so. Exactly how did T. rex eat? Did it hunt down and kill its food, or plod around looking for a nice corpse, like some giant toothy vulture? And how did that truly scary set of dentures work anyway? Those 60-odd, six-inch teeth that could double for railroad spikes-were they more for show than utility? A fright wig on a big reptilian chicken?

Such questions shaped Erickson's post—Montana training—a masters degree from Montana State University; a Ph.D. from Berkeley; a post—doc split between Stanford and Brown—which is why his current work often takes him far afield of classic dinosaur research. Today, Erickson has become the undisputed authority on the eating mechanics of crocodilians—that large family of swamp lizards whose ancestors coexisted with dinosaurs for millions of years.

By studying the teeth and bite characteristics of crocs and their American cousins—the alligators—Erickson believes he can piece together a good idea of the biting and feeding capacities of their prehistoric dinosaur neighbors. If scientists ever hope to draw a realistic picture of how dinosaurs lived—particularly the giant meat-eaters like T. rex—a good understanding of their dentition and dining habits is essential, he says.

“I'm not just interested in finding out how hard an alligator or crocodile can bite, the force involved,” he said. “I'm also interested in how and why the teeth changed size and shape during development. This can tell us a lot about how the animals' diets changed as they matured, and also about how they used their teeth and for what purpose.”

Just like today's crocs and gators, dinosaurs shed and replaced their teeth throughout their lives, he said.

The bite-force of big crocs and alligators, when factored in with their overall size and width of their jaws and skulls, have helped Erickson build models that enable him to predict the bite-force and feeding habits of T. rex and other extinct four-legged carnivores. Following the 1997 discovery of a gigantic crocodile-like fossil in the Sahara Desert by a National Geographic explorer, Erickson was called on to calculate the monster's bite force (see “Chomp Champ,”).

For his work with crocs, Erickson gets up front and personal with the most powerful specimens he can find. He's waded into swamps (“billabongs”) in northern Australia—home to the planet's biggest crocs (a 17-footer is the largest Erickson tested)—and done a lot of work right in his new home state, not exactly unknown for its native, large-lizard population.

In Florida, Erickson's is a well-recognized face at the St. Augustine Alligator Farm and Zoological Park on the east coast. With the hearty cooperation of the farm's staff, Erickson uses a device called a bite-force transducer to measure the chomp-strength of resident gators and crocs of all sizes.

The device, mounted on a bar whose length could suggest the measure of the operator's backbone, gets thrust into the wide-open maw of an aroused animal, right between the rear-most molars where bite strength is concentrated. The resulting chomp gets electronically translated into raw force on the transducer's read-out. The device works much like a bathroom scale, Erickson said.

The fiercest bite Erickson has ever recorded from one of his alligator experiments was delivered by a 13-foot, 750-lb wild animal captured from a lake in Central Florida. The big gator hammered Erickson's machine with 2,960 pounds of force, roughly the weight of a Toyota pick-up.

Erickson has used his data to try to estimate the bite-force of T. rex. Working with bioengineer Dennis R. Carter of Stanford, and using metallic replicas of T. rex teeth, he conducted a simulation using cow pelvises and data extracted from genuine T. rex bite marks found in fossil samples of its prey. At the minimum, he calculated that the T. rex that made the fossilized marks used 3,300 pounds of force.

But the bite marks he was studying didn't represent the full force of a T. rex chomp, he says. Without having a more complete range of fossilized bite marks to test, it's impossible to calculate just how hard a tyrannosaur could bite when it really wanted to.

Still, Erickson believes that pound-for-pound, a crocodile could out-bite a comparable-sized tyrannosaur. And there were crocodiles that size and larger during T. rex's day (see sidebar.

Lion or Lazy?

From the day the first T. rex fossil was unearthed in 1905 in Montana, the toothy biped was slapped with a reputation as a ferocious, take-no-quarter killing machine.

Armed with as vicious-looking an array of choppers nature ever made, this creature could stand more than 20 feet tall and weigh up to eight tons. Taking such a formidable physique into account, most scientists quickly put the beast into the predator-from-hell category.

But not all scientists bought the idea, and opined that this prehistoric beast with its nearly useless forelimbs may in truth have been a lowly scavenger instead of a ruthless, hide-and-seek killer. They trotted out bears as an example of an animal that comes equipped with sharp teeth and claws—and still chooses mainly to eat fruit, insects, carrion and other things it doesn't have to go to war with.

The T. rex-as-scavenger notion picked up support during the 1970s and '80s from some researchers who concluded that the beast's teeth, for all their wicked appearance, were simply too weak to bring down and dispatch large—and one would assume unwilling—prey. By 1991, Erickson's former graduate prof in Montana, Jack Horner-the first paleontologist to discover baby dinosaurs in North America—had become an outspoken champion of the scavenger theory.

But in 1996, Erickson made a discovery that put ideas about a fragile-toothed T. rex into serious question. In examining a set of bones collected by an amateur fossil collector in Montana, Erickson found the whole collection to be dramatically punctured and scarred by what were obviously some potent bite wounds. Using ordinary dental putty, Erickson was able to make some remarkably detailed casts of the bite marks.

The casts confirmed that T. rex had a taste for the flesh of at least two of his dino relations: Triceratops, a five-ton beast with a massive, horny head, and Edmontosaurus, a large (up to 35 feet) duck-billed plant-eater. The discovery proved that T. rex teeth were not only powerful, but were capable of being highly effective weapons if ever needed in mortal combat.

Further proof that T. rex dentition could hold up during the stress of a clash with hard material such as bone was soon to come. In 1998 the journal Nature published an analysis done by Erickson and others of an exceedingly rare find—a large (17-inch) sample of fossilized dinosaur dung. Where the dropping was found (in Southwestern Saskatchewan) and other characteristics led the researchers to conclude that the fossil—known in the paleo-trade as a coprolite—was produced by a tyrannosaur.

Using techniques involving chemicals that fluoresce under exposure to x-rays, Erickson's team found the coprolite loaded with bits and pieces of shattered bone. In fact, nearly half the sample was found to be made up entirely of bone fragments.

Since this finding, Erickson's revelations about the terrific biting strength of these gigantic reptiles has largely quieted the weak-tooth debate. The inescapable conclusion is that whatever T. rex bit, chances are it stayed bitten—and badly.

“These animals were able to simply pulverize whatever they were eating,” he said. “For decades, people have thought that to be able to crush bones, animals had to have occluding (directly opposing) teeth, but that's just not the case.”

Erickson suggested that a single T. rex crunch could carry enough force to shatter even sizeable bones like they were made of glass.

The findings, though fascinating, hardly settle the scavenger vs. predator debate, Erickson knows. But at least they demonstrate beyond any reasonable doubt that T. rex had all the tools it needed to rip into a big meal, be it dead or still kicking. The pattern of the bites—at times deep, powerful punctures, followed by mighty pulls that would have created gaping, surely lethal wounds; at others far less dramatic, suggesting leisurely nibbling—also suggests much about how T. rex made its living.

Erickson isn't yet willing to take a side on the predator vs. scavenger debate, saying that the jury may be out for years on the issue. Ultimately, some researchers say the evidence will show that T. rex was both—one day an opportunistic eater of the slow and the dead; the other an unimaginable typhoon of terror.

Living Large

Whatever the case, today Erickson has been able to coalesce more than 15 years of his own research with that of a dozen or more investigators scattered around the globe. The result is a high-tech portrait of dinosaurs in general that's far more detailed than early researchers could ever have imagined possible.

“We've been able to theorize how dinosaurs as a whole grew relative to animals of today,” he said.

Until recently, most scientists assumed that dinosaurs grew up similar to today's crocodiles and alligators—rocketing to big, brawny bodies in a flash, even rivaling the amazing growth rate of birds.

By studying teeth and bone growth lines gathered from meat-eating dinosaurs and from fossils of some of the earliest bird-like creatures known, Erickson paints a different picture.

He believes that large dinosaurs—which most researchers agree could easily put on several hundred pounds a day—substantially out-paced the development of modern reptiles (even by a factor of 40), but still had growth rates substantially below that of their modern descendents—birds. He said T. rex most likely grew at the same rate of modern elephants, while the smallest dinosaurs grew moderately faster, like marsupials, e.g. opossums.

And unlike filmmakers would have us believe, T. rex probably couldn't run much faster than today's biggest elephants, either. Recent experiments by some of Erickson's colleagues at Stanford using elephants as models have put the lie to Hollywood's depiction of a swift-footed tyrannosaur.

A modern, four-ton elephant galumphs along on four legs at a top speed of about 15 mph. To hit speeds of 45 mph or more as depicted in the movies (and as some scientists still say the beast was capable of) the Stanford team calculated that even running on its beefed-up hind legs, up to 90 percent of the body mass of a 13,000-lb T. rex would have to have been made up entirely of muscle, a physiological impossibility. Still, such a toothy monster—even clipping along at elephant speed—would figure to be all the horror show one would ever want to witness up close.

Such images have helped sustain Erickson's passion for what he does since he was a four-year-old, when his obsession with dinosaurs first impressed his dad, a wildlife biologist. He sees his latest focus on dinosaur dentition taking him down yet another evolutionary trail—the developmental history of teeth.

Although teeth were around hundreds of millions of years before dinosaurs (many species of huge, scary-toothed sharks prowled the seas eons before the first big lizard drew breath) science is still without a satisfactory explanation of how these exquisitely designed, vitally important bodily utensils came to be.

“We really don't know how nature built teeth in the first place,” Erickson said. “Believe it or not, except perhaps in humans, we don't have a very good understanding of the form and function of teeth at all.”

He said he hopes to use a better appreciation of tooth form and function to draw a more detailed picture of dinosaur biology.