Story by Jason WilliamsManaging Editor
Masculine images of a green/gray-scaled Tyrannosaurus Rex pursuingits prey down a wooded path, perhaps toying with it, but easilyovertaking its dinosaur-meal are synonymous with the idea of themassive predator — the idea that a beast so fierce in design couldbe nothing but a hunter, chasing dinosaurs across the Cretaceouslandscape with abandon.
However, recent inquiries into the giant’s gait are giving newweight to the argument that T. rex had a slower side.
A team of Stanford University researchers have seemingly put tobed the debate surrounding the apex predator’s speed: TyrannosaurusRex, the “tyrant lizard king,” could not run, but was instead limitedto a 10 mph walk.
“But speed is all relative,” said John Hutchinson, lead author ofthe findings published in the Feb. 28 edition of the journal Nature.”Ten mph is still darn fast from a human perspective, although notfrom a cheetah’s.”
The debate has divided academics along lines of speed ranging from5 and 10 mph to 45 mph, with the sexier top speeds being popularizedby Hollywood films like Jurassic Park. Many assumptions have beenbased on qualitative studies comparing the physiology and locomotionof living animals, such as elephants, to the large theropod.
Hutchinson is taking a quantitative approach.
“I didn’t really buy into the 45 mph super-tyrannosaur idea,”Hutchinson said. “The physics didn’t make sense to me.”
Using biomechanics, the researchers have designed a method fordetermining running ability by calculating the minimum mass ofextensor muscle required for fast running (an extensor is a skeletalmuscle that extends or stretches a body part).
Hutchinson’s analysis revealed that tyrannosaurs would have neededan inordinate amount of extensor muscle to be capable of reachingspeeds of 45 mph — 43 percent of the carnivore’s total body masswould have had to have been devoted to extensor muscle in each leg tosupport the predator’s 13,000 pounds at a sprint. With 86 percent ofTyrannosaurus Rex’s body comprised of nothing but leg muscle, therewould have been little room for any other functioning essentials(organs, skeleton, etc.) — an impossible scenario.
“ATyrannosaurus was just ill-suited for 45 mph sprinting,” Hutchinsonsaid.
To arrive at this conclusion, Hutchinson looked to extanttetrapods; this suborder typically devotes less than 50 percent ofits total body mass to muscle — all categories of muscle. Withinthis allocation, only 5 percent to 40 percent is comprised of limbextensor muscles.
Considering this, a biped with an extensor requirement of 25percent or greater in each leg would not have enough muscle mass topropel its entire body mass into a run.
This is the case with tyrannosaurs.
These calculations fall in line with existing trace fossilevidence. Trackways have been discovered that show smaller theropodsrunning; however, there are no known fossilized footprints depictinga larger theropod on the level of a tyrannosaur at a run.
“If Tyrannosaurus was indeed an adept runner, then it must havehad many musculoskeletal specializations that available data do notsuggest,” Hutchinson wrote in Nature.
However, looking again to contemporary examples, there is littlevariation among living vertebrates in the basic features of anatomyand locomotion; muscle exerts a relatively consistent peak force perunit area across vertebrate species — a factor in determining therequisite extensor muscle mass.
Hutchinson acknowledged the difficulties in assigningcharacteristics to an extinct species based on observations of extantfauna 65 million years removed.
“We can only do our best with what we know, and keep revising andrefining our models as we learn more,” Hutchinson said.
When considering parameters that were hard to predict based onexisting taxa, such as limb orientation and muscle fiber lengths,Hutchinson used conservative assumptions geared to underestimatemuscle mass requirements.
“Many of the parameters needed to model the limb mechanics ofextinct dinosaurs are uncertain,” Hutchinson wrote.
“Weused rigorous muscle reconstructions that minimize speculation.”
Still, even with the conservative estimates, Hutchinson found “noconvincing evidence” Tyrannosaurus could run at high speeds.
However, a walking T. rex was far from going hungry.
The data from the biomechanical model generalizes to all largerdinosaurs — including Tyrannosaurus contemporaries, which would havebeen as slow, if not slower.
“Assuming (Tyrannosaurus Rex) did at least some predation, likealmost any living carnivore does, then it had plenty of big preyaround that weren’t too fast either,” Hutchinson said.
This conclusion is significant toward resolving tyrannosaurhunting habits, and in beginning to answer the question of whetherthis 18-foot-tall, 37-foot-long carnivore was a hunter or ascavenger.
Behavior, such as how extinct species hunted, is particularly hardto decipher from skeletal remains and trace fossils. Hutchinsonbelieves Tyrannosaurus Rex was likely both a scavenger and an ambushhunter, and at times even a pursuit predator — just not a 45 mphpursuit predator.
Large theropods may have had trouble catching smaller, quickerprey, but Hutchinson does not view the reduction in TyrannosaurusRex’s perceived speed as an indication that the dinosaur wasdisadvantaged.
“I don’t think my work shows that its ecology was cruciallylimited by speed,” Hutchinson said.
As tyrannosaur ancestors evolved into the large meat-eaters of85-65 million years ago, their locomotive flexibility decreased withtheir gains in stature. It is impossible to say what evolutionaryfactors selected for size over speed, but monopolization of resourcesis a potential benefit.
“A big tyrannosaur could kick butt on smaller or weakerdinosaurs,” Hutchinson said. “And it could still wail on largedinosaurs of similar size.
“I see no ecological reason for it to be extraordinarily fast.”
A six-ton carnivore doesn’t need to run.
