But its huge legs would power a worryingly fast walk.

Credit: © SPL

Tyrannosaurus probably couldn't run. The dinosaur would have needed unfeasibly large leg muscles to generate enough power to sprint, researchers now calculate.

Those planning Jurassic Park IV needn't be too depressed. With its 2.5-metre-long legs, a brisk walk would have carried T. rex along at over 20 kilometres per hour - enough to catch many other dinosaurs.

"It could have walked as fast as a running rhino," says palaeontologist Per Christiansen of the University of Copenhagen. "If you ever need to outrun a Tyrannosaurus, I suggest you do it on horseback."

Engineers John Hutchinson and Mariano Garcia, of Stanford University, Palo Alto, California, have estimated the minimum amount of leg muscle needed for running in different-sized animals. This depends on leg and muscle-fibre length, and the animal's stance, among other factors.

"With extinct animals you can't prove anything," says Garcia. "But given what we know about other animals that are good runners, it's unrealistic that Tyrannosaurus could run."

Researchers have argued over whether T. rex was a hunter or a scavenger. Its inability to run and its puny forearms hint it could only handle carrion, suggests Andrew Biewener, who studies animal locomotion at Harvard University in Cambridge, Massachusetts.

But this needn't have disqualified Tyrannosaurus from catching live meat, he says. Its potential prey were other large dinosaurs, probably much more lumbering than itself.

Solid muscle

"Things have a harder time moving as they get bigger," explains Garcia; muscle power does not keep pace with body mass. This is why fleas can jump so much higher than we can, relative to their size.

“An inability to run and puny forearms hint T. rex could only handle carrion Andrew Biewener, , Harvard University, Massachusetts”

In running, both feet leave the ground. A 6,000 kg T. rex would have needed more than 80% of its total mass in its hind leg muscles to achieve this - an implausibly high figure, the duo concludes1. The maximum seen in living land vertebrates is about 50%.

A chicken, in contrast, could run with only about 9% of its mass in its leg muscles, Hutchinson and Garcia estimate. Real chickens have 17%. A Tyrannosaurus-sized chicken would need an impossible 99% of its body mass in each leg to run.

The finding might explain the absence of fossil footprints showing a Tyrannosaurus in full flight. Tyrannosaurus' smaller - although still fearsome - relatives could run at 30 kilometres per hour, as illustrated by a trackway recently found in the United Kingdom2.

The threshold dividing dinosaur runners and walkers probably lay between a body weight of 1,000 and 2,000 kg, says Biewener.

References 1 Hutchinson, J. R. & Garcia, M. Tyrannosaurus was not a fast runner. Nature 415, 1018 - 1021 (2002). 2 Day, J. J., Norman, D. B., Upchurch, P. & Powell, H. P. Dinosaur locomotion from a new trackway. Nature 415, 494 - 495 (2002). Download references

Authors John Whitfield View author publications You can also search for this author in PubMed Google Scholar

Additional information Harvard University, Massachusetts

Related links Related links Related external links Dinosaur speeds Dinosaur locomotion

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