Physics Offers New Look at T. rex Arms: Weapons not Waste

With a gargantuan head flaunting the largest teeth of any predatory dinosaur, Tyrannosaurus rex embodies the ideal nightmarish horror. So, what was such a ferociously large animal doing with such tiny forelimbs that look more like a humorous afterthought than an evolutionary tool?

“Some people think T. rex forearms are just vestigial organs which evolved away, but I claim no,” said Scott Lee, a professor of physics at University of Toledo, who argues that a T. rex could move its forearms quickly enough to prevent a struggling prey's escape. Therefore, the arms were an integral part of the predator’s hunting tactics, he said, and not useless stubs.

Dinosaur sculture in Germany. Image provided through Wikimedia. 

One of the key pieces of evidence supporting Lee’s notion are the stress fractures paleontologists have found on the handful of wishbones they’ve recovered from fossilized T. rex remains. Today, birds are the only animals with a wishbone, which helps them achieve flight. The boomerang-shaped wishbone, also called the furcula, of a T. rex comprises part of the forelimb, and its purpose is less understood.

Lee suggests that the stress fractures resulted from extreme struggles between T. rex and its prey. Many scientists think that, much like large, land predators today, T. rex would run down its prey from behind, grabbing the throat with its mouth upon contact. Most land predators today will latch onto the throat until the animal suffocates to death, and Lee thinks that T. rex might have done the same while using its arms to restrain the animal from struggling free.

“They would have had to grab something which was trying desperately to get away,” Lee said.

Bronze cast of a Tyrannosaurus rex wishbone (from the "Sue" specimen), at the Field Museum in Chicago. 

T. rex's forelimbs may have also helped the dinosaur to push off the ground in order to stand up from a lying position or to hold on to another T. rex while mating. But neither of these exercises would be rigorous enough to produce the level of stress fractures preserved in fossilized furcula, Lee argues. Moreover, Lee’s calculations indicate that T. rex had the agility to grab prey with its arms, supporting his argument that the hunter would have used this ability for that exact purpose.

Based on a three-dimensional representation of a T. rex forelimb, Lee calculated the acceleration at which the animal could attack with its claw-equipped hands. He determined that T. rex could shoot its hands out at an acceleration of 90 meters per second per second, or about nine times the acceleration due to Earth’s gravity. Other animals, like the Mantis shrimp, can strike with their limbs at 10,600 g’s.

Skeleton casts mounted in a mating position. Credit: Jurassic Museum of Asturias.

Although T. rex did not have the speediest forelimbs, it still would have taken a good deal of muscle to achieve an acceleration of 9 g’s. Scientists estimate that T. rex could have lifted 439 pounds with help from its bicep muscles, which were about three and half times more powerful than the average human equivalent.

“Why did T. rex have arm muscles?” Lee asked. “Why were they so strong and able to accelerate so quickly? Mating would not necessarily involve such big accelerations.”

Lee presented his results at a poster session last week at the APS March Meeting in Denver, Colorado. Stay tuned for more of Lee's work on the physics of different dinosaurs' speeds on Thursday.