T. Rex's Tiny Arms May Have Been Useful After All

The tiny arms of a T. rex made an otherwise fearsome creature look like it had a flaw. New research says that might not be the case. sruilk/Shutterstock

The Tyrannosaurus rex was considered one of the most fearsome carnivores of its time. While debate rages about whether it was a pure predator or a scavenger, we can all agree that T. rex looks like it knew how to chow down on a meal.

Detracting from its ferocious appearance, however, were a pair of tiny, two-digit claws. They looked like they belonged on a much smaller creature, not the so-called king of the tyrant lizards.

However, scientists say these puny-looking arms may have still be useful, if an analysis of distant relatives' forelimbs are any indication.

Looking at the present to understand the past

To consider the forelimb of a T. rex, Christopher Langel, an undergraduate student of geology, and Matthew Bonnan, a professor of biology, both at Stockton University in New Jersey, looked at the limbs of the domestic turkey (Meleagris gallopavo) and the American alligator (Alligator mississippiensis). They took the ulna and humerus bones of both animals and placed them inside two devices that create what's called an X-ray Reconstruction of Moving Morphology, or XROMM.

The XROMM allows researchers to create a 3D model of the bones of whatever is scanned. Movement data are then added to the scan and the result is a 3D moving image that, according to the XROMM website, allows scientists to see "rapid bone movement, such as during bird flight, frog jumping and human running."

While XROMM can be used with live specimens, Langel and Bonnan used just the limbs of turkeys and alligators for their study. The wings and arms were placed on a Plexiglass platform between the XROMM machines. The two researchers then used fishing wire to pull on the elbow of each limb while the XROMM recorded the bone movements.

A close-up of an alligator's front left forelimb
The American alligator may help scientists understand how the T. rex moved its arms. Wing-Chi Poon/Wikimedia Commons

The results showed that the elbows of both animals are complicated in their movements, much more so than ours. "When we flex our elbows, both forearm bones follow the hinge joint to fold in toward the upper arm," the researchers said on Oct. 17 during the 78th annual Society of Vertebrate Paleontology. "Our hands often rotate palm side up when we flex our elbows, because one forearm bone pivots around the other.

"The elbow joint [for turkeys and alligators] is more complex, and both bones in the forearm not only pivot around the joint, but [also] rock sideways toward the upper arm bone as the elbow is flexed," the researchers continued. "Unlike our elbows, both forearm bones cause the palm of the hand to turn inward and somewhat upward."

This, according to Live Science, surprised the researchers.

"It was especially surprising to see how much the forearm bones could rock side to side at the elbow, a movement that is essentially off-limits to mammals like us," Langel and Bonnan said. "In essence, alligators and turkeys can turn the palm of the hand inward and upward like we do, but [they do it] by using more-complex movements of the bones at the elbow. Once again, Mother Nature has solved the same problem in different ways."

Reach out and touch something

Close-up of a plastic model of T. rex's arms
The T. rex may have kept its arms facing inward. JopsStock/Shutterstock

As for what this means for the T. rex, Live Science explains that it indicates that the dinosaur was a "clapper, not a slapper." This means that the T. rex would keep its hands folded inward, like they're clapping, as opposed to facing downward and out, as if they were slapping.

The findings suggest the T. rex's arms "may have been able to rotate the palm of the hand inward and upward in such a way that the palm would face the chest when the elbow was flexed," the researchers explained.

Why the T. rex would do this, however, remains a mystery since we can't actually see the dinosaur in action.

"But we might speculate that such a movement (rotating the forearm and hand in toward the chest) could allow some theropods to bring prey in close for a bite," Langel and Bonnan told Live Science in an email.

The duo's research has not be published in a peer-reviewed journal.