Tiny T. rex arms were built for combat

SEATTLE, Wash. — No question, Tyrannosaurus rex had small arms. Still, this dino was no pushover.

It is best known for its giant head, powerful jaws and overall fearsome appearance. And then there were those comical-looking arms. One scientist now argues that they weren’t funny when it came to combat. Those roughly meter- (39-inch-) long limbs weren’t just sad reminders of a longer-armed past, concludes Steven Stanley. He’s a paleontologist at the University of Hawaii at Manoa. Those forelimbs were well-adapted for vicious slashing at close quarters, he says.

Stanley shared his assessment October 23, here, at the Geological Society of America’s annual meeting.

T. rex ancestors had longer arms, which they used for grasping. But at some point, T. rex and other tyrannosaurs began to rely on their giant jaws for grasping. Over time, their forelimbs evolved into shorter arms.

Many scientists had suggested the tinier arms were, at best, useful in mating or perhaps for pushing the dino up off the ground. Others suspected they might at this point have had no role at all.

Those arms remained, however, quite strong. With robust bones, they would have been able to slash out with forceful power, Stanley notes.

What’s more, he points out, each arm ended in two sharp claws about 10 centimeters (4 inches) long. Two claws give more slashing power than three, he notes, because each one could apply more pressure. Their edges also were beveled and sharp. That makes them more like the claws of a bear rather than the flat, grasping claws of an eagle. Such traits support the slasher hypothesis, Stanley argues.

But not all scientists buy his claim. While an interesting idea, it’s still unlikely that an adult T. rex would have used its arms as a primary weapon, says Thomas Holtz. He’s a vertebrate paleontologist at the University of Maryland in College Park. Although the arm of an adult T. rex was strong, it would barely have reached past its chest. That would have severely limited the size of its potential strike zone.

Still, fossils show that the arms on a T. rex grew more slowly than its body. So the arms would have been relatively longer in juveniles. And that, Holtz says, may have helped the young predators slash their prey.