T. rex’s silly-looking arms were built for slashing

SEATTLE — Tyrannosaurus rex may have had small arms, but it was no pushover.

This fierce dinosaur is known for its giant head, powerful jaws and overall fearsome appearance — except for those comical-looking arms. But the roughly meter-long limbs weren’t just vestigial reminders of a longer-armed past, paleontologist Steven Stanley of the University of Hawaii at Manoa said October 23 at the Geological Society of America’s annual meeting. Instead, the limbs were well-adapted for vicious slashing at close quarters, he argued.

T. rex ancestors had longer arms that the dinos used for grasping. But at some point, T. rex and other tyrannosaurs began to use their giant jaws for grasping instead, and the limbs eventually atrophied. Many people have hypothesized that the shrunken arms were, at best, used for mating or perhaps pushing the animal up off the ground; at worst, they were completely functionless.

But Stanley noted that the arms were quite strong, with robust bones that could sustain the impact of slashing. Each arm ended in two sharp claws about 10 centimeters long. Two claws give more slashing power than three, because each one can apply heavier pressure. Furthermore, the edges of the claws are beveled and sharp like those of a bear rather than flat like the grasping claws of an eagle. Those traits support the slasher hypothesis, Stanley concluded.

Many scientists aren’t convinced. While an interesting idea, it’s still unlikely that an adult T. rex would have used its arms as a primary weapon, says vertebrate paleontologist Thomas Holtz of the University of Maryland in College Park, who was not involved in the study. Although strong, the arm of a fully grown T. rex would barely reach past its chest, greatly reducing its potential strike zone. But a T. rex’s arms grew more slowly than its body, so younger dinos would have had proportionally longer arms. It’s possible the juveniles might have found them useful for slashing prey, he says.