Tiny Hands

What’s the Point of T-Rex’s Tiny Arms?

For grabbing? Slashing? Mating? Paleontologists can only agree on one thing: There’s a reason, and we don’t quite know it yet.

If you ask Steven Stanley, a geologist at the University of Hawaii at Manoa, Tyrannosaurus rex’s famously tiny arms aren’t a joke. Sure, they’re comic in how disproportionate they are for one of Earth’s most ferocious predators. But Stanley argues that T. rex’s arms were weapons of mass destruction, as big and strong as a man’s leg, with two 4-inch claws apiece, built for slashing deep wounds in dinosaur flesh in quick succession.

Not that other paleontologists agree. Stanley presented the idea at the Geological Society of America’s annual meeting last month, gaining few converts. “There’s no question that there will be a lot of people who don’t agree with me,” he told The Daily Beast. “If you come into another area and you have a fresh viewpoint, you’re not saddled with conventional wisdom—but you’re never welcome.”

The conventional wisdom, as far as T. rex arms go, is that they atrophied over evolutionary time as natural selection in the animals came to favor the bitey, bitey over the grabby, grabby. Tyrannosaurus arms may have ultimately had some use, but it wasn’t much of one. They simply had too little strength and not enough reach to do a whole lot.

Sure, some theories of function have been floated. Perhaps they used them to help themselves up from a squat, or to grasp prey in a fight, or to hold on to a mate during sex. Stanley finds these proposals largely unsupported, and frankly laughable. He, on the other hand, presents five bits of anatomical evidence that suggest that T. rex arms were custom-made for raking prey to shreds.

Notably, Tyrannosaurus has two fingers on each hand, when its close relatives tended to have three. This, Stanley suggests, would give each claw 50 percent more slashing power per unit of force. Having two instead of one provides for increased stability at the wrist. Also, the T. rex claw has a similar shape to a bear claw, and bears are the large-bodied slashers of the modern world.

In Stanley’s telling, a T. rex kill looked something like this: The predator ambushes the prey, perhaps picking a sick or young animal as an easy target, and jumps on its back. Now at close range, it lets lose those claws, inflicting several wounds in a matter of seconds. “Things would not have recovered from this,” he said. “They would have been in great pain, they would have suffered blood loss. Nerves would have been slashed, blood vessels, muscles… it would have been hideous.” Now, the prey is too injured to get away, and the T. rex goes in with its primary weapon—its impressive jaws.

Michael Habib, a paleontologist and anatomist at the University of Southern California, sees it differently. Here’s how he imagines a fight between a rex and a juvenile hadrosaur: “It walks this thing down—imagine a brisk power walk at speeds that are slower than the fastest human sprinters, but faster than most of us can run,” he tells The Daily Beast. “It gets close enough to this thing to reach in with the head, which has pretty good range, and it gets one horrible, crunching, bone-splintering bite in.

“You hear something like every piece of celery on earth breaking at once, and bits of hadrosaur go all over the place, and it’s probably enough to turn the average stomach, and then it goes still basically instantly because every part of its central nervous system has just been severed, and its body has been crushed, and the rex just tears pieces off and eats for the next 15 minutes.”

If, on some off chance, the first bite doesn’t kill it, “your best bet is not to then switch to trying to slash at it blindly with your little hands. You back up, and you bite it again. You’re a Tyrannosaurus rex. If you have bitten it twice and it’s not dead, you should run—or at least walk—away, because you have apparently attacked something too big and too armored for even you to break, which means that you are outmatched and should run away now.”

Habib doesn’t buy that the hands could have been effective secondary weapons in this scenario. In most positions the dinosaur would not have been able to bite and claw its prey at the same time, and to forgo the jaws to use claws would be rather silly. “Anything the jaws can’t destroy is going to consider a stroke from the hands laughable.”

It’s no secret that the T. rex had a fearsome jaws. Its bite strength was among the strongest of any vertebrate land animal, even relative to overall size. Habib’s research, written up recently by Kenneth Lacovara in his new book, Why Dinosaurs Matter, suggests that T. rex arms got small not because they ceased to be useful, but because they had to get small in order for the jaws to get big. Big jaws require a large skull, which require strong neck muscles. Neck muscles compete with arm muscles for space attaching to shoulder bones, so favoring one requires forgoing the other.

“If your arms can’t really contact prey anyway, the best way of maximizing your weapon capacity as a theropod dinosaur is actually to get rid of as much arm muscle as you can, and replace it with neck muscle,” says Habib.

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Stanley agrees that the arms of T. rex ancestors were basically useless for a long time, which is how they got so small. But, he argues, once they got to that close-range slashing sweet spot, they gained an accidental purpose. The anatomy of the arms began to favor slashing ability, and the arms retained enough size and strength to do damage.

As for the idea that a T. rex would never go for a prey with arms when it could use its teeth, he suggests that grabbing a prey animal running for its life with your jaws isn’t easy, and it might make sense in most cases to simply get on top of it, knock it off its feet with some painful slashes, and then go for the killshot.

It seems that Stanley and Habib can’t both be right; either T. rex arms were effective weapons, or they were not. However, Thomas Holtz, a tyrannosaur expert at the University of Maryland, offers a potential third option. Could it be that T. rex arms were useful as secondary weapons in their young life, but lost that usefulness with age?

Holtz, for the record, believes that T. rex arms were useful but not all that well-used. Fossilized bones show evidence of fractures and torn ligaments, but also of healing. This suggests that arms were put to some use, but losing that function wasn’t life-threatening.

In an 8-ton adult, Holtz doubts those claws would do much good in a fight. “For them to use those arms against a prey item, they would have to shove its chest up against something and slash there. In order to do that, it has to get one of the best-evolved weapons of destruction—its skull—out of the way.” But juvenile T. rexes had longer arms relative to their underdeveloped skulls—it could be that those animals put their paws to some use in a fight, he says.

The true answer is yet unknown but not unknowable. Digital and physical models of tyrannosaurs interacting with prey could shed light on just how a clawing match might go down. Stanley must still submit his ideas for peer review, and then to the ongoing scrutiny of scholarly knowledge-building. It can be an uncomfortable process; each scientist must make airs to welcome conflicting ideas at the same time as he wishes to prove himself right.