Why Did the T. rex Have Such Tiny Arms?

T. rex didn’t skip arm day; they redefined it.

Can’t high five, but you wouldn’t have dared to, anyway.

This dino makes you question both evolution and your gym routine.

Why are their arms so small? One scientist thinks it’s so they didn’t accidentally get bitten off by other dinos. Let’s get into it.

The first T. rex skeleton was discovered in 1902, in Montana, by Barnum Brown—a real-life Indiana Jones. When he found the first bones, he didn’t quite know what he had, but he knew it was going to be big. Literally and figuratively.

He also sometimes showed up to digs in a full fur coat. Normal stuff!

Brown didn’t just dig, though. He blew up the hill with dynamite and used horse-drawn plows to remove thousands of pounds of dirt.

And yet…it still took three years to excavate the first Tyrannosaurus rex.

The skeleton was shipped by train and reassembled at the American Museum of Natural History. More in the upright position of a kangaroo with the tail dragging, than in the standard horizontal T-Rex shape we all know and love today. But hey, they tried. It was named Tyrannosaurus Rex, Latin for a king of the tyrant lizards. Your majestry.

And for more than a century, this fearsome predator has been the butt of tiny arm jokes. Let’s be real, those little bitty arms make this giant dinosaur look…a little bitty ridiculous.

For comparison’s sake, the scale of T. rex’s arms to body—roughly 40 feet of body to about 3 feet of arms—would be like a Border Collie having …almost three-inch paws.

But here’s the thing: evolution doesn’t hang onto body parts for no reason.

If T. rex’s tiny arms were truly useless, they would’ve disappeared.

Instead, they shrank.

That tells us they had a purpose—juuuuuust probably not one you’re thinking.

Forget the memes for a second. What if those little arms were an evolutionary survival strategy (instead of just a punchline)?

Today we’re digging into the science surrounding these tiny arms: why did they evolve? What could they do? And what if this so-called weakness was actually one of T. rex’s strengths? (not that the king of dinosaurs needed another strength)

Scientists have long tried explaining the mystery of T. rex’s tiny arms ( the arm bone’s connected to the shoulder bone and the shoulder bone’s connected to the…neck bone).
We called up a real dinosaur scientist to learn more.

KEVIN PADIAN: Tyrannosaurs are part of the carnivorous dinosaur lineage, the ones we call theropods. Of those, the T. rex was the culmination, probably, of a big group of tyrannosaurs that started on the small side with longer arms than T. rex has. They became what we might call hyper predators, just animals that are capable of taking down a really big game and eating it. The T. rex, in particular, unlike the earlier Tyrannosaurs and the other carnivorous dinosaurs, did not have blade-like teeth with serrations on the edges. It had very thick, almost D-shaped teeth and a cross-section.
So it almost looks like half a banana. And the other half of the banana is lodged in the skull.
So you might say that T. rex has a mouth full of lethal bananas.

Lethal bananas and teeny tiny arms. Got it.

KEVIN: The problem of what T. rex’s arms were, as we say, good for or used for, is a really great central question to me about what actually qualifies as an explanation in science. Because you can make up a story, but how do you know if that story is any good?

KEVIN: When T. rex was first found in the early 1900s, the pieces of the arms that were found with it were so small that the people who discovered it didn’t think they belonged to the same animal. But then they realized that no, no, this fits. This is the actual set of arm bones, very small, of this huge carnivorous dinosaur with these big teeth and huge head. And so that immediately raised speculations about what it might have been doing with the arms.

One theory? What if the small arms were helpful accoutrements for uh, grasping, during mating?

KEVIN: Some fishes and sharks have claspers like this in their pelvic region that they use. But that’s a very different structure and the animals are very different and there was no reason to accept that.

Another theory suggested that the tiny arms might’ve been used as part of a mating dance.
(Maybe like today’s woodcock and sage grouse. Birds are just living tiny dinosaurs, after all.)

And still another theory speculated that maybe the arms helped them get up off the ground.

But there’s a big problem. Or two really small ones.

KEVIN: Every function you’re proposing would have worked better if the arms were bigger. So this is not really an explanation. And it ignores the fact that this was a pointed evolutionary progression from larger arms to smaller arms. So reduction in itself, we presume, should have served some kind of function.

KEVIN: It turns out that when you look at an actual skeleton of T. rex and see how the arms are structured, these little arms, when you manipulate them, could not reach each other. They could not touch each other. They couldn’t touch the mouth. They couldn’t touch the head. They couldn’t extend very far at all. I mean, maybe a foot or two on an animal that’s 40 feet long and has a head that’s 4 feet long.

KEVIN: Every one of these hypotheses about holding things or pushing up or grabbing things would not work. And if you say it could have been stabbing somebody, well, it wouldn’t be able to reach anybody to stab it. And besides the mouth, the jaws are so huge.
That’s going to be crushing whatever you’re after. I mean, these little arms are ridiculously useless in any of these functions.

Two paleontologists calculated that T. rex’s gym routine actually worked—their small arms would have been capable of bench-pressing as much as 400 pounds.
If they could ever reach anything, that is.

KEVIN: It seems to me that maybe we’re looking at the whole problem through the wrong end of the telescope. Maybe we shouldn’t worry about what the arms did, but what the reduction of the arms did for the welfare of the whole animal.

Here’s something we know: it’s possible—based on finding skeletons of multiple animals together—that Tyrannosaurus rex actually hunted in packs.

KEVIN: If we think about other animals who are predators who hunt together, Komodo dragons, lions, hyenas, wolves, crocodiles, at least congregate in a place where they attack the same food together, or eat the same food anyway. It turns out that almost always the larger individuals of the predator species get to feed first on the carcass, no matter who kills it, because they just chase the others away.
Okay, so if we think about Tyrannosaurs with these huge heads and huge teeth, having one or more of these animals, large, next to each other on a carcass, if one animal gets a little bit too close to the other animal and that animal gets a little bit upset by this, it wouldn’t take much for that animal to turn to the first one and simply bite his arm off. Did this actually happen?
If we look at the bite marks that we find on the bones of carnivorous dinosaurs, including Tyrannosaurs, what we find is that most of the bite marks are around the skull and jaws. But we haven't found a lot of bite marks on the tiny arms of T. rex, which suggests, and we need a lot more evidence to try to test this farther, but it suggests at least that the reduction of the arms may have kept them out of the way while feeding.

Recap: don’t bite the hand that feeds you. Especially if it’s your own. Or your brother’s.

But we’ll never meet a T. rex and find out for sure. So why do we care? And what makes a good scientific explanation, anyway?

KEVIN: I think it’s very important to think about what qualifies as an explanation. Because you have an idea that sounds interesting or makes an analogy to something you know from the living world does not mean that’s a good explanation. It really has to be tested. And the scientific method is based on testability, on evidence. I don’t know whether this idea about feeding and avoidance of damage to the forelimbs is going to be the best explanation of the history of the reduction.

I’m proposing it because I think it’s a good and very different way of looking at the whole problem. The reduction by itself might have been the central value of the evolution of these arms. And so why don’t we look and see why they might have been reduced?

So, it turns out, these little appendages might not be a design flaw but a feature.