Why did T-Rex have such teeny arms?

Why did T-Rex have such teeny arms? They couldn’t reach it’s mouth to help it eat. They couldn’t help it get up if it fell over. It just seems so impractical!

That is a question often debated. Some claim they are useful, that the arms were used to stand up or balance while eating at ground level or when knocked over. Another theory is that the loss of weight was necessary to maintain T form bipedal balance. Since T Rex was a successful dinosaur, the lack of longer arms wasn’t a hindrance to it’s existence.

Very likely to be vestigial, or nearly so.
Just because an organ is not fully necessary for an animal doesn’t mean it disappears completely immediately. These things take time.

Well, they weren’t vestigial. The problem is, they were actually well-muscled and functional as appendages, although small ones. If an organ is vestigial and on the way to disappearing - the pelvis of whales, eyes of blind fish - you don’t expect it to be functional at all. If T. rex’s arms were vestigial, you would expect the musculature to be minimal and the joints to be non-functional.

T. rex arms

One hypothesis I’ve heard is that they used them to hold onto each other for mating.

Those arms are not that small – they’re about the size of a full-grown man’s, and muscled to match. The thing is, they’re on a body built to a much larger scale.

One very common hypothesis is that they were used primarily as an anchor. When the T. rex rose from a resting position, the mighty hind limbs would move forward to lift the tail and butt of the enormous biped. The strong little forearms would grip the ground tightly to keep the animal from sliding forward while it got its hindlegs under its hips, and could then raise its chest and head by counterbalancing with the tail. Visualize your own posture as you attempt to get up after lying on your belly – don’t roll over or “sit out” but bring your kegs up under your hips like a sprinter. This is what T. rex was faced with, and he couldn’t bang a gong for help.

A counter-position to Chronos’ assertion that they used the short arms to get it on.

So they could reach around for things that fell behind the washing machine.

Toothpicks. Handy toothpicks.

Dodging the functional question, T. rex had small arms because it was descended from previous tyrannosaurids that had small arms. The same trait was inherited by other closely related theropods such as Albertosaurus, Tarbosaurus and Daspletosaurus.

They didn’t need to box with God, that’s why.

This isn’t a useful way to look at things. “vestigial” organs aren’t organs that are in the process of disappearing, and we just caught them at the wrong snapshot in time to see it. Such organs are still serving a purpose, and creatures which have these organs are presumably more successful than mutations lacking them.

In the case of what we think of as whale transitional forms, it’s been speculated that the “vestigial” legs were, in fact, used for mating, to help the male hold on, as Chronos says some think the T. rex used its arms.

I haven’t heard or read of that suggestion for T.rex arms before, but one that I have heard is that they might actually have helped the T. rex stand up. The huge legs can push the T. rex body forwards, but to prevent it from merely pushing the body along the ground, it has been suggested that the arms dug in and kept the body in place while it startede to rise. I don’t know if that’s really reasonable, but I do know that it has been, in al seriousness, suggested.
FWIW, in the original 1933 King Kong, shortly after the T. rex enters the scene, and just before we cut away to Kong haranguing Bruce Cabot in his cubbyhole under the cliff where the tree used to be, the T. rex pausees to scratch itself with one tiny arm as Fay Wray screams in terror. If nothing else, you’d think the arms would be good for scratching.

I know this is a joke, but T. rex couldn’t even reach its mouth with its forelimbs.

While true, this doesn’t offer any insight into why the trait developed in the first place.

This isn’t correct either. Some organs are vestigial, with no discernible function, and are in the process of disappearing. The energy cost in making them, however, has not yet produced enough selection against them to overcome the developmental program that makes them.

This may have been true of some archaic whales, in which the vestigial legs were still external. It’s not true of modern whales, in which the legs bones are internal and are truly vestigial, with no known function.

P’raps he uses his arms to wave “hello” to his buddies?

They were used to wave threateningly.

They were used to play the ukulele.

Would you want to face a T. Rex? If the answer is no, then it doesn’t really matter what its arms were like. They were just fine.

It almost seems like they were a form of “land shark” - they had a powerful body/means to get around, a huge mouth and the various sensory tools to enable them to steer the mouth with their body to get food. Everything else is just window dressing :wink:

But, per Colibri, there is a difference between vestigial and “not a primary tool for this animal.” The arms functioned and obviously did a few things sufficiently well to stick around evolutionarily.

Kinda like Humans’ sense of smell??

I disagree. It’s still a good question. Useless appendages take energy, protein, and can get infected. Mother nature is too stringent to allow useless arms for very long. I think they were either useful or (although the fossil record doesn’t support it) disappearing.

Maybe they were sexually appealing.

For giving two snaps and a hey.