Why Were T-Rex Arms So Small? One Scientist Has A Great New Theory


Rachael Funnell

Social Editor and Staff Writer

clockApr 5 2022, 17:01 UTC
why were t-rex arms so small

Hard as it is to imagine, T-rex arms may have helped them to survive dangerous feeding conditions. Image credit: Herschel Hoffmeyer /

T-rex arms and their comparatively tiny size sit slightly juxtaposed against the towering body of what’s widely considered to have been one of the most ferocious predatory dinosaurs ever to stomp across planet Earth. However, Tyrannosaurus rex isn’t the only dinosaur to have expressed this teeny arm phenotype, which brings into question if there are some perks for apex predators to have little limbs.

The mystery surrounding the petite nature of T-rex arms is one that’s been pondered by palaeontologist Kevin Padian who has repeatedly been asked by students at the University of California, Berkeley, where he is Professor of Integrative Biology, what business they had being so small? “No one knows,” had historically been his take on the topic, until he decided to float a theory of his own in the journal Acta Palaeontologia Polonica.


Why were t-rex arms so small?

It’s possible that T-rex hunted in packs like wolves, and several dinosaurs may have gathered around a carcass even if they hadn’t taken it down as a team. Here, Padian posits, having diddy T-rex arms may have come in handy.

“What if several adult tyrannosaurs converged on a carcass? You have a bunch of massive skulls, with incredibly powerful jaws and teeth, ripping and chomping down flesh and bone right next to you. What if your friend there thinks you’re getting a little too close?” Padian posited.

“They might warn you away by severing your arm."


Reptiles like crocodiles and alligators have been observed accidentally amputating their fellow reptiles’ limbs during feeding frenzies, so this kind of accidental cannibalism isn’t without precedent. If a similar threat existed for T-rex, having such tiny arms would have been helpful rather than a hinderance.

“It could be a benefit to reduce the forelimbs, since you’re not using them in predation anyway,” said Padian.

Generally, evolution moves in the direction of adaptations that make animals fitter for their environment and given T-rex predecessors had longer arms it stands to reason that the shortening of the forelimbs afforded some benefit to these animals.


“None of the [existing] hypotheses explain why [T-rex] arms would get smaller — the best they could do is explain why they would maintain the small size,” Padian concluded. “And in every case, all of the proposed functions would have been much more effective if the arms had not been reduced."

Just how small were t-rex arms?

T-rex arms were so small that when the first fossil remains were uncovered by Barnum Brown in 1900, he didn’t believe that the arm bones could possibly belong to the same dinosaur.

why are T.rex arms so small IFLScience

To put it into perspective, a 14-meter-long (45-foot) T-rex with a 1.5-meter-long (5-foot) head would’ve had arms less than a meter long. That’s the equivalent of a 1.8-meter-tall (6-foot) human having arms just 13 centimeters (5 inches) long.


As for the purpose or evolutionary pressure that inspired the miniature nature of T-rex arms, as Padian admits it’ll be hard ever pinning down an answer for sure. Demonstrating hypotheses gets a bit more complex when they center around animals that went extinct roughly 66 million years ago…

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