Why did Tyrannosaurus rex, the most fearsome and ferocious dinosaur, have such ridiculously tiny arms? Depending on which paleontologist you ask, the answers could range from holding on to a mate to slashing prey at close range. Now, UC Berkeley paleontologist Kevin Padian proposes an interesting new theory. They're small to help prevent other T. rexes from biting them off during a feeding frenzy. From UC Berkeley:
[Padian's hypothesis is that] T. rex's arms shrank in length to prevent accidental or intentional amputation when a pack of T. rexes descended on a carcass with their massive heads and bone-crushing teeth. A 45-foot-long T. rex, for example, might have had a 5-foot-long skull, but arms only 3 feet long — the equivalent of a 6-foot human with 5-inch arms.
"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? They might warn you away by severing your arm," said Padian, distinguished emeritus professor of integrative biology at the University of California, Berkeley, and a curator at the UC Museum of Paleontology (UCMP). "So, it could be a benefit to reduce the forelimbs, since you're not using them in predation anyway."
Severe bite wounds can cause infection, hemorrhaging, shock and eventual death, he said.
Padian noted that the predecessors of tyrannosaurids had longer arms, so there must have been a reason that they became reduced in both size and joint mobility[…]
He admitted that any hypothesis, including his, will be hard to substantiate 66 million years after the last T. rex became extinct.