Sometimes when there’s no obvious answer, looking the opposite direction may help. For example, for more than 20 years, paleontologist Kevin Padian has taught a freshmen seminar at UC Berkeley called “The Age of Dinosaurs.” Invariably, the questions always comes up: Why did Tyrannosaurus rex have such laughably small arms?

He would usually go down the list of possibilities: for mating, for holding or stabbing prey, for tipping over a Triceratops. Eventually, he would get to “No one knows.”

But he also suspected scientists were coming at it from the wrong angle. Why would the arms of a ferocious predator shrink over time, through evolution? Because they needed to be smaller, to help the whole animal’s chances of survival, he postulated. What if it wasn’t about what the arms did, but what they didn’t do?

According to a news release from UC Berkeley, Padian floats his new hypothesis in a paper published in the current issue of the journal “Acta Palaeontologia Polonica.”

The university said the idea goes like this: “The 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, a distinguished emeritus professor of integrative biology at the UC Berkeley and a curator at the UC Museum of Paleontology (UCMP), in a statement.

“So, it could be a benefit to reduce the forelimbs, since you’re not using them in predation anyway.” Padian said severe bite wounds can cause infection, hemorrhaging, shock and eventual death. He noted that the predecessors of tyrannosaurids had longer arms, so there must have been a reason they became reduced in both size and joint mobility. This would have affected not only T. rex, which lived in North America at the end of the Cretaceous period, but the African and South American abelisaurids from the mid-Cretaceous and the carcharodontosaurids, which ranged across Europe and Asia in the Early and Mid-Cretaceous periods and were even bigger than T. rex.

“All of the ideas that have been put forward about this are either untested or impossible because they can’t work,” Padian said. “And none of the hypotheses explain why the arms would get smaller — the best they could do is explain why they would maintain the small size. And in every case, all of the proposed functions would have been much more effective if the arms had not been reduced.”

He admitted that any hypothesis, including his, will be hard to substantiate 66 million years after the last T. rex became extinct.

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