Explanation to the tiny arms of the T-Rex: to avoid amputations

Tyrannosaurus rex evolved with conspicuously short arms to avoid amputation when a pack of them swarm over a corpse with their massive heads and bone-crushing teeth.

It is the explanation that UC Berkeley paleontologist Kevin Padian finally offers the recurring question of his students for decades about the reason for the unique anatomy of the upper extremities of this iconic predatory dinosaur.

It usually listed a variety of hypotheses proposed by paleontologists (for mating, to hold or stab prey, to capsize a Triceratops), but his students, who usually stared a life-size replica in the face, remained dubious. Padian’s usual response was, “Nobody knows.” But he also suspected the scholars who had proposed a solution to the puzzle had approached it from the wrong perspective.

Instead of asking what the T-Rex’s short arms evolved for, Padian said, the question should be what benefit those arms were to the entire animal.

In an article appearing in the current issue of the journal Polish Palaeontology Act, Padian posits a new hypothesis: T-Rex arms shrank to prevent accidental or intentional amputation when a herd of T-Rex descended on a carcass with their massive heads and bone-crushing teeth. A 14 meter long T-Rex, for example, it might have had a 1.7-long skull, but arms only 1 meter long, the equivalent of a 2-meter human with 12-centimeter arms.

“What would happen if several adult tyrannosaurs converged on one carcass? You have a bunch of massive skulls, with incredibly powerful jaws and teeth, tearing and chewing through flesh and bone right next to you. What if your friend thinks you’re getting too close? They could warn you away by cutting off your arm,” said Padian, distinguished professor emeritus of integrative biology at the University of California (UC), Berkeley, and curator of 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. Serious bite wounds can cause infection, bleeding, shock, and ultimately death.said.

Padian noted that tyrannosaurid predecessors had longer arms, so there must be a reason they were reduced in both size and joint mobility. This would have affected not only T-Rex, which lived in North America in the late Cretaceous period, he said, but also mid-Cretaceous African and South American abelisaurids and carcharodontosaurids, which spread across Europe and Asia in the Periods. early and middle Cretaceous and they were even bigger than T-Rex.

“All the ideas that have been put forward about this are either untested or impossible because they can’t work,” Padian said. “And neither hypothesis explains why the arms would get smaller; the best they could do is explain why they would keep the size small. And in all cases, all the proposed functions would have been much more effective if the arms had not been reduced”.

He admitted that any hypothesis, including his own, will be difficult to corroborate 66 million years after the last T-Rex went extinct.