A Ferocious Predator of Its Day: T. Rex’s Short Arms May Have Lowered Risk of Bites During Feeding Frenzies
A lifesize cast of T. rex in the atrium of UC Berkeley’s Valley Life Sciences Building shows how peculiarly short the forearms were, given that the creature was the most ferocious predator of its day. (Photo by Peg Skorpinski)
Over the two decades paleontologist Kevin Padian taught a freshman seminar called The Age of Dinosaurs, one question asked frequently by undergraduates stuck with him: Why are the arms of Tyrannosaurus rex so ridiculously short?
He would usually list a range of paleontologists’ proposed hypotheses — for mating, for holding or stabbing prey, for tipping over a Triceratops — but his students, usually staring a lifesize replica in the face, remained dubious. Padian’s usual answer was, “No one knows.” But he also suspected that scholars who had proposed a solution to the conundrum came at it from the wrong perspective.
Rather than asking what the T. rex’s short arms evolved to do, Padian said, the question should be what benefit those arms were for the whole animal.
In a new paper appearing in the current issue of the journal Acta Palaeontologia Polonica, Padian floats a new hypothesis: 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, 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. This would have affected not only T. rex, which lived in North America at the end of the Cretaceous period, he said, 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.
Arms and the T. rex
When the great dinosaur hunter Barnum Brown discovered the first T. rex fossils in 1900, he thought the arms were too small to be part of the skeleton. His colleague, Henry Fairfield Osborn, who described and named T. rex, hypothesized that the short arms might have been “pectoral claspers” — limbs that hold the female in place during copulation. This is analogous to some sharks and rays’ pelvic claspers, which are modified fins. But Osborn provided no evidence, and Padian noted that the T. rex’s arms are too short to go around another T. rex and certainly too weak to exert any control over a mate.
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