He admitted that any hypothesis, including his, will be hard to substantiate 66 million years after the last T. And in every case, all of the proposed functions would have been much more effective if the arms had not been reduced." "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. "All of the ideas that have been put forward about this are either untested or impossible because they can't work," Padian said. 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. 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. Severe bite wounds can cause infection, hemorrhaging, shock and eventual death, he said. "So, it could be a benefit to reduce the forelimbs, since you're not using them in predation anyway." 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). "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. 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. rexes descended on a carcass with their massive heads and bone-crushing teeth. rex's arms shrank in length to prevent accidental or intentional amputation when a pack of T. In a paper appearing in the current issue of the journal Acta Palaeontologia Polonica, Padian floats a new hypothesis: The T. rex's short arms evolved to do, Padian said, the question should be what benefit those arms were for the whole animal. 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. 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.
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