Over the course of the two decades, paleontologist Kevin Padian taught at a first-year seminar called The Age of Dinosaurs, a question often asked by students, holding on to him: Why are the arms on Tyrannosaurus rex so ridiculously short?
He would usually list a number of paleontologists’ suggested hypotheses – for mating, for holding or sticking prey, for overturning a Triceratops– but his students, who usually stared at a life-size copy in the face, remained dubious. Padian’s usual response was, “No one knows.” But he also suspected that scientists who had suggested a solution to the riddle came to it from a wrong perspective.
Instead of asking what T. rex’s short arms evolved to do, Padian said, the question should be what benefit these arms were for the whole animal.
In a paper that appears in the current issue of the journal Acta Palaeontologia PolonicaPadian hovers a new hypothesis: The T. rex’s the arms shrunk in length to prevent accidental or intentional amputation when a pack with T. rexes down on a carcass with their massive heads and bone-crushing teeth. A 45 foot long T. rexfor example, could have had a 5 foot long skull, but arms only 3 feet long – similar to 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 that tear and chop meat and bones down right next to you. What if your friend there thinks you get a little too close? They can warn you away by cutting your arm off, “said Padian, a senior emeritus professor of integrative biology at the University of California, Berkeley, and curator at the UC Museum of Paleontology (UCMP). “So it might be beneficial to reduce the front legs, since you are not using them in predation anyway.”
Severe bite wounds can cause infection, bleeding, shock and eventually death, he said.
Padian noted that the predecessors of tyrannosaurids had longer arms, so there must have been a reason why they were reduced in both size and joint mobility. This would not only have affected T. rexwho lived in North America at the end of the Cretaceous, he said, but the African and South American abelisaurids from the mid-Cretaceous and the Carcharodontosaurides, which spread across Europe and Asia in the early and middle Cretaceous and were even larger than T. rex.
“All the ideas that have been put forward about this are either untested or impossible because they can not 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 keep the small size. And in any case, all the proposed features would have been much more effective if the arms did not had been reduced. “
He admitted that any hypothesis, including his, would be difficult to substantiate 66 million years after the last. T. rex extinct.
Arms and T. rex
When the great dinosaur hunter Barnum Brown discovered the first one 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, assumed that the short arms could have been “breast buckles” – limbs that hold the female in place during mating. This is analogous to some shark and stingray brook buckles, which are modified fins. But Osborn provided no evidence, and Padian noted that T. rex’s the arms are too short to go around another T. rex and determined too weak to exercise any control over a spouse.
Over the course of more than a century, other proposed explanations of the short arms included waving for the attraction of spouses or social cues that served as an anchor to allow T. rex to rise from the ground, hold prey, stab enemies and even push over a sleeper Triceratops in the night. Think cow-tipping, Padian said. And some paleontologists suggest that the arms had no function at all, so we should not worry about them.
Padian approached the question from a different perspective and asked what benefit shorter arms could have for the animal’s survival. The answer came to him after other paleontologists had found evidence that some tyrannosaurids hunted in droves, not individually, as depicted in many paintings and dioramas.
“Several important quarry sites that have been excavated over the last 20 years are keeping adult and young tyrannosaurs together,” he said. “We can not really assume that they lived together or even died together. We only know that they were buried together. But when you find several places with the same animals, it is a stronger signal. And the possibility that other researchers have already brought up is that they hunted in groups. “
Maybe, he thought, shrinking his arms to get out of the way while packing. T. rex Young people in particular would have been wise to wait until the older adults were done.
In his new paper, Padian examines speculations from other paleontologists, none of which appear to have been fully tested. The first thing he determined, by measuring the natural size T. rex cast, which dominates the atrium outside the doors of the UCMP, is that none of the hypotheses would actually work.
“The arms are simply too short,” he said. “They can not touch each other, they can not reach the mouth, and their movement is so limited that they can not extend very far, neither forward nor upward. The huge head and neck are far out in front of them and pretty much that kind of death machine , you saw in ‘Jurassic Park’. “
Twenty years ago, two paleontologists analyzed the arms and assumed it T. rex could have bench pressed about 400 pounds with his arms. “But the thing is, it can’t get close enough to anything to pick it up,” Padian said.
Beware of komodo kites
Padian’s hypothesis has analogies in some fearsome animals today. The giant Komodo Dragon lizard (Varanus komodoensis) of Indonesia hunts in groups, and when it kills prey, the larger dragons converge on the carcass, leaving the remains for the smaller ones. Maulings can occur as they do among crocodiles during feeding. The same could be the case T. rex and other tyrannosaurids, which first appeared in the Late Jurassic and reached their peak in the Late Cretaceous before becoming extinct.
It may never be possible to establish the hypothesis, Padian said, but a connection could be found if museum samples around the world were checked for bite marks. It would be something of a feat with fossil crowdsourcing, he admitted.
“Bite wounds on the skull and other parts of the skeleton are well known in tyrannosaurs and other carnivorous dinosaurs,” he said. “If fewer bite marks were found on the reduced limbs, it could be a sign that the reduction was working.”
But Padian has no illusion that his idea will be the end of the story.
“What I first wanted to do was establish that the prevailing functional ideas simply do not work,” he said. “It gets us back to square one. Then we can take an integrative approach and think about social organization, feeding behavior, and ecological factors apart from purely mechanical considerations.”
One problem with establishing the hypothesis is that there were several groups of large carnivorous dinosaurs that independently reduced their limbs, albeit in different ways.
“The size and proportions of the limb bones in these groups are different, but so are other aspects of their skeletons,” Padian said. “We should not expect them to be reduced in the same way. This also applies to the reduced wings of our large, living, flightless ostriches, such as the ostrich, the emu, and the rhea. They apparently went different evolutionary paths for their own reasons.”
Padian sees a common thread in the story of explanations of short arms and other characteristics of T. rex.
“For me, this study of what the arms did is interesting because of how we tell stories in science and what qualifies as an explanation,” he said. “We tell many stories like this about possible features of T. rex because it is an interesting problem. But are we really looking at the problem in the right way? “
Padian’s paper is part of a Festschift honoring mammalian paleontologist Richard Cifelli, longtime director of the Oklahoma Museum of Natural History and presidential professor of biology at the University of Oklahoma in Norman.
To fill the bones of Quetzalcoatlus, the Earth’s largest fly ever
Kevin Padian, Why the Limbs of the Tyrannosaurus Were So Short: An Integrative Hypothesis, Acta Palaeontologica Polonica (2022). DOI: 10.4202 / app.00921.2021
Provided by the University of California – Berkeley
Citation: T. rex ‘short arms may have lowered the risk of bites while feeding frenzies (2022, April 1) retrieved April 1, 2022 from https://phys.org/news/2022-04-rex-short-arms- lowered-frenzies. html
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