Over more than a century, other proposed explanations for the short arms included waving for mate attraction or social signaling, serving as an anchor to allow T. rex to get up from the ground, holding down prey, stabbing enemies, and even pushing over a sleeping Triceratops at night. Think cow-tipping, Padian said. And some paleontologists propose that the arms had no function at all, so we shouldn’t be concerned with them.
Padian approached the question from a different perspective, asking what benefit shorter arms might have for the animal’s survival. The answer came to him after other paleontologists unearthed evidence that some tyrannosaurids hunted in packs, not singly, as depicted in many paintings and dioramas.
“Several important quarry sites unearthed in the past 20 years preserve adult and juvenile tyrannosaurs together,” he said. “We can’t really assume that they lived together or even died together. We only know that they were buried together. But when you find several sites with the same animals, that’s a stronger signal. And the possibility, which other researchers have already raised, is that they were hunting in groups.”
Perhaps, he thought, the arms shrank to get out of the way during pack feeding. T. rex youngsters, in particular, would have been wise to wait until the larger adults were finished.
In his new paper, Padian examines speculations by other paleontologists, none of which appear to have been fully tested. The first thing he determined, by measuring the lifesize T. rex cast that 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’t touch each other, they can’t reach the mouth, and their mobility is so limited that they can’t stretch very far, either forward or upward. The enormous head and neck are way out in front of them and pretty much form the kind of death machine you saw in ‘Jurassic Park.’”
Twenty years ago, two paleontologists analyzed the arms and hypothesized that T. rex could have bench pressed about 400 pounds with its arms. “But the thing is, it can’t get close enough to anything to pick it up,” Padian said.
Beware of Komodo dragons
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 and leave the remains for the smaller ones. Maulings can occur, as they do among crocodiles during feeding. The same could be true of T. rex and other tyrannosaurids, which first appeared in the Late Jurassic and reached their peak in the Late Cretaceous before becoming extinct.
Firmly establishing the hypothesis may never be possible, Padian said, but a correlation could be found if museum specimens around the world were checked for bite marks. That would be quite a feat of 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 reduction worked.”
But Padian has no illusion that his idea will be the end of the story.
“What I first wanted to do was to establish that the prevailing functional ideas simply don’t work,” he said. “That gets us back to square one. Then, we can take an integrative approach, thinking about social organization, feeding behavior and ecological factors apart from purely mechanical considerations.”
One problem in establishing the hypothesis is that there were several groups of large carnivorous dinosaurs that independently reduced their forelimbs, although in different ways.
“The sizes and proportions of the limb bones in these groups are different, but so are other aspects of their skeletons,” Padian said. “We shouldn’t expect them to be reduced in the same way. This is also true for the reduced wings of our large, living, flightless ratite birds, like the ostrich, the emu and the rhea. They evidently took different evolutionary paths for their own reasons.”
Padian sees a common thread in the history of explanations of short arms and other characteristics of T. rex.
“To 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 a lot of stories like this about possible functions of T. rex because it’s an interesting problem. But are we really looking at the problem the right way?”
Padian’s paper is part of a Festschift honoring mammalian paleontologist Richard Cifelli, long-time head of the Oklahoma Museum of Natural History and Presidential Professor of Biology at the University of Oklahoma in Norman.
RELATED INFORMATION
- Why tyrannosaurid forelimbs were so short: An integrative hypothesis (Acta Palaeontologica Polonica)
- Fleshing out the bones of Quetzalcoatlus, Earth’s largest flier ever (Dec. 7, 2021)
- Padian Lab website
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