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Why did Tyrannosaurus rex have such small arms?

A recently published article in Acta Palaeontologica Polonica raises a new hypothesis as to why Tyrannosaurus rex , the world’s fiercest predator, had such short arms.

The reasons that had been considered so far pointed to the fact that the animal needed its “little arms” to mate, to hold or stab prey or even to overturn a Triceratops. Paleontologist Kevin Padian of the University of California Berkeley has found a new reason, and it is this: T. rex arms were shortened to prevent accidental or intentional amputation when a herd of T. rex descended on a carcass with their arms. huge heads and bone-crushing teeth.

“What if multiple adult tyrannosaurs converge on a carcass? You have a bunch of huge 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 to stay away by cutting off your arm ,” said Padian, professor emeritus of integrative biology at the University of California, Berkeley, and curator of the UC Museum of Paleontology (UCMP). “So it could be a benefit to reduce the forelimbs, since it doesn’t use them in predation anyway.” The expert also points out that “serious bite wounds can cause infection, hemorrhage, shock and eventually death.” Avoiding them would be like life insurance.

Padian pointed out that the predecessors of tyrannosaurs had longer arms , so there must have been 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, he said, but mid-Cretaceous African and South American abelisaurids and carcharodontosaurids, which spread across Europe and Asia in the early and mid-Cretaceous. Cretaceous and were even larger than T. rex.

“All the ideas that have been raised about it 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 keep the size small. And in all cases, all of the proposed functions would have been much more efficient if the arms were not would have reduced.”

The expert admits that any of the hypotheses raised, including his, is difficult to corroborate 66 million years after the last T. rex became extinct.

A giant with “little arms”

When the first Tyrannosaurus rex fossils were discovered in 1900, the arms were thought to be too small to be part of the skeleton. At that time, the hypothesis was raised that these could have been a kind of “pectoral hooks” that the animal used to hold the female while copulating. Padian, however, claims that T. rex’s arms are too short to encircle another member of the species and too weak to exert any control over a mate.

Other explanations that have been given contemplate the hypothesis that they were used to greet and thus attract the couple or as social signaling , that they served as an anchor to allow the T. rex to get up from the ground, hold the prey, stab the enemies and even push a sleeping triceratops at night.

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 and not alone, as depicted in many paintings and dioramas.

“Several major quarry sites unearthed in the last 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 multiple sites with the same animals are found, it’s a stronger signal. And the possibility, already raised by other researchers, is that they hunt in a group.

Perhaps, he thought, the arms were shrunken to avoid incidents when the animals ate in packs. The young T. rex, in particular, would have been cautious and not start eating until the larger adults had finished.

In his new paper, Padian examines the speculations of other paleontologists, none of which have been 100% confirmed. The first thing he determined, by measuring the life-size T. rex cast that dominates the UCMP atrium, was that neither hypothesis would really work.

“The arms are too short,” he said. “They can’t touch each other, they can’t reach their mouths, and their mobility is so limited that they can’t stretch very far, either forward or up. The huge head and neck are way ahead of them and form more or less the kind of machine of death that you saw in Jurassic Park .”

Twenty years ago, two paleontologists analyzed the arms and hypothesized that T. rex could have lifted about 400 pounds with its arms. “But the thing is, you can’t get close to anything to pick it up,” Padian said.

Padian’s hypothesis bears similarities to what happens to living animals such as the Komodo dragon . It turns out that this giant lizard hunts in packs, and when it kills prey, the larger dragons converge on the carcass, leaving the remains for the smaller ones. Mutilations may occur, as occurs among crocodiles during feeding. The same could happen with 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 firmly establish the hypothesis, Padian said, but a correlation could be found if museum specimens around the world are checked for bite marks. “Bite bites to the skull and other skeletal parts 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 worked.”

However, the researcher is under no illusions that his hypothesis is the final explanation as to why the ferocious tyrannosaurus had such small arms.

 

Reference:

Padian, K. 2022. Why tyrannosaurid forelimbs were so short: An integrative hypothesis. Acta Palaeontologica Polonica. DOI: 10.4202/app.00921.2021

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