A new hypothesis holds that the reason for the reduction in the size of its forelimbs was a ‘defense mechanism’ against bites from other tyrannosaurs
It became extinct 66 million years ago, along with the rest of the dinosaurs, after the impact of a meteorite that also killed more than 75% of life on Earth. It lived in what is now North America, and ever since Edward Drinker Cope discovered the first specimen in 1892, both its ferocious behavior and certain features of its anatomy continue to intrigue scientists.
And it is that the Tyrannosaurus Rex had strangely short forelimbs, with limited mobility and that, certainly, ‘do not fit with the rest of the body of one of the largest predators that have set foot on our planet. At over 40 feet long, with a huge skull and the most powerful jaws that ever existed, T.
rex was capable of biting with a force that paleontologists estimate between 20,000 and 57,000 newtons. The same, for example, that an elephant exerts on the ground when sitting down. For comparison, suffice it to say that the bite force of a human rarely exceeds 300 newtons.
Why such short arms?
Now, why did the T. Rex have such ridiculously small arms? For more than a century, scientists have been proposing various explanations (for mating, holding their prey, or capsizing the animals they attacked…), but for Kevin Padian, a paleontologist at the University of California, Berkeley none of them is correct.
In an article just published in ‘ Acta Paleontologica Polonica ‘, in fact, Padian maintains that the arms of T. rex were reduced in size to avoid irreparable damage caused by the bite of one of their congeners. Evolution does not maintain a certain physical trait if not for a good reason. And Padian, instead of wondering what such short upper limbs could do, focused on finding out what possible benefits they could bring to the animal. In his paper, the researcher hypothesizes that T. rex arms ‘shrank’ to prevent accidental or intentional amputation when a herd of tyrannosaurs lunged at a carcass with their massive heads and bone-crushing teeth.
A 13-meter T. rex, for example, with a 1.5-meter-long skull, had arms no longer than 90 centimeters. If we apply these proportions to a human being 1.80 meters tall, his arms would barely measure 13 centimeters.
Avoiding bites
“What would happen if several adult tyrannosaurs gathered around a carcass? Padian wonders. We’d have a bunch of huge skulls, with incredibly powerful jaws and teeth tearing and chewing through flesh and bone right next to each other. And what if one of them thinks the other is getting too close? He could warn her away by cutting off her arm. So reducing the forelimbs could be of great benefit, since they weren’t used in predation anyway.”
A serious bite wound can cause infection, bleeding, shock, and eventually death. In his study, Padian points out that the ancestors of tyrannosaurs had longer arms, and therefore their subsequent reduction in size must be for good reason. This reduction, furthermore, did not affect only T. rex, which lived in North America, but also other large carnivorous dinosaurs that lived in Africa, South America, Europe, and Asia at different times in the Cretaceous, some of them even larger than the T.rex.
According to Padian, all the ideas in this regard presented so far “have not been tried or are impossible because they cannot work. And neither hypothesis explains why the arms should get smaller. In all cases, the proposed functions would have been much more effective if they had not been reduced to considering them as weapons.
They hunted in packs
The idea proposed in his study occurred to the researcher when other paleontologists found evidence that the T.rex was not a solitary hunter, as previously thought, but often hunted in packs.
Several major sites discovered over the last 20 years, Padian explains, show adult and juvenile tyrannosaurs together. «Really -he points out we cannot assume that they lived together or even that they died together. We only know that they ended up buried together. But when multiple sites are found where the same thing happens, the signal becomes stronger. And the possibility, which other researchers have already raised, is that they were hunting in a group.
In his study, the Berkeley paleontologist examines and discards one by one the solutions to the enigma proposed so far. “Simply,” he explains, “the arms are too short. 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 the kind of death machine that we saw in Jurassic Park .” Twenty years ago, a team of paleontologists analyzed the arms and hypothesized that T. rex could have lifted about 400 pounds with them. “But the thing,” says Padian, “is that they couldn’t get close enough to anything to pick it up.”
Current analogies
Padian’s hypothesis has analogies with some current animals, such as the giant Indonesian Komodo dragon, which hunts in groups and, after killing prey, the largest specimens pounce on it and leave the remains for the smaller ones. . In the process, it is not uncommon for one of the dragons to sustain serious injuries. And the same goes for crocodiles. For Padian, the same scene could have played out with T. rex and other families of tyrannosaurs millions of years ago.
However, Padian himself admits that it will never be possible to prove his hypothesis, although a correlation could be found if all T.rex specimens in museums around the world are examined for bite marks. “Bite wounds to the skull and other parts of the skeleton,” he explains, “are well known in tyrannosaurs and other carnivorous dinosaurs. If fewer bite marks were found on the shrunken limbs, it could be a sign that the sizing worked.”
