The tyrannosaurus rex still on the throne as the great predator of all time. About 12 meters long and almost six meters tall, he could exert up to six tons of pressure with one bite of his powerful jaw of 60 sharp teeth, able to split a car in half. However, for one of the scariest beasts to ever exist on Earth, it has been mocked in endless memes. Basically because of a surprising body feature: his short and useless arms.
Why were they so small? Science has come up with various theories.
Scientists have been racking their brains trying to figure out the mystery ever since archaeologist Barnum Brown unearthed the first complete T. rex skeleton in 1904. And of course, the puzzle is even more complicated considering that this creature is 68 million years old. Still, it hasn’t deterred scientists from pursuing its evolutionary benefits. The hypotheses range from grabbing the female during sex to a solution to not bite themselves. What is clear is that their ancestors had larger arms, so there is a clear selective evolution that caused their limbs to shrink.
Some paleontologists even propose that the arms had no function. They were too short, they couldn’t touch each other or reach their mouths, and their mobility was so limited that they couldn’t stretch very far, either forward or upward.
Now, new research from UC Berkeley led by paleontologist Kevin Padian and published in the journal Acta Palaeontologica Polonica suggests that Tyrannosaurus rex’s arms shortened over time. to prevent accidental or intentional amputationl. That way, when a herd of T. rex gathered over a carcass to feed with their massive heads and powerful jaws, they wouldn’t sever their own limbs in the process.
Imagine a hunt of that magnitude and several adult T. rexes swarming around the victim tearing and shredding their flesh. It would not be advantageous to have loose limbs that could also suffer cuts. Padian hypothesizes that natural selection favored shorter forelimbs, since arms were of little use to carnivores. However, its arms were known to be quite strong: analyzes have shown that a T. rex arm could lift 400 pounds, despite being pretty useless in coordination.
Henry Fairfield Osborn, who described and named T. rex, hypothesized that the short arms could have been “chest braces”, limbs that held the female in place during copulation. Something similar to the pelvic clips of some sharks and rays, which are modified fins. But there is no evidence about it. Also, the females have the same limbs and the arms are very close to the body, so it’s not clear how effective they would be in holding or surrounding another such large animal.
Other scientists have theorized that the arms were used as a lever for T. rex to get up from the ground in the event of a fall. Like a cane. Or, perhaps, they slept in such a way that they must have a way to stand up again. Other paleontologists believe that the arms were used to grasp wriggling prey before being dispatched. Steven Stanley of the University of Hawaii at Manoa noted that they were claw-like, capable of slashing prey and inflicting fatal wounds on prey.
However, the hypothesis that has received the most weight apart from avoiding its accidental amputation is that related to allometric growth. That is the uneven evolution of the organs of an animal We see that uneven rhythm of the body in humans themselves and most vertebrates. Babies’ limbs, for example, grow larger than their heads, which are usually quite large at birth.
Finally, some argue that they are vestigial: an evolutionary remnant. The arms were simply no longer needed, like the wings of today’s flightless birds such as ostriches and emus. As discussed in this National Geographic article, evolution sometimes works by subtraction. It is stripping things, not adding. Older organisms had more segments, so there is a constant loss of elements. That is, if an organism, being simple, survives its environment, it endures.
However, none of these theories and speculations seem to have been fully proven. And neither explains why the arms would evolve like this. The mystery will continue, surely forever.