Why did dinosaurs like T. Rex have such small arms?

Tyrannosaurus rex is perhaps the most famous dinosaur, known for its enormous teeth and gigantic size, but also for its relatively small arms. The fearsome predator was about 40 feet tall and yet its arms were only 3 feet long. However, this ridiculous ratio wasn’t just reserved for the king of the dinosaurs: there are plenty of other theropods – the clade to which T. rex is part of it – also had measly appendages. But why?

Why did some theropods have such small arms?

The truth is, no one knows for sure. Theropods, like all other non-avian dinosaurs, went extinct 66 million years ago, so we will never be able to see for ourselves how they handled their small limbs. Fortunately, paleontologists do have some theories.

T. rex small arm shell

A 14 meter long (45 feet) T. rex would have had arms less than a meter long. That’s the equivalent of a 1.8 meter tall human with arms only 13 centimeters long.

Image credits: © IFLScience, Mateusz Atroszko / Bowonpat Sakaew / nale / Shutterstock

One of the first suggestions came from Henry Fairfield Osborn, who described and named it T. rex in 1905. He suggested that the dinosaur’s stubby arms may have been used as a kind of mating device, giving it an advantage during sexy times.

The hypothesis is alive and well today. In 2022, after the discovery of Meraxes gigas – another small-armed theropod – project leader Juan Canale speculated about this.

“They may have used the arms for reproductive behavior, such as holding the female during copulation,” Canale argued in a statement. Short arms may have even helped M. gigas, and other theropods, to attract potential mates. “Sexual selection is a powerful evolutionary force. But since we cannot directly observe their behavior, it is impossible to be certain.”

Meraxes gigas

Meraxes gigas rocked with small arms 20 million years ago T. rex even existed.

Canale also suggested that despite being so small, their small arms could have helped dinosaurs get back up after a fall. Weighing about 7,000 kilograms (15,500 pounds) – the weight of a large African elephant – it was probably difficult for T. rex to raise himself from the ground, so even the smallest weapon would have been a help.

“When they got up from a crouched position, they could use the arms to perform a little tyrannosaur push-up,” Scott Persons, chief curator at the Mace Brown Museum of Natural History, told BBC Future. “[But] you have to understand that this really only helps the tyrannosaurus with the first two feet [0.6 meters]. And then it’s about 15 feet [4.5 meters] to get off the ground.”

Another idea is that stubby forelimbs may have helped theropods avoid accidental amputations – which sounds like a big bonus to us.

“What if several adult tyrannosaurs converge on a carcass? You have a set of enormous skulls, with incredibly powerful jaws and teeth, tearing and chewing flesh and bone, right next to you. What if your friend over there thinks you’re getting a little too close? They can warn you by cutting off your arm,” paleontologist Kevin Padian explained in a 2022 press release.

In this case, having arms that don’t get in the way can be the difference between getting mauled by your partner and dodging being chewed.

Believe it or not, some scientists have made that claim T. rexThe pathetic looking limbs could actually have been used as weapons. According to a 2017 study, they may have been adapted for ‘vicious sshing’ – although this view is disputed, given how ridiculously small they are. Other theories about why some theropods had such tiny arms include nest digging and nest care.

Of course, there is also the possibility that the weapons served no purpose at all and were simply a remnant of their former ancestors. Maybe, faced with evolutionary pressures, like T. rex spent less energy maintaining arm size, instead leading to longer legs or a larger head.

“An animal can only devote so much of its body volume to one thing or another,” John Hutchinson, a biologist at the Royal Veterinary College of the University of London, told LiveScience. ‘He cannot be an all-rounder. So you either have a very general organization where everything is equally specialized for a general ecological niche, or you really specialize like T. rexwhich is super specialized to be a front-end predator.”

The end result is a vestigial feature, like theropod arms or the human tailbone.

Carnotaurus sastrei

Carnotaurus sastrei taking little arms to their limits.

But perhaps our favorite suggestion is that some dinosaurs, like Carnotaurus, may have used their tiny arms in a bizarre arm-twisting courtship. Phwoar.

Hopefully, future research and fossil discoveries will help answer some of the remaining questions about theropods’ absurd appendages. The reason behind this can be debated, but at least we can all agree that they are really, really stupid.

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