Why did the monstrous T. rex have such tiny arms?

The Tyrannosaurus rex may have been the most feared dinosaur of its time, but what’s up with those arms? A 45-foot-long T. rex might have had a huge, five-foot-long skull, yet its arms were only three feet long — the equivalent of a six-foot human with five-inch arms. According to a recent study, scientists believe the reason that the iconic job T. rex had such short arms was to protect them during feeding frenzies on carcasses.

The latest theory is that the predator’s arms shrank to their tiny size to prevent accidental or intentional amputation when a pack of T. rexes descended on prey with their massive heads and bone-crushing teeth.

Paleontologist Kevin Padian, a professor of integrative biology at the University of California, Berkeley, has been pondering the question or decades. He focused his study on possible benefits of such tiny arms.

“What if several adult tyrannosaurs converged on a carcass? You have a bunch of massive skulls, with incredibly powerful jaws and teeth, ripping and chomping down flesh and bone right next to you. What if your friend there thinks you’re getting a little too close? They might warn you away by severing your arm,” he says in a statement. “So, it could be a benefit to reduce the forelimbs, since you’re not using them in predation anyway.

Such severe bite wounds could lead to infection, hemorrhaging, shock and even death for the T. rex.

Padian, also a curator at the UC Museum of Paleontology, says that T. rex’s ancestors, some of which were even bigger than T. rex, had long arms. Hence, there must have been a reason that they became reduced in both size and joint mobility.

“All of the ideas that have been put forward about this are either untested or impossible because they can’t work,” he explains. “And none of the hypotheses explain why the arms would get smaller. The best they could do is explain why they would maintain the small size. And in every case, all of the proposed functions would have been much more effective if the arms had not been reduced.”

The scientists admits that the theory will be hard to substantiate.

When the great dinosaur hunter Barnum Brown discovered the first T. rex fossils in 1900, he thought the arms were too small to be part of the skeleton. His colleague, Henry Fairfield Osborn, who described and named T. rex, hypothesized that the short arms might have been “pectoral claspers” — limbs that hold the female in place during copulation. This is similar to some sharks and rays’ pelvic claspers, which are modified fins, but Osborn provided no evidence.

But Padian concludes that T. rex’s arms are too short to go around another T. rex, and certainly too weak to exert any control over a mate.

Previous proposed explanations for the short arms included waving for mate attraction or social signaling. Scientists have also suggested they serve as an anchor to allow T. rex to get up from the ground. Others believe they were used for holding down prey, stabbing enemies, and even pushing over a sleeping Triceratops at night.

Some paleontologists even propose that the arms had no function at all.

“The arms are simply too short. 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,” says Padian. “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.”

The answer came to the professor after other paleontologists unearthed evidence that some tyrannosaurids hunted in packs, not singly, as often depicted.

“Several important quarry sites unearthed in the past 20 years preserve adult and juvenile tyrannosaurs together. We can’t really assume that they lived together or even died together. We only know that they were buried together,” adds Padian. “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. Tyrannosaurus rex youngsters, in particular, would have been wise to wait until the larger adults were finished.

Twenty years ago, two paleontologists analyzed the arms and hypothesized that T. rex could have bench pressed about 400 pounds with its arms. Even so, Padian says the beast still couldn’t get close enough to pick anything up.

Although not conclusive, crocodiles and Komodo dragons descend in packs, leaving the smaller bits for their young when they have finished. They often suffer maulings, however.

“Bite wounds on the skull and other parts of the skeleton are well known in tyrannosaurs and other carnivorous dinosaurs. If fewer bite marks were found on the reduced limbs, it could be a sign that reduction worked,” says Padian. “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. 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?”

The study is published in the journal Acta Palaeontologia Polonica.