How long are a typical adult’s arms? The length can vary of course, but on average, a human man’s wingspan is about two inches longer than his height, and a woman’s is about half an inch longer than her height. By contrast, perhaps the most well-known dinosaur, Tyrannosaurus rex, has bizarrely puny arms despite having a 45-foot-long length and a towering height of 12 feet. If our arms shared the same proportion, they would be only five inches long.
So how did an animal so huge have forelimbs that were only three feet long—even shorter than some basketball players’ arms? This disparity has had paleontologists scratching their heads since famed dinosaur hunter Barnum Brown unearthed the first “reasonably complete” T. rex skeleton in Hell Creek, Montana back in 1902. In the decades since the discovery, the dino with the short arms has been portrayed as the butt of jokes in popular culture favorites such as the Toy Story films, in which a toy plastic version named Rex suffers from anxiety.
For the real dinosaurs, whose Latin name means “King of the Tyrant Lizards,” their tiny limbs were probably a survival feature. They likely fed on carcasses together, evolutionary biologist Kevin Padian of the University of California, Berkeley says in a news release.
“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,” Padian says. Imagine the aftermath of your bones being crushed by those teeth—it would have been a painful death, probably involving great blood loss and infection, he says.
Padian’s hypothesis, published in the current issue of the journal Acta Palaeontologia Polonica, describes his rationale. Tyrannosaurid forerunners had longer arms, and they must have shortened for a good reason, explains Padian, a curator at the UC Museum of Paleontology. “It could be a benefit to reduce the forelimbs, since you’re not using them in predation anyway.”
This adaptation would have shaped several species from the Cretaceous period, Padian says, including the African and South American abelisaurids from the mid-Cretaceous, and the carcharodontosaurids, which were even bigger than T. rex and ranged across Europe and Asia in the Early and Mid-Cretaceous periods.
It’s difficult to prove an idea about an animal that went extinct 66 million years ago. However, Padian proposes that T. rexes had so much predatory power with six-inch teeth and powerful jaws, plus heads up to five feet long, that fighting over the same prey posed a real threat.
The oldest known tyrannosauroids, such as Dilong, were much smaller, only three to six feet long, and their forelimbs were more than half the length of their hindlimbs. In general, observations of T. rex predecessors and other predator dinosaur species show that as their skulls, teeth, and other predator attributes grew more dominant, their forelimbs diminished. At a certain point, the arms of predators that may have fed in packs became a liability in the face of such intimidating jaws.
The T. rex remains one of the outliers in its extremely disproportionate arm size. (The albertosaurids dinosaurs have arms that are even smaller, proportionally.)