Tyrannosaurus Rex and Other Terrifying Predatory Dinosaurs Had Itty-Bitty Arms. Scientists May Have Finally Figured Out Why

A new study suggests that certain theropods—two-legged, mostly meat-eating dinosaurs—had shrunken forelimbs as an evolutionary trade-off for their strong skulls

Sara Hashemi | Daily Correspondent

May 29, 2026 11:53 a.m.

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Tyrannosaurus rex’s tiny arms might seem out of place on a massive predator, but there must have been an evolutionary reason for the limbs’ almost comical size. Now, scientists think they’ve figured it out.

Shortened forelimbs in T. rex and some other theropods—two-legged, mostly meat-eating dinosaurs—seem to be a result of the ancient animals’ powerful jaws and skulls, according to a study published May 20 in the journal Proceedings of the Royal Society B. As the terrifying predators increasingly used their heads to bite prey, their arms became less useful.

“People have long been fascinated by why big meat-eating dinosaurs like T. rex have such tiny arms,” says study co-author Elizabeth Steell, a paleontologist at the University of Cambridge in England, in a statement. “We’ve just confirmed what many people have suspected, which is that if you’ve got a big skull and you’re tackling big prey, then you don’t need your arms as much, and arms become a bit redundant.”

T. rex, which lived around 66 million to 68 million years ago, was one of the largest known carnivorous dinosaurs. They are thought to have grown to about 40 feet long and weighed as much as an African elephant. But their arms were merely around three feet long—on average, less than one-third the length of the giant creatures’ legs, writes Sarah Sheffield, a paleobiologist at Binghamton University, who was not involved in the study, for the Conversation. Other theropods, such as abelisaurids, also had strangely small arms relative to their body sizes.

So, Steell and her colleagues gathered data on the sizes and masses of the bodies, forelimbs and skulls of 85 non-avian theropod species. Then, they calculated the ratios between most of the studied species’ forelimbs and skulls. The team also came up with a way to quantify the strength of a skull based on factors like its shape, bite force and how the bones fit together.

T. rex earned the highest cranial strength score, followed by the Tyrannotitan, another giant theropod that lived in modern-day Argentina during the early Cretaceous period, more than 30 million years before T. rex’s time, the researchers found. Overall, skull-forelimb ratio and cranial strength were strongly correlated among the analyzed species, hinting that shorter arms often accompanied more robust heads. In other words, powerful jaws came at the price of claws.

“If you’re a dinosaur with a very strongly put together skull, chances are you’re going to have very small forelimbs,” study co-author Charlie Scherer, a paleontologist at University College London, tells CNN’s Jacopo Prisco. “And it doesn’t really matter how big you are—you could be one ton in weight, or ten tons in weight. If you have a strong skull, you’re going to have relatively small arms.”

What’s more, the analysis revealed the relationship between arm and skull strength existed not only in T. rex’s fellow tyrannosaurids, but also in abelisaurids, carcharodontosaurids, ceratosaurids and megalosaurids. The latter two groups hadn’t previously been recognized as subjects of this evolutionary trend, says Fion Waisum Ma, a paleontologist at the Beipiao Pterosaur Museum of China, who wasn’t involved in the study, to Jake Buehler at New Scientist.

The researchers suggest that an increase in the size of the theropods’ prey may have driven this evolutionary change. As herbivores, like long-necked sauropods—the biggest known dinosaurs—grew larger, predators had to adapt their hunting tactics.

“Everything was approached headfirst, so the head just became what came into contact with the prey,” Scherer tells CNN. “That was the easiest way to bring them down, as opposed to jumping around or fighting with claws.”

Andre Rowe, a paleontologist at the University of Bristol in England, who was not involved in the research, tells New Scientist that he is curious about the mechanical function of the theropods’ arms. Even though they were tiny, they weren’t necessarily useless, he says.

The findings emphasize the evolutionary diversity of dinosaurs, Rowe adds. “It reminds me of why I fell in love with dinosaurs in the first place. They were some of the most innovative and successful animals to ever exist.”

Sara Hashemi | Read More

Sara Hashemi is a science writer and fact-checker currently based in New York City. Her work has appeared in Sierra, The Body, Maisonneuve magazine and more.