One of the things that has baffled dinosaur experts for over 100 years is why some meat-eating dinosaurs like Tyrannosaurus rex had such small arms. For a fearsome predator of its size, T.rex had comically small arms: they were less than a metre long on a body that measured about 12 metres from nose to tail. What could such tiny arms be used for? A number of theories have been discussed over the years. Some argued it was for balance, others for holding or slashing at prey, but ultimately no-one has come up with a convincing explanation for this strange trait…until now. Here’s the latest reason dinosaur experts are giving for T. rex’s tiny arms. 

Small arms evolved

Many scientists now believe that T. rex had such small arms because of evolution. While having forelimbs was once useful to these creatures, they gradually became obsolete over time, meaning they shrank in size. Just like our ancient primate ancestors used to have tails, leaving modern humans with a tailbone, some theropods evolved to not need their arms. A study published in May 2026 found that tyrannosaurs weren’t the only ones with tiny arms. In fact, this trait evolved in several groups of theropods (meat-eating dinosaurs that walked on two legs). They also found it was likely connected to these animals developing strong, powerful heads. 

A team of researchers studied data from 82 species of theropod and discovered that the shortening of the forelimbs occurred across five groups including the tyrannosaurids, of which T. rex is a member. As these dinosaurs developed larger heads and more powerful jaws their arms got shorter. 

Why did small arms evolve?

The researchers think dinosaurs like T. rex had to grow bigger heads and jaws because their prey was getting bigger. During the Cretaceous period, Earth’s climate got hotter. The warm, dry environment meant the large herbivores that theropods ate thrived and grew to huge sizes. As these herbivores got bigger, the theropods had to find more effective ways of hunting them. The evolutionary answer was a shift from hunting with forelimbs and claws to hunting with heads and jaws. Lead researcher Charlie Roger Scherer from University College London (UCL) said, “The head took over from the arms as the method of attack. It’s a case of ‘use it or lose it’ – the arms are no longer useful and reduce in size over time…Trying to pull and grab at a 100ft-long sauropod with your claws is not ideal. Attacking and holding on with the jaws might have been more effective.”

Some scientists even think that if the dinosaurs had lived for another few million years, T. rex may have lost its arms altogether. 

Meet T. rex in London

If you’re fascinated by T. rex, don’t miss the award-winning dinosaur children’s show, Dinosaur World Live. Stomping its way to the Troubadour Wembley Park Theatre in London this summer, it’s one of the best dinosaur attractions in the UK for learning about these wonderful creatures. At 50 minutes long with no interval, it’s perfect for kids aged three and up, and there’s even a meet and greet afterwards. Search ‘dinosaur attractions near me’ and book your tickets today for a roarsome adventure!