Scientists have explained why Tyrannosaurus had such small hands

Scientists from University College London and Cambridge University have studied 82 species of theropods - bipedal predatory dinosaurs, which includes T. rex. They concluded that the reduction of the forelimbs was closely linked not just to body growth, but to the development of large and powerful skulls.

Simply put, in some predatory dinosaurs the main weapon gradually became not the front paws with claws, but the head and jaws. If prey was more effectively attacked and held with the jaws, the arms may have lost their former importance and diminished over time.

Important: the study shows a link, not direct proof of a cause. Scientists cannot 'test' dinosaur behaviour experimentally. But the coincidence of short forelimbs with powerful skulls in several different groups of theropods makes this theory convincing.

Details

T. rex is the best-known example of a dinosaur with tiny arms, but it's far from the only one. According to the study, shortened forelimbs independently appeared in at least five groups of theropods: in Tyrannosauridae, Abelisauridae, Carcharodontosauridae, Megalosauridae, and Ceratosauridae. Supplementary material to the paper also indicates that reduction of the forelimbs relative to the skull occurred in at least five theropod lineages.

The authors tested whether arm reduction was more strongly associated with overall body size, skull length, or skull strength. To do this, they developed a way to estimate the "power" of the skull: they took into account the shape of the head, the connection of the bones and the strength of the bite.

T. rex got the highest score. This is not surprising: it had a huge head, large teeth and a very strong bite. London's Natural History Museum notes that T. rex lived 68-66 million years ago, reached about 12 metres in length, had about 60 teeth, and its bite was about three times as powerful as a lion's.

But the key finding of the study is not that large dinosaurs simply had small hands. The link to a strong and powerful skull was stronger than to total body mass. This means that the arms may have shrunk not as a side effect of gigantic size, but because the role of the main weapon was taken over by the head.

A good example is Majungasaurus, a predator from Madagascar. It was much smaller than T. rex, but it too had a powerful skull and greatly reduced forelimbs. This shows: it wasn't just about the dinosaur getting huge.

Scientists also suggest that this evolution could have been influenced by prey. In worlds where giant sauropods and other large herbivorous dinosaurs lived, grabbing prey with the front legs was not the most convenient way to hunt. A powerful bite and a strong skull may have been more effective.

At the same time, different groups of dinosaurs reduced their forelimbs in different ways. In Abelisauridae, the hands and the lower part of the arm were particularly shrunken. In tyrannosaurids, the elements of the forelimb were reduced more evenly. This suggests that a similar result - small hands - could have arisen through different evolutionary pathways.

Why it matters

The small hands of T. rex are often seen as a comical detail. But for palaeontologists, it's an important example of how an animal's body changes when the way it hunts changes.

If a predator relies more and more on its head and jaws, other parts of the body may lose their former role. This doesn't mean that the hands were completely "useless". They may have retained some functions, such as helping with lifting, holding a mate or other movements. But in attacking large prey, they were probably no longer the primary tool.

The study also shows how evolution can come to a similar decision several times. Different groups of predatory dinosaurs had their forelimbs shrunk independently, but each time, powerful heads and jaws appeared alongside. This recurring scenario is particularly interesting: it shows a general pattern rather than a random oddity in one species.

Background

Theropods are a large group of bipedal dinosaurs. They ranged from small predators to giants like T. rex, Tyrannotitan, and other large forms. Many theropods were meat-eaters, although different types of diets and lifestyles were found within the group.

The question of the arms of T. rex has long been debated. Some hypotheses have suggested that they may have helped to hold prey. Others have suggested that they were used when getting up from the ground or during mating. There is also a version that they gradually decreased, because the main work in the hunt took on the head.

The new work does not overturn all previous ideas, but emphasises the wider evolutionary context. Small hands were not only in T. rex, and the link to a powerful skull can be traced back to several lineages of predatory dinosaurs. So the question is no longer "why would a Tyrannosaurus have such strange hands?". Rather, "why did different predatory dinosaurs have their bodies rearranged again and again in favour of the head and jaws?".

Source

Charlie Roger Scherer et al, "Drivers and mechanisms of convergent forelimb reduction in non-avian theropod dinosaurs", Proceedings of the Royal Society B: Biological Sciences, 2026. DOI: 10.1098/rspb.2026.0528.

In the study, the authors analysed data from 82 species of non-bird theropods. They compared forelimb length, skull size and strength, overall body size, and evolutionary relationships between species. The work showed that a reduction in forelimbs was more strongly associated with a powerful skull structure than with just body size.