It's a question that has baffled scientists for decades: why did Tyrannosaurus. rex have such tiny arms?

The 45ft dinosaur had arms measuring just three feet long – the equivalent of a 6ft human with five–inch arms.

However, despite decades of research, scientists have remained clueless as to why this is the case. 

Now, researchers from University College London believe they've solved the mystery. 

They claim that the evolution of tiny arms was likely driven by the development of strong, powerful heads. 

'Everyone knows the T. rex had tiny arms but other giant theropod dinosaurs also evolved relatively small forelimbs. The Carnotaurus had ridiculously tiny arms, smaller than the T.rex,' Charlie Roger Scherer, lead author of the study.

'We sought to understand what was driving this change and found a strong relationship between short arms and large, powerfully built heads.

'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.' 

T. rex first appeared in the Late Jurassic period, and reached its peak in the Late Cretaceous before becoming extinct about 65.5 million years ago.

Previous studies have shown that the species – also known as the 'King of the Dinosaurs' – had tiny arms. 

However, until now, the reason why has remained a mystery. 

To get to the bottom of it, the researchers analysed data on 82 species of theropod – two–legged, mainly meat–eating dinosaurs. 

Their analysis revealed that shortening of the forelimbs occurred across five groups –tyrannosaurids, abelisaurids, carcharodontosaurids, megalosaurids and ceratosaurids.

While the researchers initially expected that smaller arms would be linked to larger overall body size, surprisingly this wasn't the case. 

Instead, tiny arms were found to be closely linked to the development of powerful skulls and jaws. 

The secondary importance of overall body size was highlighted by the fact that some theropods with strongly built heads and tiny arms were not very large. 

For example, the Majungasaurus, an apex predator in Madagascar 70 million years ago, weighed just 1.6 tonnes – about a fifth of the T. rex. 

The reason for this limb shortening remains unclear. 

However, the researchers suggest that as prey increased in size, T. rex shifted to hunting using its jaws and head instead of its claws – resulting in a shrinking of the arms.

'These adaptations often occurred in areas with gigantic prey,' Mr Roger Scherer explained. 

'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.

'While our study identifies correlations and so cannot establish cause and effect, it is highly likely that strongly built skulls came before shorter forelimbs. 

'It would not make evolutionary sense for it to occur the other way round, and for these predators to give up their attack mechanism without having a back–up.' 

Previous studies have suggested a range of weird and wonderful theories to explain T. rex's tiny arms. 

In 2022, researchers from the University of California, Berkeley, claimed that T. rex had short arms to stop them getting bitten by other dinosaurs during feeding frenzies. 

Meanwhile, a wacky study later that year by experts at the Ernesto Bachmann Paleontological Museum in Neuquén, Argentina claimed the tiny arms allowed the dinosaurs to grip each other tightly during sex. 

Speaking at the time, project leader Dr Juan Canale said: 'Actions related to predation were most likely performed by the head. 

'I am inclined to think their arms were used in other kinds of activities.

'They may have used the arms for reproductive behaviour such as holding the female during mating or support themselves to stand back up after a break or a fall.'