Why did the T.rex have tiny arms?

They were once one of the most fearsome dinosaurs to roam the earth, with an enormous set of teeth, but why did the T.rex have such tiny arms?

Well, scientists now think they might have an answer.

A team of researchers at University College London (UCL) and Cambridge University studied 82 species of theropods - two-legged, mainly meat-eating dinosaurs.

They discovered that shorter arms were present in five groups of dinosaurs, including tyrannosaurids - the family that includes Tyrannosaurus rex - and found that smaller arms were closely linked to the development of large, powerful skulls and jaws.

"Everyone knows the T.rex had tiny arms but other giant theropod dinosaurs also evolved relatively small forelimbs," said lead author Charlie Roger Scherer, a PhD student at UCL Earth Sciences.

The Carnotaurus had ridiculously tiny arms, smaller than the T.rex."

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

The scientists think that the link between an increased head size and shrinking arm size could be down to their diet.

The dino's main food source was plant-eating dinosaurs, like the gigantic sauropods, who around this time were growing in size.

They think that the predators would have relied less on their arms and more on their fearsome jaws to hunt the large dinos, causing their arms to shrink over time, as they weren't used as much, and their jaws to grow.

"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," said Charlie Roger Scherer.

"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."