The evolution of tiny arms in several groups of meat-eating dinosaurs was likely driven by the development of strong, powerful heads, which were used to attack prey, according to a new study led by researchers at UCL (University College London) and Cambridge University.

The study, published in the journal Proceedings of the Royal Society B, looked at data for 82 species of theropod (two-legged, mainly meat-eating dinosaurs), finding that shortening of forelimbs occurred across five groups, including tyrannosaurids, the family that included Tyrannosaurus rex.

The team, including Dr. Elizabeth Steell at Cambridge and Professor Paul Upchurch at UCL, found that smaller arms were closely linked to the development of large, powerful skulls and jaws, more so than to larger overall body size, indicating that tiny arms were not just a by-product of bodies getting bigger.

The researchers suggested that the increasing size of prey, in the form of gigantic sauropods (long-necked, long-tailed plant-eaters) and other large herbivores, may have resulted in a shift to hunting using jaws and head instead of claws.

How tiny arms and big heads evolved
Lead author Charlie Roger Scherer, a Ph.D. student at UCL Earth Sciences, said, "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.

"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.

"These adaptations often occurred in areas with gigantic prey. 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."