T. rex’s tiny arms may have evolved for a surprisingly brutal reason

The research, published in Proceedings of the Royal Society B, examined 82 species of theropods, a group of mostly carnivorous, two legged dinosaurs. The scientists found that reduced forelimbs evolved independently in at least five dinosaur lineages, including tyrannosaurids, the group that included T. rex.

Rather than simply being a side effect of growing larger bodies, the study suggests that shrinking arms were closely connected to the evolution of massive, powerful skulls and jaws.

Giant Skulls Took Over the Hunt

The researchers discovered that dinosaurs with shorter arms tended to have especially robust skulls. That connection was stronger than the link between tiny arms and overall body size.

According to the team, this may reflect a major evolutionary shift in hunting strategy. As giant plant eating dinosaurs such as sauropods became more common, predators may have relied less on grasping prey with claws and more on delivering devastating bites.

Lead author Charlie Roger Scherer, a PhD 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."

Scherer added that the evidence points to skulls becoming stronger before the arms began shrinking.

"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."

Measuring Dinosaur Skull Strength

To investigate the relationship between arm size and skull power, the researchers developed a new method for measuring skull robustness. Their approach considered several factors, including bite force, skull shape, and how tightly the bones of the skull were connected. Compact skulls were considered stronger than longer, narrower ones.

Using this system, T. rex ranked as the most robust skull in the study. Close behind was Tyrannotitan, another enormous theropod that lived in what is now Argentina more than 30 million years before T. rex during the Early Cretaceous period.

The team believes giant prey animals may have triggered an "evolutionary arms race" in which predators evolved stronger jaws and skulls to overpower increasingly massive herbivores. In many cases, these hunters also grew to enormous sizes themselves.

Multiple Dinosaur Groups Evolved Tiny Arms

The researchers compared forelimb length with skull length and identified five dinosaur groups with notably reduced forelimbs. These included tyrannosaurids, abelisaurids, carcharodontosaurids (which included Tyrannotitan), megalosaurids, and ceratosaurids.

Their analysis showed that tiny arms were more strongly associated with skull robustness than with either skull size or total body size.

The study also highlighted that not all of these predators were gigantic. Majungasaurus, for example, had a heavily built skull and extremely small arms despite weighing only about 1.6 tons, roughly one fifth the weight of T. rex. The dinosaur lived in Madagascar around 70 million years ago and was still considered an apex predator.

Different Paths to the Same Result

The scientists also found that dinosaur groups reduced their forelimbs in different ways over time.

Among abelisaurids, the hands and lower sections of the arms beyond the elbow became dramatically smaller, with later species such as Majungasaurus developing exceptionally tiny hands. Tyrannosaurids, however, showed a more balanced reduction across the entire forelimb.

The researchers concluded that separate dinosaur lineages likely reached the same outcome through different evolutionary and developmental pathways.

The study was carried out by a broader research group focused on dinosaur evolution at UCL, working closely with the Natural History Museum. The group includes research fellows, postdoctoral scientists, and more than 10 PhD students studying dinosaurs and other vertebrates such as crocodiles and birds.