Why Did the Tyrannosaurus Have Such Short Arms?

Photo: CFOTO/Future Publishing via Getty Images

Tyrannosaurus rex belonged to the theropods — a variety of predominantly carnivorous bipedal dinosaurs.

Despite their enormous size, some theropods had "ridiculously small arms," says lead author of the study Charlie Roger Scherer, a PhD student at University College London (UCL) in the UK.

There are a number of theories explaining the purpose of such small arms, including the assumption that they played some role in mating or helped the tyrannosaur get up from the ground.

But the authors of the new study, scientists from the University of Cambridge and University College London, suggest that these limbs might have originally been long but then shrunk as a result of evolutionary development.

The reason might be that they ceased to be useful for hunting, unlike the large and powerful head and jaws.

"Everyone has always wondered why large carnivorous dinosaurs like the tyrannosaur had tiny forelimbs," said co-author of the study Dr. Elizabeth Steele from Cambridge University on BBC Radio's Today program.

The research group studied 82 species of theropods and found that a reduction in forelimb size was observed in five groups, including tyrannosaurids, the family to which the tyrannosaur belongs.

Scientists developed a method for measuring skull strength, based on factors such as its size and how the bones fused.

"In addition, we studied the relationship between skull size and body size, as well as the length of the forelimbs in comparison to skull size and body size... and looked for patterns," Steele explained.

Evolutionary Arms Race

It turned out that the reduction in forelimbs more often correlated with the development of large skulls and strong jaws, but not with an increase in overall body size. This suggests that the small arms were not simply a "by-product" of increasing overall body size.

Some theropods, such as Majungasaurus, which lived in Madagascar, also had short forelimbs, and these animals themselves were not very large. However, they had powerful heads.

"They often adapted similarly in areas where giant prey was present," Scherer explained. He suggests this is related to hunting methods.

Part of the theropod diet consisted of herbivorous dinosaurs, such as long-necked and long-tailed gigantic sauropods.

Researchers suggest that the predators' skulls grew larger as the size of these herbivores increased. Theropods had to switch to using jaws for hunting instead of claws to win the "evolutionary arms race."

"Trying to pull and grab a 30-meter sauropod with claws is not the best option. Attacking and holding with jaws could be more effective," Scherer explained. "The head replaced the forelimbs in attack. This is a 'use it or lose it' situation — the forelimbs cease to be useful and decrease in size over time."

According to Steele, some dinosaurs still used their forelimbs, but such animals evolved differently. "Their skulls changed; they became more elongated and slightly more delicate," she said.

Scherer acknowledges that this study only establishes a correlation between skull strength and forelimb length but does not directly prove that one causes the other.

However, he says, it is "very likely" that stronger skulls appeared before shorter forelimbs.

"From an evolutionary point of view, the reverse sequence is illogical: for predators to abandon their attack mechanism without a backup," he explained.

Researchers found that in different groups of theropods, the forelimbs seemed to shorten differently: some had particularly shortened wrists and forearms, while in others, the shortening was more uniform throughout the limb. This means that different groups could have achieved the same result through parallel evolutionary paths.

"We confirmed a fact that many already suspected: with a large skull, arms are not very necessary," said Steele.

She suggests that similar methods for measuring skull strength could also be applied to the study of other animals.

"It would be really interesting to apply this to birds, as they are also theropod dinosaurs, but still existing today," she reminded.

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