Why were T. rex’s arms so tiny?
Tyrannosaurus rex is iconic for its ferocity and large teeth, as well as its tiny arms. However, the Cretaceous apex predator was not the only carnivore with underdeveloped forelimbs. At least five groups of two-legged, primarily carnivorous theropod dinosaurs experienced shortening of their upper arms during their evolutionary journey. But why did they have such small claws? A team of researchers thinks the answer is simple.
“It’s a case of ‘use it or lose it,'” Charlie Scherer, a paleontologist at University College London, said in a statement.
Scherer and colleagues recently examined data from 82 theropod species, including those of T. rexThe tyrannosaurid family. Their study published today in the Proceedings of the Royal Society B Biological Sciences argues that a combination of massive skulls and crushing jaws, coupled with increasingly larger prey, caused many theropods to rely less and less on their forearms.
“We sought to understand the causes of this change and discovered a close relationship between short arms and large, powerful heads,” Scherer explained. “The head has taken precedence over the arms as a method of attack.”
The team based their findings on a new system for assessing dinosaur skull strength, based on attributes such as overall dimensions, how well the bones fit together in the head, and bite force. Unsurprisingly, T. rex came in first place for bite force, followed by Tyrannotitan. Almost as big as a T. rexTHE Tyrannotitan lived in present-day Argentina at the beginning of the Cretaceous, more than 30 million years before his famous descendant. In each example, the reason for short guns likely coincided with hunting larger and larger targets for dinner.
“Trying to pull and grab a 100-foot-long sauropod with your claws is not ideal. Attacking and holding with the jaws could have been more effective,” Scherer added.
Overall, the team identified a greater correlation between skull strength and smaller arms than with skull or body size. This conclusion is also supported by some theropod dinosaurs with strong heads, tiny forelimbs, and relatively small stature. For example, Majungasaurus roamed modern-day Madagascar 70 million years ago and weighed about 1.75 tonnes, about a fifth the size of T. rex.
The limbs of all dinosaurs also did not shrink in the same way. Abelisaurids like Majungasaurus featured smaller arms beyond their elbows as well as their hands, while tyrannosaurs’ arms were reduced proportionally. In each case, it appears that theropods were first much more successful at holding on to their prey with their powerful jaws, and then evolution did the rest of the work.
As for which dinosaur had the smallest forearms, the answer according to Scherer is clear.
“THE Carnotaurus had ridiculously small arms, smaller than the T. rex“, he said.
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