We Finally Have the Answer for T. Rex’s Tiny Arms
Tyrannosaurus rex may have been a fearsome predator, but we can’t ignore how ridiculous its little arms looked. Research published today in Proceedings of the Royal Society B Biological Sciences sheds light on why they and other fearsome meat eaters evolved tiny forelimbs.
T. rex and other tyrannosaurids are theropods, a large group of primarily carnivorous dinosaurs with hollow bones and three toes. As theropod species diversified during the Jurassic and Cretaceous periods, notable trends emerged. Many of them grew larger, developed relatively massive heads, and sported shrunken forelimbs.
To examine the phenomenon more closely, paleontologists looked at data from 82 different theropod species and found evidence of arm shortening in five distinct lineages. They also quantified skull robustness – or the strength and thickness of the skull – and found that this was more closely related to shorter arms than to skull size or body size. Essentially, it appears that theropods’ small arms are due to a beefier skull and not just larger growth.
“We sought to understand what was driving this change and discovered a close relationship between short arms and large, powerful heads,” study author Charlie Roger Scherer of University College London said in a statement. “The head has taken over the arms as a method of attack. It’s a matter of ‘use it or lose it’: the arms are no longer useful and diminish in size over time.”
Read more: “T. Rex was a slacker”
Other studies have shown that forelimb shortening is just a byproduct of allometric growth: different parts of the body grow at different rates. In this scenario, selective pressure favoring something like head size or leg size during development would result in relatively small forelimbs. Scherer and his coauthors argue that this is not the case, because the forelimbs did not show the same pattern of shortening. Some theropods had shorter arms because all of their arm bones were reduced, while others were smaller because some bones were reduced.
So what caused T. rex and its ilk to develop such powerful heads?
Researchers believe it was an evolutionary arms race (pun inevitable) necessary to take out larger prey. Theropods like T. rex tended to live among huge herbivores that grew even larger during this period. Perhaps their best chance of getting a meal was to put all their eggs in one powerful, tooth-filled basket.
“These adaptations often occurred in areas with gigantic prey. Trying to pull and grab a 100-foot-long sauropod with your claws is not ideal,” Scherer said. “Attacking and holding with the jaws could have been more effective.”
Hey, when you have a bite like a T. rex, no one makes fun of your little arms.
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Main image: Nobu Tamura / Wikimedia Commons
