Why Were the Arms of Tyrannosaurus Rex So Small Compared to Its Massive Body?
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A recent study has uncovered the mystery behind why the arms of Tyrannosaurus rex (T. rex) and two other bipedal dinosaurs remained remarkably small in proportion to their enormous bodies.
“Theropods primarily refer to a group of two-legged carnivorous dinosaurs. Although there were many enormous species among them, their arms were ridiculously small,” says Charlie Roger Scherer, a PhD student at University College London.
T. rex individuals measured about 12 to 13 meters in length, yet their arms were only about 1 meter long.
Numerous theories have been proposed to explain this peculiarity.
One suggestion is that the small arms may have had reproductive functions, or evolved to assist in rising from the ground.
Photo source, Roger Harris/SPL via Getty Images
New research from Cambridge University and University College London (UCL) shows that theropods with longer arms lost them over time because they were unable to grasp prey effectively, while their powerful heads and large jaws evolved simultaneously.
Speaking on the BBC’s Today programme, Dr. Elizabeth Steele from Cambridge said, “Anyone encountering such a large carnivorous dinosaur might be surprised by its small arms.”
The research team studied 82 species of theropod dinosaurs, discovering that five groups, including T. rex, had notably reduced arm lengths.
They also developed a new method to measure biomechanical stress based on arm size and bone structure.
“We compared head and body size ratios as well as arm length to head strength… then analyzed how those conditions influenced morphologies,” Dr Steele explained.
What Evolution Reveals
Researchers found a close relationship between small forearms and powerful heads and jaws, noting that small arms were not merely a consequence of increasing body size.
Dinosaurs like Majungasaurus from Madagascar also had small arms despite having relatively smaller bodies, yet they too possessed powerful heads.
“Such adaptations were less consistent but appeared in habitats with large prey,” Scherer said—the body form likely linked to hunting techniques.
Photo source, Kostyantyn Ivanyshen/Stocktrek Images via Getty Images
Theropods preyed upon gigantic herbivorous sauropods, which had long necks and tails.
Scientists speculate that the development of strong jaws and massive heads arose because these large herbivores grew in size and predators needed to use their mouths to compete and dominate instead of brute strength alone.
“Trying to grab a sauropod over 30 meters long with your arm just wasn’t practical. Attacking with the mouth was much more effective,” Scherer said.
“This shift in predation style to using the head instead of arms led to their gradual reduction in size as they became less needed.”
Dr. Steele added that some dinosaurs still made limited use of their arms.
“You can observe that their claws were shaped differently, longer and more delicate,” she said.
Photo source, Kevork Djansezian/Getty Images
While the study demonstrated a connection between jaw strength and arm length among theropods, there was no clear evidence one directly influenced the other, Scherer noted.
However, he added that it’s quite likely that a powerful bite existed before the arms began shrinking significantly.
“From an evolutionary perspective, it doesn’t seem that arms became small first and then the jaws became stronger. Rather, these predators likely changed their hunting strategy,” he said.
The research found that different theropod groups shortened their forearms in various ways: some reduced arms mainly from the wrist down, while others shrank the entire limb proportionally. This suggests that these evolutionary changes arose independently but with a similar outcome.
Dr. Steele said, “We confirmed what was previously suspected — if the jaws are large, there is no necessity for long arms.”
She added, “This method can also be applied to check whether other animals possess strong jaws.”
“It would be fascinating to investigate this in birds, as they are modern theropod dinosaurs.”