tyrannosaurus rex was a vicious hunter with the strongest bite of any animal that ever walked the earth. The beast was prowling the end Cretaceous desert more than 66 million years ago, in search of a Triceratops or Edemonosaur to snack.
The only thing that wasn’t threatening about the tyrant lizard king was his small arms. T. rex wasn’t the only one dinosaur with small arms compared to the rest of his body; many of its theropod cousins – a group of bipedal, mostly carnivorous dinosaurs – shared this trait. But why did many theropods develop such stubby arms?
Scientists have offered a few possible explanations.
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A 2021 study published in the journal Polish Paleontological Journal (opens in a new tab) have suggested that bone-crushing theropods such as T. rex developed small arms so that they would not bite their arms when feeding. Paleontological Evidence Suggests These Animals Devoured Their Prey in bundle (opens in a new tab)so maybe they evolved the little limbs to avoid accidentally tearing their arms off as a mob of theropods descended on a tackle Triceratopssuggested the author of the study.
For now, however, this is just a guess. “It’s a great story,” said John Hutchinson, a biologist at the Royal Veterinary College, University of London, who was not involved in the study. “But I think at the end of the day, we don’t really know.”
Hutchinson, who studies the biomechanics of movement in large land animals – living and extinct – looks at the forelimbs of dinosaurs evolution in a different way: In the evolution of theropods, “the arms didn’t really get shorter, but the legs got longer,” he said.
“As the animals grow, the forelimbs get smaller and the head gets bigger,” he continued. Tyrannosaurs, in particular, “adapt this killer bone-crushing bite in their heads, so they really specialized their heads, and then they really, really shrunk their forelimbs.”
As tyrannosaurs and their theropod cousins developed larger heads and a bipedal posture, they used their forelimbs less. They began to use their heads more to catch and kill prey. As a result, the forelimbs did not grow as much as the rest of their body, according to this idea.
“An animal can only devote a portion of its body volume to one thing or another,” Hutchinson said. “He can’t be a jack-of-all-trades. So either you have a very generalized body where everything is equally specialized for a general ecological niche, or you really specialize like T. rexwhich is super specialized to be a frontal predator.”
T. rex the arms were too short to help him hunt and kill. These huge dinosaurs used a “puncture-extraction” method to bring down their prey, in which T. rex would bite off “big chunks of them, tearing back with their strong necks as they did,” Hutchinson said. This is how modern Komodo dragons (Varanus komodoensis) also hunt, he added. And their broad hind legs would have helped stabilize them. There is no evidence – currently – that their small arms helped in any way.
It’s tempting to assume that every trait an animal possesses has some sort of evolutionary role in helping the creature survive. But sometimes traits appear (or disappear) that don’t necessarily confer a clear evolutionary advantage. In this case, this trait – the length of the forelimbs – did not change, unlike the other traits. Other T. rex body parts have grown to colossal sizes to help them survive in their ecological niche. There might not have been a need for the arms to grow with the rest of T. rex bodies, making them look comically small in comparison.
But that might not be the whole story either, Hutchinson said; T. rex and other theropods may have used their arms for something, and it will take a lot more research and well-preserved fossils to find out what.
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