Tyrannosaurus rex was a vicious hunter with the strongest bite of any animal that ever walked ashore. The beast roamed late chalk Wilderness more than 66 million years ago, in search of a triceratops or eDmontosaurus to snack.
The only thing that wasn’t threatening about the king of the tyrant lizards were his tiny little 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 two-legged, mostly carnivorous dinosaurs — shared this trait. But why did many theropods evolve such stub arms?
Scientists have suggested a few possible explanations.
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A 2021 study published in the journal Acta Paleontologica Polonica (opens in new tab) suggested that bone-crushing theropods such as t rex developed small arms so they wouldn’t bite each other’s arms off while eating. Paleontological evidence suggests these animals devoured their prey as a pack (opens in new tab)perhaps they evolved the small limbs to avoid accidentally snapping the arms when a crowd of theropods swooped down on a tackle triceratopssuggested the study author.
For now, however, this is just a hypothesis. “It’s a beautiful story,” said John Hutchinson, a biologist at the University of London’s Royal Veterinary College who was not involved with the study. “But I guess ultimately we don’t really know.”
Hutchinson, who studies the biomechanics of movement in large land animals—both living and extinct—considers the front legs of dinosaurs evolution Anders: In the evolution of theropods, “the arms didn’t really get shorter, but the legs got longer,” he says.
“As the animals get bigger, the front legs get smaller and the head gets bigger,” he continued. Tyrannosaurs in particular “fit this bone-crushing killer bite into their heads so they really specialize their heads and then really, really shrink their front legs.”
As tyrannosaurs and their theropod cousins evolved larger heads and a bipedal posture, they used their front legs less. They began using their heads more for catching and killing prey. As a result, according to this conception, the front legs did not grow as much as the rest of their body.
“An animal can only use a certain part of its body volume for 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 in a general ecological niche, or you really specialize that way t rexwho’s super specialized at being a front-end predator.”
T. rex The arms were too short to help him hunt and kill. These giant dinosaurs used a “puncture-pull” method of taking down prey in which t rex would “bite large chunks out of them, tearing backwards with their strong necks,” Hutchinson said. How Modern Komodo Dragons Work (Varanus komodoensis) also hunt, he added. And their large hind legs would have helped stabilize them. There is currently no evidence that their handguns helped in any way.
It is tempting to assume that every trait possessed by an animal plays an evolutionary role in helping the creature survive. But sometimes traits just appear (or disappear) that don’t necessarily confer a clear evolutionary advantage. In this case, that trait—the length of the front legs—didn’t change, while other traits did. Miscellaneous t rex Body parts grew to colossal sizes to help them survive in their ecological niche. It might not have been necessary for the arms to grow with the rest of T. rex body, which makes them look comically small in comparison.
But that might not be the whole story, Hutchinson said; t rex and other theropods might have used their arms for something, and it will take a lot more research and well-preserved fossils to figure out what for.
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