Tyrannosaurus rex and many of its theropod cousins were large but had small arms. Why is that?
Theropod dinosaurs
Theropoda (Latin name: Theropoda) belongs to the order Sauropoda, which appeared in the Middle Triassic. Most of them were carnivorous dinosaurs, walking on two legs, with sharp claws at the ends of their toes, sharp teeth like daggers or knives in their mouths, and often serrated on the front and back edges of their teeth. Tyrannosaurus rex is its famous representative. Theropods (all non-avian dinosaurs) all disappeared in the mass extinction 65 million years ago.
Tyrannosaurus rex had a huge head and hind legs, but weak arms. There are many reasons why this can happen.
Tyrannosaurus rex was a ferocious hunter and the most biting apex predator of all animals on land. More than 66 million years ago, this beast wandered the wilderness of the late Cretaceous in search of triceratops or Edmonton dragons to feast.
Edmonton Dragon
The only thing where such an Earth Overlord is not threatening is its tiny arms. Compared to the rest of the body, Tyrannosaurus rex was not the only dinosaur with a small arm; Many of its theropod cousins share this trait. But why did many theropods evolve such stubby arms?
Tyrannosaurus rex
Scientists have come up with several possible explanations.
A 2021 study published in the Polonian Journal of Paleontology showed that theropods such as Tyrannosaurus rex caught their prey in groups. So the study authors suggest that perhaps they evolved tiny arms to avoid accidentally biting off each other's arms when a group of theropod dinosaurs attacked Triceratops.
Tyrannosaurus rex is preying on triceratops
However, this is only a hypothesis at the moment.
Some scientists believe that in the evolution of theropod dinosaurs, "the arms did not become shorter, but the legs became longer."
As tyrannosaurs and their theropod cousins evolved larger heads and bipedal walking postures, they used their forelimbs less often. They began to use their heads more to catch and kill their prey. Therefore, according to this view, the forelimbs did not grow as well as the rest of the body, so the arms of the Tyrannosaurus rex appeared particularly small.
The tiny arms of the Tyrannosaurus rex
The arms of the Tyrannosaurus rex were too short to help it hunt and kill. As a result, these huge dinosaurs would knock out their prey with a "prick-and-pluck" method, and the Tyrannosaurus rex would bite off large chunks of its prey's flesh while tearing it back with its strong neck. This is also how the modern Komodo dragon (Varanus komodoensis) hunts. Their large hind legs help stabilize the body. But there is no evidence yet that their tiny arms play a role in any way.
Tyrannosaurus rex has a powerful biting ability
It's easy to think that every trait of an animal played a role in the evolution that helped it survive. But in fact, sometimes the emergence of traits does not necessarily lead to a clear evolutionary advantage. Other body parts of Tyrannosaurus rex would grow to huge sizes to help them survive in the age of dinosaurs. Tyrannosaurus rex arms may not need to grow with the rest of the body, which makes them ridiculously small in comparison.
Of course, these are still possible explanations that scientists think. It is also possible that T. rex and other theropod dinosaurs did something with their arms, and finding out what they were used for may require more well-preserved fossils and further research.