Why did Tyrannosaurus rex have short arms? No one really knows. They could have no reason and be left over from longer arms that evolved away. They could have been used for slashing, gripping prey, and longer arms might have been bitten off by other Tyrannosauruses. They may have been used to help the dinosaurs stand up, as well.
The Tyrannosaurus rex is probably the most famous of all dinosaurs and the one nearly all children will know. They lived in the Cretaceous period, from roughly 83.6 million years ago and they were wiped out by the asteroid 66 million years ago. Tyrannosaurus rex was the largest land carnivore that has ever lived, and they grew to be about 12.5 m long and 3.96 m tall. We can only know the heights of the fossils that have been found and there may very well have been much larger Tyrannosauruses. They are also often pictured as standing vertically, but they would have had to keep their bodies parallel with the ground, using their tails as a counterweight for balance. Standing upright would have damaged their hips and other joints. In fact, biomechanical models have shown that the Tyrannosaurus rex was not able to run as fast as was previously thought. Looking at its weight, length, and the strength of its bones, experts have calculated that it probably couldn’t exceed 20 km/h. That is still faster than most people on Earth, but it is not as fast as was thought. The reason is, if the Tyrannosaurus rex actually ran, it would have to have both legs off the ground at the same time and landing each time would shatter the bones in its legs. It could go no faster than a very fast walk.
So, why did Tyrannosaurus rex have short arms? One theory was that the short arms were left over from longer arms or legs that had evolved away over time, similar to our wisdom teeth. A vestigial feature, something that used to have a purpose, but not longer does. The Tyrannosaurus rex had massive neck muscles, which they used to power their jaws, and to rip and shake prey, tearing pieces off. Larger arms would have required larger muscles, which might have got in the way of the neck muscles. It is thought that the Tyrannosaurus rex didn’t need longer arms because it did everything with its neck muscles.
However, it seems that they did use their short arms for pose and if longer arms were an evolutionary advantage and shorter arms were a hindrance, it is most likely that the tyrannosaurus rex would have evolved longer arms over time. The fact that they didn’t shows that they either don’t need their arms much, or shorter arms are better. So, what did they use their arms for?
The first thing they used their short arms for was slashing. Each arm was about 1 m long and had two very long claws on the end of each one. Each claw was about 10 cm long. The bones of the arms were also incredibly thick and sturdy. Tyrannosaurus rex could have used these arms and claws to slash prey at close range while it held them with its jaws. Two 10 cm claws on a 1 m long arm could inflict long and deep cuts on a prey animal. The Tyrannosaurus would have been able to move them up and down very quickly as well.
The second reason could have been to grip prey. The Tyrannosaurus could have used its forearms and claws to hold onto the prey so it couldn’t move while it killed it. Once the prey was dead, the Tyrannosaurus would have used its teeth and strong neck muscles to rip flesh off it. However, there is some argument here because it is likely that the Tyrannosaurus rex hunted in the same way as Komodo dragons today, which is to use their powerful neck muscles to tear pieces off the prey until they died. In which case, they wouldn’t need their arms.
A third reason is possibly to help them stand up. Tyrannosauruses must have lain down to sleep or sat down at some point. They wouldn’t be able to sleep standing up, as four legged animals can. That means, they would have needed to get back up and fossils of footprints found in New Mexico appear to show that the Tyrannosaurus rex stood up by using the elbows of its tiny arms to balance and push off the ground.
A final reason is to avoid having them bitten off. Archaeological evidence suggests that Tyrannosauruses hunted in packs. If a number of Tyrannosauruses were using their powerful teeth and jaws to rip prey to shreds, it is safe to assume that they might have accidentally bitten each other’s arms off. Having short arms could have been protection against this.
It does seem that the tiny arms did have some uses but the arms weren’t tiny for these reasons, rather they were tiny and the Tyrannosaurus rex used them. They were really too small for slashing and the Tyrannosaurus would have to get too close to anything. It seems more likely that the Tyrannosaurus rex gradually evolved to get bigger and its arms didn’t. It sacrificed its arms for more muscle strength in the neck. And this is what I learned today.
Photo by icon0 com: https://www.pexels.com/photo/dinosaur-statue-730433/
Sources
https://www.livescience.com/why-tyrannosaurus-rex-theropod-dinosaurs-small-arms
https://www.history.com/news/there-was-a-dangerous-purpose-behind-t-rexs-tiny-arms
https://www.universityofcalifornia.edu/news/why-t-rex-has-tiny-arms
https://en.wikipedia.org/wiki/Tyrannosaurus
https://www.nationalgeographic.com/science/article/tyrannosaur-trex-running-speed
https://www.newscientist.com/article/dn25418-t-rex-didnt-need-proper-arms-thanks-to-its-neck/
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