Why Did the T. Rex Have Tiny Arms?

(Last Updated On: August 30, 2022)

If you’ve ever seen an artist’s rendition of a dinosaur, or the skeleton of a T. rex, you probably know the tyrannosaurus rex had really tiny arms. Almost comically tiny. But when it comes to one of the most iconic dinosaurs around, especially as the one marketed with the really big jaw, has anyone ever thought about why its arms were so… Pointless? Seriously, why did the T. rex have tiny arms?

Further Reading: Did Dinosaurs Have Feathers?

How Big Were T. Rex Arms Really?

Well the answer is not very big. The arms of the tyrannosaurus rex were only like one foot long–in relation to the 40 foot length of the rest of its body. These arms wouldn’t have been long enough to reach the mouth of our small-armed dinosaur, and the elbow couldn’t bend more than 90 degrees and the wrists weren’t durable enough to assist in getting up should a T. rex be toppled. Perhaps paradoxically, evidence also suggests that the T. rex’s arms supported considerable muscle mass, stress fractures in some fossils also (might) suggest that the T. rex may have used its arms for… something at least. 

Some have argued that these arms were used to either hold prey down or assist in copulation, but almost any time a function for the T. rex arm is proposed someone else disputes it. Given the 35,000 Newton bite force and the likely ability to splinter bones with its teeth, it’s not an uncommon argument that the T. rex’s mouth substituted many functions we’d attribute to arms. 

How Special Were T. Rex Arms Really?

The T. rex isn’t even that unique. If you’ve picked up on how species are named, you know that the first word in a species name is more general than the second one. That’s just taxonomy. So tyrannosaurus (the tyrant lizard) is just the genus for our famous T. rex. This is why, as we learn more about dinosaurs, we’re finding more tyrannosaurus species. Like the tyrannosaurus imperator, which still has small arms (but with two incisors). Also, “imperator” is just an objectively cool name for a dinosaur, we will not be taking questions.

Tyrannosaurs were likely the apex predators of their time period, running around at least as of 80 million years ago though the Cretaceous Period and remained dominant until the dinosaurs were killed by the giant meteor. That doesn’t stop us from naming them cool things like tyrannosaurus imperator or, if you were discovered in 2020, the Reaper of Death

Asking the Wrong Questions

Asking the question “what were T. rex arms for?” might be the wrong one to begin with. We know that the small arms of the T. rex probably didn’t hinder their survival–that’s evolution. Contemporary research asks the following: “what overriding benefit would small arms have for a T. rex?” That is to say, if the T. rex’s small arms didn’t provide direct utility, did they prevent some other massive con that made their (potentially useless) arms worth it? What larger purpose could stubby arms serve?

The most simple answer is that the T. rex simply didn’t use its arms. If you never needed them in the first place, there’s no real reason to keep putting energy into developing them. Simple disuse is not, in itself, an explanation though–since we then have to figure out why the arms weren’t getting used in the first place. 

One of the more recent theories, though difficult to substantiate, suggests that the T. rex’s arms were small as a protective thing. If the T. rex already wasn’t using its arms, and multiple T. rexes were to feed on a single carcass, then those arms may have been liable to get bitten. If these arms were as useful short as they were long (that is, not at all), then it would have benefit the T. rex to have smaller arms that were less likely to be injured. 

See if you know more dinosaur name origins here.

About the Author:

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Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.