Why T. rex arms were so short, and how strong they were

The short answer

T. rex arms were about three feet long on an animal forty-five feet long, and palaeontologists are still working out why. The measurements are the easy part. A 45-foot-long T. rex might have had a 5-foot-long skull but arms only 3 feet long, which UC Berkeley palaeontologist Kevin Padian scales down to "the equivalent of a 6-foot human with 5-inch arms."

Hold your own arm out and imagine it stopping a few inches past the shoulder. That is the shape of the problem.

They were short, and they were strong

Two palaeontologists estimated that T. rex could have "bench pressed about 400 pounds with its arms," which is more than most gym memberships will ever see.

Padian thinks the strength is beside the point. "The arms are simply too short," he says. "They can't touch each other, they can't reach the mouth, and their mobility is so limited."

That is the part people miss when they laugh at the arms. A powerful limb that cannot reach your own face, or the other limb, or anything in front of your jaws, is a limb with nowhere to put its power.

Every explanation anyone has tried

The list of proposed functions is long. The arms have been proposed as equipment for mating, for holding or stabbing prey, for tipping over a Triceratops, for waving at a potential mate, for signalling to other tyrannosaurs, and for anchoring the animal while it heaved itself up off the ground.

All of them are plausible. None of them has stuck, which is roughly where things sat until 2022.

The 2022 paper comes at it from the other side

Padian's Why tyrannosaurid forelimbs were so short stopped asking what a short arm is good for and started asking what a long arm would have cost.

He describes the scene this way.

"What if several adult tyrannosaurs converged on a carcass? You have a bunch of massive skulls, with incredibly powerful jaws and teeth, ripping and chomping down flesh and bone right next to you. What if your friend there thinks you're getting a little too close? They might warn you away by severing your arm."

Severe bite wounds can cause infection, hemorrhaging, shock and eventual death. An animal whose arms end well short of the dinner keeps less of itself in reach of somebody else's jaws. Run that over enough generations and the arms get shorter.

The paper makes the same case for other giants. Abelisaurids and carcharodontosaurids shrank their forelimbs too, so whatever drove the change was not unique to tyrannosaurs.

How you would actually test it

The hypothesis leaves a mark somebody could go and look for, and Padian names the test himself. "Bite wounds on the skull and other parts of the skeleton are well known in tyrannosaurs and other carnivorous dinosaurs. If fewer bite marks were found on the reduced limbs, it could be a sign that reduction worked."

Somebody would have to check museum specimens around the world and count. Padian is upfront that firmly establishing it may never be possible, and that he has no illusion his idea will be the end of the story.

The best skeleton we have is still being studied

SUE is the largest and most complete Tyrannosaurus rex skeleton ever discovered, at more than 40 feet long. Its right arm went through a CT scan so researchers could build digital models of the range of motion and force.

Palaeontologist Jingmai O'Connor put it plainly: "Jokes aside, the proportions of these forelimbs are very strange."

Decades of study on the best specimen anyone has, and the arms are still unexplained.

Which brings us to our own oversized predator

Coop is a bernedoodle. In his other form he stands thirty-one feet tall, weighs about thirty-one thousand pounds, carries a spiky mane, walks upright on his hind legs, runs on all four, and goes by Cooposaurus Rex.

He kept the front legs. So he stands up like a tyrant lizard with two full-length dog arms attached, using them for exactly what he used them for at ninety pounds.

Field note: the tyrant king's arms shrank until they could not reach his own mouth. Our thirty-one-foot bernedoodle kept both paws at full length and spends them on leaning his entire weight against somebody who was in the middle of something.

If you want to keep going

That hoodie and the Cooposaurus Rex New King tee both crown him King of Trash Pandas, which is a separate matter and also correct. The rest of his shirts are in the Coop Dog Collection.

For the dinosaur in context, the chapter called The Dinosaur Visit is the obvious place to start. The Atomic Critters Universe page introduces everyone else.