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Why did Tyrannosaurus have such short arms?

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Illustration showing various dinosaur species drawn to scale and grouped by geologic era: Late Cretaceous, Early Cretaceous, Jurassic, and Triassic. Dinosaurs range from large species like Brachiosaurus and Tyrannosaurus to smaller ones like Velociraptor and Compsognathus, with a human silhouette included for size comparison. Measurement bars at the bottom indicate 1 meter and 3 feet for scale reference. — dinosaur A selection of dinosaurs grouped by the geologic interval in which they lived.

The forelimbs of Tyrannosaurus were notably small, measuring less than the length of the shoulder blade. While their musculature may have allowed them to lift significant weight, they were too short to reach the mouth or grasp prey.1

Paleontologists have proposed several theories regarding the evolutionary advantage of these reduced limbs:

Injury Prevention: Some researchers argue that small forelimbs evolved to keep the appendages out of the way of the dinosaur's powerful, snapping jaws during group feeding. This would have reduced the risk of accidental injury, such as amputation or blood loss, which could have been fatal.1
Alternative Functions: Other theories suggest the limbs may have been used to strike or slash at prey during close encounters, or that they played a role in social displays or mating.1

While early, more primitive tyrannosaurs retained three fingers, later forms like T. rex saw these appendages reduced to only two digits.12