Why T. rex has little arms

Study finds links in dinosaur anatomy - 1st June 2026

The riddle of why Tyrannosaurus rex's forelimbs appear to be so out of proportion may have been solved by researchers from the University of Cambridge and University College London. Their latest research shines light on an often-arbitrary debate.

A bipedal carnivore with a body up to 13 metres long, T. rex measured almost four metres tall up to the hip. Despite having forelimbs less than a metre long, this fearsome predator stalked the Earth from 68 to 66 million years ago.

Multiple theories have attempted to account for this anomaly, including that T. rex used them to grip its prey. One suggests they play a role in courtship rituals, while another imagines their length stopped them being bitten off by another T. rex.

This research broadened out the question to 85 different species of carnivorous dinosaur. Its authors, Charlie Roger Scherer, Paul Upchurch and Elizabeth Steell, catalogued forelimb length in theropods from 5 diverse lineages and identified a common thread.

The dinosaurs catching the largest prey consistently had shortened forelimbs, since attacking them with their heads was more effective. Therefore, the strength of the predator's skull mattered more than its upper limbs. For Scherer, evolution takes a simple "use it or lose it" approach.

The team compared species' skulls, developing a new method for quantifying 'cranial robusticity'. They graded skull size, bite force and how the bones fitted together. This placed each species on the same scale, with T. rex scoring the highest. Tyrannotitan, a dinosaur from 30 million years earlier, took second place.

The study revealed high cranial robusticity consistently aligned with having shorter forelimbs. This pattern crossed theropod lineages over 180 million years. The result brings uncovering these forelimbs' purpose a step closer. Scherer believes "They obviously served some sort of function, otherwise they wouldn't have them. ... Hopefully, we can find that out with a bit more work."