Why T. rex has little arms

Study of predators puts T. rex ahead - 1st June 2026

The debate surrounding Tyrannosaurus rex's contentious forelimbs may be settled following a recent study undertaken by palaeontologists at the University of Cambridge and University College London.

Reaching up to 13 metres in length and nearly 4 metres in height in its horizontal posture, T. rex roamed the Earth between 68 and 66 million years ago. Against this reptile's vast proportions, one incongruous anatomical feature's its forelimbs, which measure under a metre long.

Their likely function's prompted debate, giving rise to numerous theories across the decades. Hypotheses have ranged from pinning down prey to courting potential mates, or that their reduced size avoided another T. rex biting them off during feeding frenzies.

The study's authors, Charlie Roger Scherer, Paul Upchurch and Elizabeth Steell, analysed forelimb length in 85 species of carnivorous dinosaurs across 5 disparate theropod clades. They concluded that a common driver for reduced forelimbs was the size of the carnivore's prey, and that blows from its head overpowered larger prey more successfully than using its arms and claws.

Therefore, cranial robusticity took precedence over having long or strong forelimbs. It's a case of "use it or lose it," claims Scherer, the study's lead author.

"If you're a dinosaur with a very strongly put-together skull, chances are you're going to have very small forelimbs." Findings suggest that "evolution doesn't like to have everything all at once."

The researchers devised a novel approach for gauging skull strength, considering diverse factors including overall size, degree of bone fusion, and estimated bite force. This enabled them to rank every species on a unified scale of cranial robusticity, with T. rex taking pole position and Tyrannotitan, from another lineage and living 30 million years before T. rex, placed second.

The study evidenced strong correlation between cranial robusticity scores and reduced forelimbs beyond the tyrannosaurids lineage, which includes T. rex. It was identified across five distinct clades of carnivorous theropods in multiple continents over a 180-million-year period.

The function vestigial forelimbs performed remains unclear. Scherer contends that "They obviously served some sort of function, otherwise they wouldn't have them. ... Hopefully, we can find that out with a bit more work."