Tiny Arms, Mighty Jaws: New Study Links Theropod Forelimb Reduction to Skull Power

A paper in *Proceedings of the Royal Society B* reports a strong correlation between the extreme shortening of forelimbs and the evolution of massive, robust skulls in five lineages of large predatory dinosaurs. The authors argue that as prey grew larger, the jaws became the primary killing tool, making the arms redundant and subject to gradual loss.

The Independent

Theropods such as tyrannosaurids, abelisaurids, carcharodontosaurids, ceratosaurids and megalosaurids each independently evolved disproportionately small arms while simultaneously increasing skull size and bite force. This pattern coincides with the rise of gigantic herbivores—especially long-necked sauropods—whose size made grasping with claws impractical.

New ScientistThe Independent

The study was led by Charlie Roger Scherer, a PhD candidate at University College London, with contributions from colleagues at multiple institutions. Independent commentary was provided by Andre Rowe (University of Bristol) and Fion Waisum Ma (University of Hong Kong). The analysis covered 85 theropod species, highlighting tyrannosaurids (including *Tyrannosaurus rex*), the Argentine *Tyrannotitan*, and representatives of the other four groups.

New Scientist

Researchers measured forelimb length, skull dimensions, body mass and biomechanical proxies such as bite force and cranial rigidity. A skull-to-forelimb ratio was calculated for each species. Results show that skull robustness predicts forelimb reduction more strongly than overall body size or skull length alone. *T. rex* achieved the highest robustness score, followed closely by *Tyrannotitan*. The statistical relationship held across all five lineages, indicating convergent evolution rather than a single phylogenetic trend.

The Independent

The findings suggest an energetic trade-off: maintaining both a powerful bite and strong forelimbs would be costly, so natural selection favored the development of massive jaws at the expense of the arms. This “use it or lose it” scenario clarifies why similar body plans emerged in disparate continents and epochs, and it underscores the role of prey size in shaping predator morphology.

The Independent

Scherer explained that the data reveal “a strong relationship between short arms and large, powerfully built heads,” and that “forelimb reduction was a product of their redundancy in prey capture.” Rowe noted the study “tackles one of the big evolutionary questions in theropod dinosaurs” and expressed curiosity about the residual mechanical function of the vestigial limbs. Ma highlighted that “hidden evolutionary signals can be revealed when traits are quantified in this way,” emphasizing the methodological advance.

New Scientist

Rowe cautioned that “tiny, vestigial arms… do not necessarily mean they were completely useless,” suggesting that reduced forelimbs may have retained secondary roles, perhaps in feather display or balance. No direct opposition to the core hypothesis was presented, but the comment signals an unresolved aspect of limb function.

The Independent

• “The head took over from the arms as the method of attack. It’s a case of ‘use it or lose it’. The arms are no longer useful and reduce in size over time,” — Charlie Roger Scherer, PhD candidate, University College London

The Independent

• “We postulate that forelimb reduction was a product of their redundancy in prey capture in these large theropods,” — Charlie Roger Scherer, University College London

The Independent

• “Nature doesn’t like to have everything all at once,” — Charlie Roger Scherer, University College London

New Scientist

• “Yes, tyrannosaurs had tiny, vestigial arms, but that does not necessarily mean they were completely useless,” — Andre Rowe, University of Bristol

New Scientist

• “This shows how hidden evolutionary signals can be revealed when traits are quantified in this way, she says.” — Fion Waisum Ma, University of Hong Kong

New Scientist

The authors propose further work on the mechanical role of reduced forelimbs, including their possible contribution to feather evolution and early flight. Clarifying whether the vestigial arms served any functional purpose remains a key unanswered question.

The Independent