Paleontologists from University College London and the University of Cambridge have unveiled that the small arms of large predatory dinosaurs evolved in tandem with their massive heads and powerful bone-crushing jaws. This evolutionary shift indicates that ancient predators increasingly relied on biting rather than grasping their prey.
Reconstruction of Tyrannosaurus macraiensis. Image credit: Sergey Krasovskiy.
“While everyone recognizes Tyrannosaurus rex, other giant theropod dinosaurs also featured small arms, indicating a trend towards reduced forelimb size,” stated Charlie Roger Scherer, a PhD student at University College London.
“For instance, Carnotaurus showcases surprisingly small arms, even in comparison to Tyrannosaurus rex.”
“We aimed to understand the driving factors behind this evolution and discovered a strong correlation between shorter arms and the development of larger, more robust heads.”
In their comprehensive study, Scherer and colleagues analyzed data from 82 species of carnivorous theropod dinosaurs.
They determined that reductions in forelimb size occurred across five key lineages: abelisaurids, carcharodontosaurids, ceratosaurids, megalosaurids, and tyrannosaurids.
The evidence suggests that smaller arms are directly linked to the evolution of a larger, more powerful skull and jaw, rather than simply being a consequence of overall body size increases.
Torbosaurus gurneyi. Image credit: © Sergey Krasovskiy.
The researchers propose that the increased size of prey, including giant sauropods and other large herbivores, may have driven theropods to rely more on their jaws and heads instead of their claws for hunting.
This shift in attack strategy highlights the evolutionary principle of “use it or lose it,” as the arms gradually became less useful and diminished in size.
“While our study illustrates a correlation, we cannot definitively establish causation; however, it strongly suggests that theropods developed more solidly built skulls in conjunction with shorter forelimbs,” Scherer explained.
“It is unlikely that these predators would have abandoned their effective methods of attack without developing alternatives.”
Ecosystem reconstruction of the Late Jurassic Dry Mesa dinosaur quarry, roughly 150 million years ago in Colorado, USA. Image credit: Sergey Krasovskiy / Pedro Salas.
For their research, the paleontologists developed a novel method to quantify skull robustness, evaluating factors such as bone connectivity, skull dimensions (with compact shapes being superior in strength), and bite force.
Among the species studied, Tyrannosaurus rex achieved the highest score for skull robustness, followed closely by Tyrannotitan, another significant theropod that existed in present-day Argentina over 30 million years ago during the Early Cretaceous period.
“The enormous size of prey may have instigated an evolutionary arms race, prompting theropods to evolve more powerful skulls and jaws to subdue increasingly gigantic prey,” the researchers concluded.
Read their full paper, published today in Proceedings of the Royal Society B.
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Charlie Roger Scherer et al. 2026. Driving forces and mechanisms of convergent forelimb reduction in non-avian theropod dinosaurs. Proc Biol Sci 293 (2071): 20260528;doi: 10.1098/rspb.2026.0528
Source: www.sci.news
