A collaborative study by paleontologists from University College London and the University of Cambridge posits that the diminutive forelimbs of large predatory dinosaurs evolved concurrently with substantial craniums and osteophagic mandibles, suggesting a strategic shift in ancient predators towards employing biting as their primary method of subduing prey, rather than prehensile capture.
Life reconstruction of Tyrannosaurus mcraeensis. Image credit: Sergey Krasovskiy.
“The diminutive appendages of Tyrannosaurus rex are widely recognized, yet it’s noteworthy that other colossal theropod lineages also developed comparatively reduced forelimbs,” commented Charlie Roger Scherer, a doctoral candidate at University College London.
“Consider the Carnotaurus, which sported exceptionally small arms, even smaller than those of Tyrannosaurus rex.”
“Our objective was to ascertain the underlying impetus for this evolutionary trend, and we discovered a significant correlation between abbreviated limbs and the development of large, robust cranial structures.”
In their comprehensive investigation, Scherer and his associates analyzed data pertaining to 82 distinct species of carnivorous theropod dinosaurs.
The research revealed the convergent evolution of forelimb reduction across five separate clades: Abelisauridae, Carcharodontosauridae, Ceratosauridae, Megalosaurinae, and Tyrannosauridae.
The miniaturization of forelimbs demonstrated a strong association with the augmentation of massive, powerfully constructed skulls and jaws—a connection more pronounced than that with overall body size. This finding implies that diminutive arms were not merely an incidental consequence of increasing body mass.
Torvosaurus gurneyi. Image credit: © Sergey Krasovskiy.
The research team theorizes that the escalating size of prey, exemplified by colossal sauropods and other large herbivorous fauna, might have catalyzed a paradigm shift towards hunting strategies emphasizing the use of jaws and the head, superseding reliance on claws.
The head effectively assumed the role of the primary offensive instrument, displacing the forelimbs. This phenomenon can be understood through the principle of ‘use it or lose it’—as the arms became less functionally indispensable, their size gradually diminished over evolutionary timescales.
“While our research elucidates correlations, it is important to acknowledge that it cannot definitively establish causality. Nevertheless, it is highly probable that robust cranial anatomy predated the reduction in forelimb length,” Scherer stated.
“From an evolutionary standpoint, it would be counterintuitive for the reverse scenario to occur, whereby these apex predators would relinquish their established offensive mechanism without possessing a viable alternative.”
Ecosystem reconstruction of the Late Jurassic Dry Mesa Dinosaur Quarry around 150 million years ago in Colorado, the United States. Image credit: Sergey Krasovskiy / Pedro Salas.
To facilitate their investigation, the paleontologists devised an innovative methodology for quantifying cranial robustness. This approach incorporated metrics such as the degree of cranial bone fusion, the overall dimensions of the skull (with a more compact form conferring greater strength than an elongated one), and an assessment of bite force potential.
Based on this evaluative framework, Tyrannosaurus rex registered the highest score. Following closely was Tyrannotitan, a theropod comparable in magnitude to Tyrannosaurus rex, which inhabited the region now known as Argentina during the Early Cretaceous epoch, more than thirty million years prior to the emergence of Tyrannosaurus rex.
“The increasing gigantism of prey likely instigated an evolutionary arms race. Theropods consequently developed formidable skulls and jaws to more effectively subdue their quarry, and in numerous instances, these predators also attained immense bodily proportions,” the researchers concluded.
Their findings have been documented in a publication released today in the esteemed journal, Proceedings of the Royal Society B.
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Charlie Roger Scherer et al. 2026. Drivers and mechanisms of convergent forelimb reduction in non-avian theropod dinosaurs. Proc Biol Sci 293 (2071): 20260528; doi: 10.1098/rspb.2026.0528
