When Dinosaurs Stopped Using Their Hands: The Evolutionary Arms Race Behind Forelimb Reduction

Many people’s strongest impression of Tyrannosaurus is its pair of disproportionately tiny forelimbs. These short arms not only became one of the most iconic features of Tyrannosaurus, but have also long fascinated both scientists and the public: why would such a gigantic predator possess such reduced forelimbs?

Tyrannosaurus（Image source：Steveoc 86， CC BY-SA 4.0 。）

However, when we broaden our perspective to the entire Mesozoic Era, it becomes clear that forelimb reduction was not unique to tyrannosaurs. Similar trends also evolved in several other theropod groups, most notably Abelisauridae and Carcharodontosauridae. This suggests that forelimb reduction was not a random occurrence, but rather the result of broader evolutionary pressures acting across multiple lineages.

To investigate this phenomenon, a recent study analysed forelimb proportions, skull structure, and body mass data from 85 theropod species in order to determine what anatomical features were associated with reduced forelimbs in predatory dinosaurs.

The results showed that, in addition to the already well-known examples among tyrannosaurids, carcharodontosaurids, and abelisaurids, reduced forelimbs were also present in Megalosauridae and Ceratosauridae. This indicates that forelimb reduction was actually a widespread evolutionary trend among theropods.

Eoabelisaurus（Image source：Paleocolour， CC BY-SA 4.0 。）

Early members of Abelisauroidea such as Eoabelisaurus still possessed overall forelimb proportions similar to earlier theropods like Dilophosaurus and Ceratosaurus, but their hands had already become noticeably reduced. In later abelisaurids such as Majungasaurus and Carnotaurus, both the hands and forearms became even more reduced, with Majungasaurus possessing some of the most extremely reduced forelimbs known among large predatory dinosaurs.

Majungasaurus（Image source：Primeval Artist， CC BY-SA 4.0 。）

Previous studies suggest that abelisaurid forelimb evolution began with the reduction of distal elements such as the hands, before later affecting the more proximal parts of the arm. Compared with other giant predators such as carcharodontosaurids and tyrannosaurids, later abelisaurids may have possessed the most extreme degree of forelimb reduction of all large theropods.

Ceratosaurus（Image source：DiBgd， CC BY 2.5 。）

Analysis of Ceratosaurus revealed a similar but less extreme pattern. Its forelimb reduction was concentrated mainly in the shortening of the metacarpals and finger bones, while the remainder of the arm retained relatively primitive proportions. Earlier studies often linked forelimb reduction with gigantism, but Ceratosaurus demonstrates that reduced forelimbs did not evolve exclusively in gigantic theropods. Juvenile specimens of Gorgosaurus also possessed relatively small forelimbs, suggesting that gigantism merely amplified an already existing trend rather than directly causing it.

Torvosaurus（Image source：Fred Wierum， CC BY-SA 4.0 。）

Because most Megalosauridae fossils are fragmentary, their forelimb evolution remains difficult to reconstruct. Nevertheless, available material suggests that some megalosaurids had proportionally shortened forelimbs. For example, although Torvosaurus was far larger than Afrovenator, both possessed forelimbs of nearly identical size, while Torvosaurus even had a shorter radius. This indicates that the larger animal had proportionally smaller forelimbs.

Spinosaurus（Image source：Connor Ashbridge， CC BY 4.0 。）

Closely related Spinosauridae, however, retained relatively long forelimbs. This was likely related to their fish-eating lifestyle, as spinosaurs probably relied heavily on their arms to help capture prey.

Within Allosauroidea, only the carcharodontosaurids developed strongly reduced forelimbs, whereas earlier forms such as Allosaurus retained more primitive arm proportions.

Carcharodontosaurus（Image source：Fred Wierum， CC BY-SA 4.0 。）

In carcharodontosaurids, the forelimbs became progressively shorter relative to both skull size and body size. Unlike abelisaurids, which reduced the hands first, carcharodontosaurids initially shortened the forearm before later reducing the hands. Overall, forelimb reduction in this group evolved alongside increasing body size.

Guanlong（Image source：Durbed， CC BY 3.0 。）

Because the fossil record of Tyrannosauroidea is relatively complete, tyrannosaurs display perhaps the clearest evolutionary trend. Early tyrannosauroids such as Guanlong still possessed normal forelimbs, but later tyrannosaurids such as Gorgosaurus began showing clear reduction. By the time of Tyrannosaurus, the forelimbs had become extremely reduced. Interestingly,Tyrannosaurus forelimbs did not appear to shrink one section at a time; instead, the entire limb shortened more or less simultaneously. Researchers also found that the rate of forelimb reduction accelerated during the Late Cretaceous, particularly from the middle Campanian to the Maastrichtian.

Alvarezsaurus（Image source：User:Karkemish， CC BY 3.0 。）

Although Alvarezsauridae are often considered another example of forelimb reduction, this study did not support that interpretation. This may be because alvarezsaurids also evolved smaller skulls and reduced body sizes, meaning their arm proportions differed from the other five groups. This further suggests that forelimb reduction evolved through different mechanisms in different theropod lineages.

So why did these groups independently evolve reduced forelimbs? The study found that forelimb reduction was most strongly correlated with cranial robusticity: the more robust the skull, the smaller the forelimbs became. Body mass played a secondary role. Although many gigantic theropods possessed short forelimbs, smaller species such as Majungasaurus also evolved extremely reduced arms, demonstrating that gigantism alone was not the primary driver.

Because all five groups coexisted with giant herbivorous dinosaurs, the researchers proposed that a predator–prey arms race occurred throughout the Mesozoic. As herbivores evolved larger body sizes, predatory theropods evolved larger and more robust skulls capable of delivering increasingly powerful bites. Over time, the head became the primary weapon during hunting, while the forelimbs gradually lost their original predatory function and eventually became energetically unnecessary, leading to their reduction.

Overall, this study demonstrates that forelimb reduction evolved independently in at least five separate non-avian theropod lineages, each following different evolutionary pathways. This represents a striking example of convergent evolution.

More importantly, the evolution of theropod forelimbs was not simply a matter of becoming larger or smaller, but rather reflected differences in predatory strategy. When the skull became the primary weapon, the forelimbs gradually lost importance. In lineages where the forelimbs remained essential for grasping prey, however, long arms and strong grasping abilities were retained — and in some descendants, those forelimbs may eventually have taken to the skies.

（Author: Bai Leng）

Reference：

Scherer, C. R., Steell, E., Upchurch. (2026). Drivers and mechanisms of convergent forelimb reduction in non-avian theropod dinosaurs. Proceedings of the Royal Society B.