Although they may have retained some residual function, what the forelimbs of Tyrannosaurus rex were almost certainly not used for was grabbing and holding large prey. They were far too short and mechanically limited for that role, especially in a predator whose real killing equipment was a massive skull, powerful jaws and bone-crushing bite. So, creationists need to explain why an intelligent designer would have equipped one of the most formidable predators ever to walk the Earth with such apparently inadequate little arms in the first place.
These apparent design failures are, of course, entirely understandable as the result of an evolutionary process operating over deep time. Just such an explanation has now been proposed by three researchers from University College London (UCL) and the University of Cambridge, who have published their findings in Proceedings of the Royal Society B. It almost goes without saying that their explanation is an application of the Theory of Evolution, with no suggestion that the authors are about to abandon it in favour of creationism — as creationists have been confidently predicting for the best part of half a century, despite the singular lack of any peer-reviewed scientific movement in that direction.
The researchers found a strong association between the evolution of large, robust skulls and the reduction of forelimbs in several groups of non-avian theropod dinosaurs. In other words, the tiny arms of T. rex were not merely a side-effect of the whole body becoming larger. They were more closely linked to the evolution of powerful heads and jaws, suggesting a shift in hunting strategy in which the skull became the principal weapon and the forelimbs became less important.
The authors are careful to point out that correlation does not prove causation. But the pattern is consistent with an evolutionary arms race in which large predatory dinosaurs increasingly relied on massive skulls and crushing bites to tackle large prey, rather than on grasping forelimbs. As lead author Charlie Roger Scherer put it, trying to grab and hold a huge herbivorous dinosaur with claws would not have been ideal; attacking and holding with the jaws may have been far more effective.
For their study, the researchers developed a new way to quantify skull robustness, using factors such as how tightly the bones of the skull were connected, the compactness of the skull, and bite force. On this measure, T. rex scored highest, followed by Tyrannotitan, a large South American theropod that lived more than 30 million years earlier.
The study also showed that forelimb reduction evolved independently in at least five theropod groups: tyrannosaurids, abelisaurids, carcharodontosaurids, megalosaurids and ceratosaurids. That makes this a case of convergent evolution: different lineages arriving at a similar anatomical result because similar selective pressures favoured a similar functional solution.
The evolutionary sequence is straightforward in this case: as the prey became larger so the jaw and skull needed to become larger to kill and consume the prey. The mouth then became the primary means of gripping and killing the prey and the forelimbs, which are not needed for locomotion, became increasingly redundant but liable to injury, so there was an advantage in reducing their size. The fact that there was convergence in different lineages, is strongly suggestive that this mechanism evolved for the same reasons, multiple times.
The Evolution of the Tyrannosaurs. Tyrannosaurus rex was not the starting point of the tyrannosaur story, but one of its final and most extreme products. The wider group, Tyrannosauroidea, had a long evolutionary history stretching back into the Middle Jurassic, more than 100 million years before T. rex. For much of that time, tyrannosauroids were not gigantic apex predators, but mostly small to medium-sized, lightly built theropods living alongside, and often in the shadow of, other large carnivorous dinosaurs.[1]The publication in Proceedings of the Royal Society B is accompanied by a news release from UCL.
Early tyrannosauroids included animals such as Proceratosaurus from Jurassic Britain and Guanlong from Jurassic China. These were not simply miniature versions of T. rex. Some had crests, longer arms and more generalised predatory bodies. Their importance lies in showing that tyrannosaurs did not appear suddenly as fully formed, giant, short-armed killing machines. The famous late Cretaceous body plan was assembled gradually, piece by piece, over tens of millions of years.[1,2]
Several Early Cretaceous tyrannosauroids also show how different the early members of the group were from their later descendants. Dilong paradoxus, from China, was small and gracile, with relatively long arms and three-fingered hands. It also preserved evidence of filamentous protofeathers, showing that at least some early tyrannosauroids were not the purely scaly monsters of older popular reconstructions.[3]
The discovery of Yutyrannus huali, also from Early Cretaceous China, pushed that point further. This was a much larger tyrannosauroid, yet it too preserved long filamentous feathers. That does not prove that an adult T. rex was fully feathered, and skin impressions from later tyrannosaurids suggest at least some scaly areas. But it does show that feathers were part of the wider tyrannosauroid evolutionary background, not an irrelevant bird-like novelty.[4]
By the Late Cretaceous, especially in Asia and western North America, tyrannosaurids had become the dominant large predators. Genera such as Albertosaurus, Gorgosaurus, Daspletosaurus, Tarbosaurus and Tyrannosaurus show the familiar trend towards massive skulls, powerful jaws, thick teeth, strong hind limbs, keen senses and reduced forelimbs. This was not a single act of design, but a long evolutionary sequence in which the skull and jaws increasingly took over the role of subduing prey.[1,5]
Recent work has added further detail to this picture. In 2025, researchers described Khankhuuluu mongoliensis, a Mongolian tyrannosauroid from the lower Upper Cretaceous, as a form close to the origin of Eutyrannosauria — the group that includes the large, late Cretaceous tyrannosaurs. Their analysis suggests a complex history of dispersal between Asia and North America, with tyrannosaur evolution involving migration, ecological opportunity and divergent growth patterns, rather than a simple straight-line progression from small ancestor to giant descendant.[6]
So the tiny arms of T. rex are not an isolated oddity needing to be excused as good design. They are part of a broader evolutionary pattern in which tyrannosaurs changed from relatively small, long-armed predators into large, skull-dominated apex predators. The result looks puzzling if imagined as the work of a designer starting from scratch, but it makes sense as the outcome of descent with modification, changing ecological pressures, and the evolutionary reworking of inherited anatomy.
Why meat-eating dinosaurs like T. rex evolved tiny arms
The evolution of tiny arms in several groups of meat-eating dinosaurs was likely driven by the development of strong, powerful heads, which were used to attack prey, according to a new study led by researchers at UCL and Cambridge University.
The study, published in the journal Proceedings of the Royal Society B, looked at data for 82 species of theropod (two-legged, mainly meat-eating dinosaurs), finding that shortening of forelimbs occurred across five groups, including tyrannosaurids, the family that included Tyrannosaurus rex.
The team, including Dr Elizabeth Steell at Cambridge and Professor Paul Upchurch at UCL, found that smaller arms were closely linked to the development of large, powerful skulls and jaws, more so than to larger overall body size, indicating that tiny arms were not just a by-product of bodies getting bigger.
The researchers suggested that the increasing size of prey, in the form of gigantic sauropods (long-necked, long-tailed plant-eaters) and other large herbivores, may have resulted in a shift to hunting using jaws and head instead of claws.
Everyone knows the T. rex had tiny arms but other giant theropod dinosaurs also evolved relatively small forelimbs. The Carnotaurus had ridiculously tiny arms, smaller than the T. rex. We sought to understand what was driving this change and found a strong relationship between short arms and large, powerfully built heads. 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. These adaptations often occurred in areas with gigantic prey. Trying to pull and grab at a 100ft-long sauropod with your claws is not ideal. Attacking and holding on with the jaws might have been more effective.
While our study identifies correlations and so cannot establish cause and effect, it is highly likely that strongly built skulls came before shorter forelimbs. It would not make evolutionary sense for it to occur the other way round, and for these predators to give up their attack mechanism without having a back-up.
Charlie Roger Scherer, lead author.
Department of Earth Sciences
University College London
London, UK.
For the study, researchers developed a new way to quantify skull robustness, based on factors including how tightly connected the bones of the head were, the dimensions of the skull (a more compact shape is stronger than an elongated shape), and bite force.
On this measure, the T. rex scored highest, followed by the Tyrannotitan, a theropod nearly as massive as T. rex who lived in what is now Argentina in the Early Cretaceous period (more than 30 million years earlier than T. rex).
The team said that increasingly gigantic prey may have resulted in an “evolutionary arms race”, where theropods developed strong skulls and jaws to better subdue this prey, and in many cases grew to gigantic sizes themselves.
Separately, the team compared forelimb length to skull length, classifying five groups of dinosaurs as having reduced forelimbs: tyrannosaurids, abelisaurids, carcharodontosaurids (including the Tyrannotitan), megalosaurids and ceratosaurids.
They found reduced forelimbs had a stronger link with skull robustness than with skull size or overall body size. The secondary importance of overall body size was illustrated by the fact that some theropods with strongly built heads and tiny arms were not very large, the researchers said, citing the Majungasaurus, an apex predator in Madagascar 70 million years ago, but weighing a mere 1.6 tonnes, about a fifth of the T. rex.
The researchers noted that the forelimbs appeared to reduce in size in different ways, with hands and the lower part of the arm (past the elbow) shortening the most in abelisaurids (with late abelisaurids such as the Majungasaurus having exceptionally tiny hands). In tyrannosaurids, on the other hand, each element of the forelimb was reduced at a similar rate.
The team concluded that the same outcome (tiny forelimbs) was likely achieved through potentially different developmental pathways in different species.
A team of five academics work on different aspects of dinosaur evolution at UCL, with strong collaborative links to the Natural History Museum. The extended research group comprises four research fellows and postdoc researchers, and more than 10 PhD students. At least four of the PhD students are working on dinosaur evolution, with the others looking at a wider array of other evolutionary questions relating to vertebrates, including crocodiles and birds.
Publication:
So the famously tiny arms of Tyrannosaurus rex are not an embarrassment for evolutionary biology; they are exactly the sort of thing evolution explains. They are the result of history, contingency and trade-offs: inherited anatomy being modified over time as natural selection favoured a different way of killing prey. As the skull became larger, stronger and more effective as the main predatory weapon, the forelimbs became less important, and so there was no evolutionary pressure to maintain them as large, powerful grasping organs.
That is why this feature is so difficult to explain as the work of an intelligent designer. A designer starting from scratch could simply have produced an animal with both a massive, bone-crushing skull and proportionately useful forelimbs, or dispensed with the arms altogether. Instead, what we see is the familiar evolutionary pattern: not perfect engineering, but modified inheritance; not clean-sheet design, but anatomical compromise shaped by changing selection pressures.
Creationism has no scientific explanation for this. It can only wave the problem away by declaring, without evidence, that the tiny arms must have had some unknown purpose, or that the designer’s motives are beyond human understanding. But that is not an explanation; it is an excuse for not having one. It predicts nothing, explains nothing, and adds nothing to our understanding of tyrannosaur biology.
The Theory of Evolution, by contrast, not only explains why such apparently odd features exist, but also provides a framework in which they can be tested. The prediction is that similar ecological and functional pressures should produce similar anatomical trends in separate lineages — and that is exactly what this study found. Forelimb reduction evolved independently in several theropod groups, associated not with divine whim, but with the repeated evolution of large, robust skulls and powerful jaws.
In other words, the tiny arms of T. rex are not a mystery for science; they are evidence of evolution doing what evolution does — adapting existing structures to changing circumstances, often imperfectly, always historically, and never with the foresight or tidiness that intelligent design would require. Once again, the evidence fits the evolutionary model and leaves creationism with nothing more substantial than incredulity, special pleading and the hope that no one looks too closely.
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