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 by researchers at UCL and Cambridge.
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," said lead author Charlie Roger Scherer from UCL. "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 backup."
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.
Elizabeth Steell is a Fellow of Girton College, Cambridge.
Reference:
Charlie Roger Scherer, Elizabeth Steell and Paul Upchurch. 'Drivers and mechanisms of convergent forelimb reduction in non-avian theropod dinosaurs.' Proceedings of the Royal Society B (2026). DOI: 10.1098/rspb.2026.0528
Adapted from a story published by UCL.
