Giants with stubby arms: Many large predatory dinosaurs of the Cretaceous period had surprisingly short front legs – but why? Paleontologists may now have found the reason for this. Accordingly, the arms of T. rex and co became shorter the larger the prey of the predatory dinosaurs became. In order to be able to overwhelm and kill this prey, the carnivores needed, above all, strong jaws and large skulls. The clawed front legs were less helpful – and were therefore reduced over time, as the team determined.
The stubby arms of Tyrannosaurus rex, Allosaurus and other large predatory dinosaurs of the Cretaceous period are still a mysterious phenomenon today. The arms of some of these carnivores were so short that the animals could no longer even reach their own snouts. This raises the question of why several theropod lineages developed such extreme shortening of their forelimbs – but not all large predatory dinosaurs.
What drove the arm shortening?
In search of an explanation, paleontologists led by Charlie Scherer from University College London have now examined the relationships, forelimbs, heads and body sizes of 85 predatory dinosaurs from different tribal lines and times in more detail. “We wanted to understand what drove this arm shortening,” says Scherer. Although there are many hypotheses about this so far, there is no clear explanation.
In their analyses, the paleontologists paid particular attention to the relationship between arm length and the size and robustness of the skull. “It is very likely that these predatory dinosaurs first developed their strong, robust skulls before their forelimbs shortened,” explains co-author Elizabeth Steell from Cambridge University. That’s why the paleontologists suspected a connection here.
Parallel trend in five different predatory dinosaur groups
In fact, there were striking correlations: “We found a close connection between shortened arms and large, robust skulls,” reports Scherer. In carnivores with high bite forces and powerful heads, the forelimbs were particularly severely reduced. This connection was most extreme in Tyrannosaurus rex, closely followed by Tyrannotitan, a predatory dinosaur that occurred in southern South America 30 million years before T. rex.
The researchers identified this trend toward ever larger, more robust skulls and smaller arms in five different theropod lineages. Some of these predatory dinosaurs, such as the abelisaurids, shortened their hand and finger bones disproportionately. In tyrannosaurs, however, all parts of the forelimbs shrank equally. According to the analyses, the theropod groups must have developed their stubby arms independently of one another.
“This suggests that this trend was promoted by similar ecological and evolutionary circumstances,” state Scherer and his colleagues.
Was the size of the prey important?
But what drivers were these? The comparative analyzes also provided new evidence for this. Because stubby arm carriers like Tyrannosaurus rex, Carnotaurus, Meraxes or Tyrannotitan had one thing in common: they hunted and ate particularly large prey. “The co-occurrence of giant theropods and gigantic herbivores suggests an evolutionary arms race between predators and prey,” explain the paleontologists.
Because their prey became ever larger, the predatory dinosaurs had to adapt. They also grew larger and developed increasingly robust, powerful skulls in order to be able to grab their massive, large prey and bite them to death. “Trying to hold on to a 30-meter-long sauropod with your claws is not really ideal,” says Scherer. “It was probably far more effective to attack and hold this prey with teeth and jaws.” Crocodiles and some large cats use a similar tactic today.
“Rationalized away” by evolution
However, this adaptation had further consequences: Because the large predatory dinosaurs relied more and more on their powerful skulls and teeth to hunt prey, their forelimbs became increasingly unimportant. “This is a case of ‘use it or lose it’: the arms were no longer useful and were therefore reduced over time,” explains Scherer. This is consistent with the fact that predatory dinosaurs, which primarily hunted smaller prey or caught fish, kept longer forelimbs.
The stubby arms of T. rex and co were therefore an evolutionary “rationalization measure”: They shrank because these predatory dinosaurs could better use the energy and resources for their forelimbs in other ways – including their powerful skulls and teeth.
Source: Charlie Scherer (University College London) et al., Proceedings of the Royal Society B – Biological Sciences, 2026; doi: 10.1098/rspb.2026.0528