The famously tiny forelimbs found in several carnivorous dinosaurs, including tyrannosaurs, carnotauruses, spinosaurs, and velociraptors, were most likely a consequence of the evolution of their large, powerful heads, which they used to attack prey. That is the conclusion of a new study led by researchers at University College London and the University of Cambridge, published in the journal Proceedings of the Royal Society B Biological Sciences.
The study analyzed data from 82 species of theropod dinosaurs, a group that was predominantly bipedal and carnivorous, and found that forelimb reduction appeared independently in five separate lineages, among them the tyrannosauridae, the family to which Tyrannosaurus rex belonged.
To carry out the research, the team developed a new method for measuring skull robustness, taking into account factors such as how tightly the bones of the skull were connected, cranial dimensions (since a more compact shape is structurally stronger than an elongated one), and bite force. Using this metric, T. rex scored highest, followed by Tyrannotitan, a theropod nearly as massive as T. rex that lived in what is now Argentina during the Early Cretaceous period, more than 30 million years before T. rex.
The researchers also compared forelimb length to skull length across the five dinosaur families with reduced forelimbs: tyrannosaurids, abelisaurids, carcharodontosaurids, megalosaurids, and ceratosaurids. They found that forelimb reduction had a stronger correlation with skull robustness than with skull size or overall body size, a pattern also visible in some theropods with small arms and powerful heads that were not particularly large animals overall.
Forelimb adaptations tended to appear in regions where oversized prey was available. The researchers suggest that hunting increasingly large animals may have prompted a shift in predatory strategy, with these dinosaurs relying more heavily on their heads and jaws rather than their claws.
The scientists also noted that forelimb reduction did not follow the same evolutionary path across all lineages. In abelisaurids, the hands and the lower portion of the arm below the elbow shrank most dramatically. In tyrannosaurids, by contrast, all parts of the forelimb were reduced at roughly the same rate.
The team concluded that the same evolutionary outcome, namely miniature forelimbs, was most likely achieved through different evolutionary mechanisms in different dinosaur species.
