We may finally know why dinosaurs like T. rex evolved tiny arms

A recent study proposes that dinosaurs such as Tyrannosaurus rex developed smaller forelimbs as their enormous, powerful heads became the primary tool for hunting large prey, making their arms less necessary. This evolutionary strategy was independently adopted by five distinct lineages of sizable theropod dinosaurs.

Researchers have long observed that many large, predatory theropods evolved towards larger bodies, bigger heads, and smaller arms over time. However, the reason behind this recurring trend across various dinosaur families globally and spanning millions of years remained unclear, according to Charlie Scherer from University College London. Additionally, there was limited knowledge on how the structure of their heavier skulls changed as their arms shrank.

“This study addresses a major evolutionary question concerning theropod dinosaurs,” notes Andre Rowe from the University of Bristol, UK, who was not involved in the research.

Scherer and his team gathered information on the forelimb and skull proportions of 85 theropod species, alongside body mass data. This enabled them to calculate the ratio between skull size and forelimb length, assessing how comparatively small the arms were. The researchers then analyzed this ratio in conjunction with other body measurements and the skulls’ strength, considering factors like bite force and skull rigidity.

The findings indicated that robust skulls were linked to smaller arms, irrespective of the species’ position in the theropod evolutionary lineage. “A predatory theropod with a strong skull likely has relatively small forelimbs,” Scherer explains.

This head-arm divergence emerged separately in five theropod groups: tyrannosaurids, short-snouted abelisaurids, knife-toothed carcharodontosaurids, ceratosaurids, and megalosaurids. Fion Waisum Ma from the University of Hong Kong, who was not part of the study, highlights that this evolutionary pattern had not been previously identified in the last two groups. She notes that quantifying traits in this manner can uncover hidden evolutionary signals.

The study provides insights into why these dinosaurs’ arms continued to shrink. As these predators’ skull strength and body size increased, so did the mass of their prey. The theropods developed massive, durable skulls to handle their large, challenging prey, reducing the need for strong arms. Scherer remarks that their heads were clearly doing most of the work, minimizing the necessity for robust, grasping arms.

“Nature doesn’t like to have everything all at once,” Scherer adds, noting the energy cost of maintaining both a powerful head and strong forelimbs.

This resulted in a trade-off between jaws and claws. Other theropods, such as megaraptorans and spinosaurs, were also large predators but opted for long arms and slender skulls instead.

Rowe expresses interest in understanding the function of the short arms in jaw-centric theropods. “Yes, tyrannosaurs had tiny, vestigial arms, but that does not necessarily mean they were completely useless,” he comments.

He emphasizes that the study underscores the evolutionary diversity of dinosaurs. “It reminds me of why I fell in love with dinosaurs in the first place,” he reflects. “They were some of the most innovative and successful animals to ever exist.”