Why T. rex had such puny arms

The famously fearsome Tyrannosaurus rex is known not only for its immense jaws but also for its disproportionately small arms. Now, a new study published in Proceedings of the Royal Society suggests a clear connection between the size of a carnivorous dinosaur's skull and its limbs.

Researchers analyzed the anatomy of 176 species of predatory dinosaurs from the Jurassic and Cretaceous periods. They observed a trend, particularly in families like Abelisauridae, Carcharodontosauridae, and Tyrannosauridae, where species evolved to become larger with more massive skulls and jaws, while their arms simultaneously became smaller and seemingly less functional.

This culminated in giants like T. rex, which weighed an estimated 8.5 tons and possessed incredibly powerful jaws. The study posits that the evolution of massive heads and jaws was driven by a shift towards hunting larger prey. Instead of using their arms to grasp prey, these apex predators likely relied on swift, powerful bites to subdue and tear apart their meals, rendering their small arms obsolete and saving energy by not developing limbs that were no longer needed.