Here’s how Tyrannosaurus ended up with a big head and small arms

Tyrannosaurus rex possessed a preposterously massive skull – 5 feet long (1.5 meters) and built to enable bone-crunching bite force – but presented preposterously puny arms. And many other meat-eating dinosaurs shared this mismatched combination of traits. So how did this come to be?

Here’s how Tyrannosaurus ended up with a big head and small arms. Tyrannosaurus rex possessed a preposterously massive skull – 5 feet long (1.5 meters) and built to enable bone-crunching bite force – but presented preposterously puny arms. And many other meat-eating dinosaurs shared this mismatched combination of traits. So how did this come to be? Researchers studying this phenomenon have now documented how skull robustness in meat-eating dinosaurs started to evolve first, as plant-eating dinosaurs that they hunted ​became bigger soon after dinosaurs became Earth’s dominant land animals, leading to a subsequent reduction in the arms. They identified five lineages of theropods – ‌the two-legged group that spans all the meat-eating dinosaurs – in which this phenomenon independently appeared, underscoring the evolutionary advantages that dr...