A new study suggests that certain theropods—two-legged, mostly meat-eating dinosaurs—had shrunken forelimbs as an evolutionary trade-off for their strong skulls Researchers weren't sure what drove some theropods, like T. rex , to evolve tiny arms relative to their body sizes. Alex Segre / UCG / Universal Images Group via Getty Images Tyrannosaurus rex’ s   tiny arms might seem out of place on a massive predator, but there must have been an evolutionary reason for the limbs’ almost comical size. Now, scientists think they’ve figured it out. Shortened forelimbs in  T. rex  and some other theropods—two-legged, mostly meat-eating dinosaurs—seem to be a result of the ancient animals’ powerful jaws and skulls, according to a study published May 20 in the journal  Proceedings of the Royal Society B . As the terrifying predators increasingly used their heads to bite prey, their arms became less useful. “People have long been fascinated by why big meat-eating dinosaurs like T.…