>>5018652>because it doesnt need its hands anymoreSo why didn't they just atrophy away into nothing over the millions of years that the genus lasted? There are loads of reasons for them to lose their arms, but I can't see any convincing ones for retaining them. Birds do fine manipulating everything they need to with beaks. T. rex apparently had pretty sensitive lips, so it should have been even better at getting stuff done with its mouth. The mystery is not why didn't they have big arms, it's why did they have those specific arms rather than just nothing?
They aren't just vestigial either. They didn't shrink at all once they got to that size. They retained claws, and are obviously adapted to a specific (very limited) set of motions, with significant strength. Don't seem to be related to mating (no sexual dimorphism). Not big enough to help standing up. Grasping prey just doesn't seem useful enough to justify them.