EVIDENCE THAT THE ARMS OF TYRANNOSAURUS REX WERE NOT FUNCTIONLESS BUT ADAPTED FOR VICIOUS SLASHING
Its short, strong forelimbs and large claws would have permitted T. rex, whether mounted on a victim’s back or grasping it with its jaws, to inflict four gashes a meter or more long and several centimeters deep within a few seconds -- and it could have repeated this multiple times in rapid succession. Infliction of damage by slashing was widespread among other theropod taxa, so in light of its formidable weaponry, why should T. rex not have engaged in this activity?
Tyrannosaur ancestors used long arms primarily for grasping. These atrophied during the evolution that led to the tyrannosaurids because the jaws took over their grasping function. No longer being selected for, the arms were selected against: the expansion of the head deprived them of nutrition in a zero-sum game. Then, as the arms approached their final size, natural selection kicked in opportunistically and put them to good use for slashing at close quarters.