Extinct for 66 million years, T. rex still inspires speculation about its anatomy, behavior and fierce physique. One conundrum: its peculiarly short forelimbs, which have relatively limited mobility. While scientists have suggested explanations, a paleontologist puts the proposals to the test and finds them wanting. He hypothesizes that the arms reduced in size to limit damage, possibly fatal, when a pack of T. rexes descended on a carcass with their bone-crunching teeth.