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Running into a Tyrannosaurus rex in the wild would have been a truly frightening thing for just about any animal that roamed the Earth between 65 million and 80 million years ago, and for an obvious reason. The mighty meat-eater was huge and had a mouth built to turn bones into powder. If it snagged you with its jaws, you were probably going to have a bad time, but nobody was afraid of its puny little arms … or were they? Just how T-Rex used its arms and for what purpose has been hotly debated for years.
This latest round of research approaches things from a different angle, seeking to determine the range of movement of the arms as a clue to their usefulness.
Perhaps the best example of truly vestigial structures are the remnants of five-toed feet that can be identified in the skeletons of snakes which is how naturalists realized that snakes evolved from five-toed vertebrate ancestors. However, it's also often the case that biologists or paleontologists describe a structure as "vestigial" simply because they haven't figured out its purpose yet.
For example, the appendix was long thought to be the classic human vestigial organ, until it was discovered that this tiny sac can "reboot" the bacterial colonies in our intestines after they've been wiped out by disease or some other catastrophic event.
Presumably, this evolutionary advantage counterbalances the tendency of human appendixes to become infected, resulting in life-threatening appendicitis. As with our appendixes, so with the arms of Tyrannosaurus Rex. The most likely explanation for T. Rex's oddly proportioned arms is that they were exactly as big as they needed to be.
This fearsome dinosaur would quickly have gone extinct if it didn't have any arms at all -- either because it wouldn't be able to mate and produce baby T. Rexes, or it wouldn't be able to get back up if it fell to the ground, or it wouldn't be able to pick up small, quivering ornithopods and hold them into its chest close enough to bite off their heads!
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