Tyrannosaurus Rex may or may not have been the most fearsome dinosaur that ever lived--you can also make a good case for Allosaurus or Spinosaurus--but whatever its ranking on the all-time viciousness charts, this predator had one of the smallest arm-to-body-mass ratios in the prehistoric world. For decades, paleontologist and biologists have debated how T. Rex used its arms, and whether a further 10 million or so years of evolution (assuming the K/T Extinction hadn't happened) might have caused them to disappear entirely.
The Arms of T. Rex Were Tiny Only in Relative, not Absolute, Terms
Before exploring this issue further, it helps to define what we mean by "tiny." Because the rest of T. Rex was so huge--adult specimens of this dinosaur measured about 40 feet from head to tail and weighed anywhere from 7 to 10 tons--its arms only seemed small in proportion to the rest of its body, and were still pretty impressive in their own right. In fact, T. Rex's arms were over three feet in length, and a recent analysis has shown that they may have been capable of bench-pressing over 400 pounds each. Pound for pound, this study concludes, T. Rex's arm muscles were over three times more powerful than those of an adult human!
There's also a fair degree of misunderstanding about the range of T. Rex's arm motion and the flexibility of this dinosaur's fingers. T. Rex's arms were fairly limited in their scope--they could only swing across an angle of about 45 degrees, compared to smaller, more flexible theropods like Deinonychus--but then again, disproportionately small arms wouldn't require a wide angle of operation. And as far as we know, the two large fingers on each of T. Rex's hands (a third, the metacarpal, was truly vestigial in every sense) were more than capable of snatching live, wriggling prey and holding tight.
How Did T. Rex Use its "Tiny" Arms?
This leads us to the million-dollar question: given their unexpectedly wide range of functionality, combined with their limited size, how did T. Rex actually use its arms? There have been a few proposals over the years, all (or some) of which may be true:
1) T. Rex males mainly used their arms and hands to grab onto females during mating (females still possessed these limbs, of course, presumably using them for the other purposes listed below).
2) T. Rex used its arms to lever itself off the ground if it happened to be knocked off its feet during battle (which can be a tough proposition if you weigh eight or nine tons).
3) T. Rex used its arms to clutch tightly onto squirming prey, before it delivered a killer bite with its jaws. (This dinosaur's powerful arm muscles lend further credence to this idea).
At this point you may be asking: how do we know T. Rex used its arms at all? Well, nature tends to be very economical in its operation: it's unlikely that the tiny arms of theropod dinosaurs would have persisted into the late Cretaceous period if these arms didn't serve at least some purpose. (The most extreme example in this respect wasn't T. Rex, but the two-ton Carnotaurus, the arms and hands of which were truly nubbin-like; even so, this dinosaur probably needed its stunted limbs to at least push itself off the ground.)
In Nature, Structures that Seem to be "Vestigial" Often Aren't
When discussing the arms of T. Rex, it's important to understand that the word "vestigial" is in the eyes of the beholder. A truly vestigial structure is one that served a purpose on some point of an animal's family tree, but was gradually reduced in size and functionality in response to millions of years of evolutionary pressure. 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.
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, and that this fearsome dinosaur would have quickly 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 on the ground, or it wouldn't be able to pick up small, quivering ornithopods and hold them in to its chest close enough to bite off their heads!