Why Did Tyrannosaurus Rex Have Tiny Arms?

Vestigial Structures in the Dinosaur Kingdom

Tyrannosaurus rex holotype specimen at the Carnegie Museum of Natural History, Pittsburgh
ScottRobertAnselmo/Wikimedia Commons/CC BY-SA 3.0

Tyrannosaurus Rex may or may not have been the most fearsome dinosaur that ever lived (you can also make a good case for Allosaurus, Spinosaurus or Giganotosaurus), but however high it ranks on the all-time viciousness charts, this meat-eater had one of the smallest arm-to-body-mass ratios of the entire Mesozoic Era. 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 way they have in modern snakes.

The Arms of Tyrannosaurus Rex Were Tiny Only in Relative 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. The arms of T. Rex were fairly limited in their scope--they could only swing across an angle of about 45 degrees, compared to a much wider range for smaller, more flexible theropod dinosaurs 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 pretty much every sense) were more than capable of snatching live, wriggling prey and holding it 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). Given how little we currently know about dinosaur sex, this is an iffy proposition at best!

2) T. Rex used its arms to lever itself off the ground if it happened to be knocked off its feet during battle, say, with an eager-not-to-be-eaten Triceratops (which can be a tough proposition if you weigh eight or nine tons), or if it slept in a prone position.

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, but once again, we can't adduce any direct fossil evidence for this behavior.)

At this point you may be asking: how do we know if 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 limbs didn't serve at least some useful 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 if it happened to fall down.)

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 at some point far back in an animal's family tree, but was gradually reduced in size and functionality as an adaptive 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. (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 on 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!