Few things in paleontology generate as much speculation, and ridicule, as the arms of Tyrannosaurus rex. In a culture where “bigger” is confused with “better,” we can’t seem to get our heads around why such a large predator would have such small forelimbs. Most puzzling of all is that the dinosaur’s arms were not vestigial–they were muscular appendages that must have had some function. But what?
Our understanding of tyrannosaur arms is constrained by what we think that dinosaurs were capable of. The trick is parsing the difference between what T. rex could do and what it actually did. Even though it appears that the forelimbs of the tyrant dinosaurs became smaller as they developed heavier heads capable of crushing bites, this doesn’t necessarily tell us what T. rex and kin used their arms for, if anything.
When I was a kid, though, there was one possibility that popped up in the dinosauriana I loved to browse. As seen in the clip above, from the documentary Dinosaur!, some paleontologists thought that tyrannosaurs could have used their arms to raise themselves off the ground after resting or–as in this case–embarrassingly being knocked to the ground by an Edmontosaurus. For a creature with such tiny arms, researchers speculated, T. rex might have been surprisingly skilled at push-ups.
The idea goes back to Barney Newman, a paleontologist who worked at what is now London’s Natural History Museum. In 1970, after overseeing a reconstruction of T. rex at the museum, Newman wrote a short paper on the posture of the famous dinosaur. Not only did the tyrant have a more bird-like posture than previously thought, Newman wrote, but he finally found a use for those short arms. The heavy construction of the dinosaur’s arms and shoulder girdle showed that the chest and arms of T. rex were surprisingly beefy, and, in Newman’s view, all that muscle and bone acted as a set of brakes.
At rest, Newman suspected, T. rex sat in a kind of crouch with its legs “folded under the body in much the same way as a hen’s,” lower jaw on the ground and palms flat. When the dinosaur stood up, Newman suggested, “The role of the fore-limbs was that of a brake holding the body, so that the force exerted by the extension of the hind-limbs was transmitted to the pelvic region, thus pushing it upwards.”
Newman didn’t say that T. rex pushed the fore-part of its body off the ground. Artists and filmmakers confused what Newman had hypothesized–that the seemingly overbuilt arms of the dinosaur acted as stabilizers as T. rex extended its legs to stand. But, the T. rex stretch meme aside, there’s no reason to think that the theropod actually behaved as Newman supposed.
In Newman’s reconstruction, the wrists of T. rex make the dinosaur’s hands face palms-down. That would have given the tyrant some grip as it stood. But we know that theropod wrists didn’t articulate this way. As paleontologists frequently point out, theropods were clappers, not slappers–their palms faced inwards, towards each other, and flexed more like bird wrists. A wonderful sitting trace of an Early Jurassic theropod confirms this position, as do other smaller theropod skeletons preserved in the act of nesting or resting. In order to achieve a palms-down grip on the ground, T. rex would have had to swing its arms far out to the sides so that the dinosaur’s hands came into the right position.
Dinosaur traces and roosting skeletons also tell us something else. Newman was right that T. rex, like other theropods, probably sat in a very bird-like position. But, like both avian and other non-avian theropods, there’s no indication that tyrannosaurs needed extra stabilization to stand up. The thick heads and heavy tails of tyrannosaurs were counterbalanced over their hips, and they probably sat down and stood up in the typical theropod manner without the need for brakes. Newman’s hypothesis was a clever one for a long-running paleo problem, but what T. rex used those small, strong arms for remains as contentious as ever.
Newman, B. 1970. Stance and gait in the flesh-eating dinosaur Tyrannosaurus. Biological Journal of the Linnean Society, 2. 119-123