Why Were Tyrannosaur Forelimbs So Ridiculously Puny?

Looking at the complete skeleton of an adult Tyrannosaurus rex, with its massive body, the powerful legs and large skull, the forelimbs seem strangely out of proportion.

Paleontologists have proposed various hypotheses to explain the small size and apparent lack of mobility of the arms. Perhaps they were helpful to the males to keep their balance on the female during mating. Or they were needed during the final moments of the hunt, keeping the prey in reach of the powerful jaws. Or maybe the forelimbs came in handy when the massive animal stood up after a brief nap.

Rather than asking what the T. rex’s short arms evolved to do, Kevin Padian – distinguished emeritus professor of integrative biology at the University of California, Berkeley, and a curator at the UC Museum of Paleontology (UCMP) – wondered what evolutionary benefit such short arms could provide.

“The arms are simply too short. They can’t touch each other, they can’t reach the mouth, and their mobility is so limited that they can’t stretch very far, either forward or upward. And none of the hypotheses explain why the arms would get smaller—the best they could do is explain why they would maintain the small size,” Padian explains.

According to an open access study published by Padian, smaller arms would be less exposed to loss or injury during a feeding frenzy between large predators. Infected wounds can lead to the death of an animal, so smaller arms and a reduced risk of injury could result in an evolutionary advantage over time.

“What if several adult tyrannosaurs converged on a carcass? You have a bunch of massive skulls, with incredibly powerful jaws and teeth, ripping and chomping down flesh and bone right next to you. What if your friend there thinks you’re getting a little too close? They might warn you away by severing your arm. So, it could be a benefit to reduce the forelimbs, since you’re not using them in predation anyway.”

Padian’s hypothesis has analogies in some animals today. The giant Komodo dragon lizard (Varanus komodoensis) of Indonesia hunts in groups, and when it kills prey, the larger dragons converge on the carcass and leave the remains for the smaller ones. Maulings can occur, as they do among crocodiles during a feeding frenzy.

Mass burial sites suggests that some theropods, like the famous Tyrannosaurs rex, may have lived and traveled in packs including adults and young members.

“Bite wounds on the skull and other parts of the skeleton are well known in tyrannosaurs and other carnivorous dinosaurs,” he said. “If fewer bite marks were found on the reduced limbs, it could be a sign that reduction worked.”

Padian himself notes that this is not the end of the debate, as his hypothesis doesn’t explain why several groups of large carnivorous dinosaurs – and even some modern birds – have independently reduced their forelimbs.

“The sizes and proportions of the limb bones in these groups are different, but so are other aspects of their skeletons. We shouldn’t expect them to be reduced in the same way. This is also true for the reduced wings of our large, living, flightless ratite birds, like the ostrich, the emu and the rhea. They evidently took different evolutionary paths for their own reasons.”


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