The Tyrannosaurus rex, the infamous tyrant lizard, was probably one of the most ferocious predators of the Cretaceous period. But despite its reputation, it’s also one of the more ridiculed dinosaurs — and it’s all thanks to its famously tiny arms. Now a University of California, Berkeley professor has come up with a new hypothesis for the T-Rex’s small limbs, and it’s not particularly flattering either.
In the journal Acta Palaeontologia, paleontologist Kevin Padian, an emeritus professor of integrative biology and curator at the University of California Museum of Paleontology, is floating a new hypothesis. Padian believes the T. rex’s arms shrank in length to prevent accidental or intentional amputation when a pack of his peers fed.
“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,” Padian said. “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 noted that the predecessors of tyrannosaurids had longer arms, which means there must have been a reason that they became reduced in both size and joint mobility. This would have affected not only T. rex, which lived in what is now North America at the end of the Cretaceous period, but the African and South American abelisaurids from the mid-Cretaceous and the carcharodontosaurids from Europe and Asia in the early and mid-Cretaceous eras.
The Tyrannosaurus rex first appeared in the Late Jurassic period and reached their peak in the Late Cretaceous before becoming extinct during the Cretaceous–Paleogene extinction event. They resided in the western United States and generally measured in at 45-feet long. However, while they might have had a five-foot head, their arms only extended three feet.
This by no means indicates that they had weak limbs. A T. rex could have bench pressed about 400 pounds on each arm and curled 420 — which is still a lot by most standards, but Padian says it didn’t them much good if they couldn’t actually pick anything up with them.
“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,” the researcher explained. “The enormous head and neck are way out in front of them and pretty much form the kind of death machine you saw in ‘Jurassic Park.'”
Padian’s hypothesis has analogies in some predators that roam the Earth today. Like the T-rex, the giant Komodo Dragon lizard of Indonesia hunts in groups, and when it kills prey, the larger dragons converge on the carcass. Maulings can occur, as they do among crocodiles during feeding. The same could be true of T. rex and other tyrannosaurids, though more research is needed. Padian says that a correlation could be found if museum specimens around the world were checked for bite marks. That would be quite a feat of fossil crowdsourcing, he admitted.
“Bite wounds on the skull and other parts of the skeleton are well known in tyrannosaurs and other carnivorous dinosaurs,” Padian said. “If fewer bite marks were found on the reduced limbs, it could be a sign that reduction worked…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.”
It’s an intriguing hypothesis, but we still don’t have any convincing evidence pointing in this direction. For now, the jury is still out on why exactly this fierce dinosaur had such underwhelming arms. In the meantime, we expect the memes to continue as well.