Why did the T-rex have short arms? A New Hypothesis Says “So Other T-Rexes Don’t Bite Them”

Tyrannosaurus rex’s famously tiny arms may have grown so small to protect the predator from accidental — or even intentional — amputations during feeding binges.

Image via Pixabay.

Ask people on the street to name a fearsome dinosaur, and chances are “T-rex” will be mentioned often. Yet this “dinosaur king” (rex means “king” in Latin) had one less than distinguished feature: his ridiculously short arms. Researchers, but also students, pupils or simple dinosaur aficionados, wonder, at one time or another, why this huge dinosaur sported such short arms. A new study attempts to answer that.

According to the findings, T. rex could have evolved to have short arms for occupational health and safety.

Short for a cause

“What if multiple adult tyrannosaurs converge on a carcass? You have a bunch of massive skulls, with incredibly powerful jaws and teeth, ripping and chewing through flesh and bone right next to you. What if your friend over there thinks you’re getting a little too close? They might warn you by cutting off your arm,” said lead author Kevin Padian, distinguished professor emeritus of integrative biology at the University of California, Berkeley and curator at the UC Museum of Paleontology (UCMP).

“So it might be beneficial to cut down the forelimbs, since you’re not using them for predation anyway.”

There is no shortage of hypotheses as to why T. rex arms were so short; these cover everything from mating to catching prey or some other predatory role. But none of these ideas has managed to impose itself on the others, nor to collect particularly important votes. Padian thinks researchers have tackled the question poorly so far. Instead of wondering what the dinosaur’s short arms were for, we should rather think about what advantage these arms were for the animal.

His hypothesis is that T. rexThe arms of have grown so small to prevent accidental or intentional amputation during the feeding frenzy of a pack of these dinosaurs. Serious injuries — such as the loss of a limb to an overly impatient or aggressive pack mate — can lead to life-threatening complications, including bleeding, shock, or infection, Padian says.

He adds that the ancestors of tyrannosaurs had longer arms, suggesting that the transition to shorter limbs with less joint mobility and strength had a purpose, otherwise the trade-off would not have given the animals an evolutionary advantage. For context, a 45-foot-long (13.7 meters) T. rex reportedly has a 5-foot-long (1.5-meter) skull but only 3-foot (0.9-meter) long arms, equivalent to a 6-foot (1.8-meter) tall human having arms 5 inches (12.7 cm).

This change would have affected both T. rexthat lived in North America during the Late Cretaceous, as well as their related species of abelisaurids and carcharodontosaurids, which ranged across South America and Eurasia, respectively, during the early and mid-Cretaceous.

“All the ideas that have been put forward on this are either untested or impossible because they can’t work,” Padian said. “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. And in any case, all the functions offered would have been much more efficient if the weapons had not been reduced.

Padian admits that his hypothesis, along with all others on this subject, would be difficult to prove so long after the extinction of the dinosaurs. Yet, based on recent theories that T. rexis hunted in packs, his hypothesis seems to have merit.

“Several significant quarry sites discovered over the past 20 years preserve both adult and juvenile tyrannosaurs together,” he said. “We can’t really assume that they lived together or even died together. We only know that they were buried together. But when you find multiple sites with the same animals, it’s a stronger signal. And the possibility, which other researchers have already mentioned, is that they hunted in groups.

Padian’s hypothesis seems to have analogues today. He cites the giant lizards of the Komodo dragon (Varanus komodoensis) from Indonesia, which hunt in groups. Larger individuals descend on the carcass after a hunt, leaving only modest remains for younger and smaller dragons. Crocodiles can also seriously injure each other while feeding.

One item that could help push the hypothesis forward is checking T. rex fossils in museums around the world for bite marks on their front limbs. That being said, a very small percentage of specimens of a species become fossilized, so we could be looking for a needle in a haystack.

“Bit bites 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, this could be a sign that the reduction worked.”

“What I wanted to do first was to establish that mainstream functional ideas just don’t work. This brings us back to square one. Then we can take an integrative approach, thinking about social organization, feeding behavior and ecological factors apart from purely mechanical considerations.

The article “Why the forelimbs of tyrannosaurs were so short: an integrative hypothesis” was published in the journal Acta Palaeontologica Polonica.

Martin E. Berry