A new hypothesis for the evolution of the T. rex’s short arms was formulated by Kevin Padian, UC Berkeley professor emeritus of integrative biology and curator of paleontology at the UC Museum of Paleontology.

His findings were recently published in the Acta Palaeontologica Polonica journal March 30. Padian suspects that group hunting and feeding habits played a role in the shortening of tyrannosaurs’ forearms over time. For a dinosaur present in North America at the end of the Cretaceous period, shorter limbs meant a lower risk of amputation when these massive creatures descended their large jaws and sharp teeth to devour freshly captured carcasses side-by-side.

“The question people want to know the answer to is why did tyrannosaurs evolve such small arms, or why did the arms shorten so much in the evolution of the tyrannosaur lineage writ large,” said Temerty Chair in Vertebrate Paleontology at the Royal Ontario Museum David Evans in an email. “Dr. Padian tackles the question in this way, and his hypothesis is an interesting one that does make intuitive sense.”

Although there are simple hereditary hypotheses for why the T. rex maintained short arms, Evans noted, Padian tackled the evolutionary question of why tyrannosaurs adapted this trait in the first place.

Padian’s hypothesis took a novel approach from its predecessors, according to a Berkeley News article. Inspired by his students’ questions while teaching the campus freshman seminar “The Age of Dinosaurs” for 20 years, Padian began to rethink existing explanations for T. rex’s short arms, such as social signaling, mating and mobility.

In turn, Padian adopted a holistic standpoint, asking what benefits shorter arms could pose to the dinosaur’s survival — including social and ecological factors. Emerging evidence among the paleontological community that some tyrannosaurs hunted in groups ultimately led to Padian’s breakthrough, the article noted.

Padian drew a comparison to the Komodo dragon to demonstrate the dangers of bite wounds and maulings during group feedings. He hypothesized that the T. rex’s arms shrank to prevent these severe bite wounds subject to infection or hemorrhaging and amputations.

In the article, Padian noted that bite marks present in tyrannosaur fossils could serve as evidence of his hypothesis — if the fossils had fewer bite marks on their reduced forearms, the adaptive trait was likely successful.

Gathering and interpreting tyrannosaur fossil evidence, however, would require crowdsourcing across a variety of museum collections, according to the article.

“Covid has made that very difficult,” Padian said in the email. “Many museums are still closed, or have limited access, including our own.”

Padian recognizes there are still more questions to answer, the article states. Reduced limb sizes for similar carnivorous dinosaurs may be due to different reasons and evolutionary paths.

Ultimately, Padian noted the importance of a holistic perspective when postulating scientific explanations.