Why Are Tyrannosaurus Rex's Arms So Short? UC Berkeley Paleontology Professor Floats New Forelimb Theory
BERKELEY (CBS SF) -- A paleontology professor at the University of California, Berkeley has come up a new answer to a question that has stumped dinosaur experts and armchair observers alike: Why are the Tyrannosaurus rex's forelimbs so ridiculously short?
The question has frequently come up in a freshmen seminar called The Age of Dinosaurs taught by paleontologist Kevin Padian, according to UC Berkeley News. Padian, a distinguished emeritus professor of integrative biology and curator at the school's Museum of Paleontology, has authored a new study appearing in the current issue of the journal Acta Palaeontologia Polonica in which he theorizes that the T. rex's arms evolved to such tiny lengths to prevent accidental or intentional amputation during T. rex feeding frenzies.
Other paleontologists have unearthed evidence that some tyrannosaurids hunted in packs, not singly, as depicted in many paintings and dioramas, according to the Berkeley News report. Padian's report said since T. rex's massive heads and bone-crushing teeth provided all necessary predatory mechanisms, "during group-feeding on carcasses, limb reduction was selected to keep the forelimbs out of the way of the jaws of large conspecific predators, avoiding injury, loss of blood, amputation, infection, and death."
Instead of what many observers would theorize on what T. rex's short arms evolved to do, Padian said the question should be what benefit those arms were for the whole animal.
"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," Padian told Berkeley News. "So, it could be a benefit to reduce the forelimbs, since you're not using them in predation anyway."
With a 45-foot length and a 5-foot-long skull, a T. rex's 3-foot-long arms would be the equivalent of a 6-foot human with 5-inch arms. Other hypotheses to explain the short limbs include their use to hold onto a female in place during mating, holding or stabbing prey, or tipping over a Triceratops.
Padian says the T. rex predecessors had longer arms, so there must be a reason that they became reduced in both size and joint mobility.
"All of the ideas that have been put forward about this are either untested or impossible because they can't work," Padian told Berkeley News. "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 every case, all of the proposed functions would have been much more effective if the arms had not been reduced."
Padian acknowledged that his hypothesis would not be the last word on the mystery of the undersized appendages, but said the theory could be strengthened by finding correlations in fossil bite marks in museum specimens around the world.
"The testing will require many objective eyes and hands, working with specimens in collections, with field notes, and in quarries, because the animals in question are found widely around the world, and data will need to be compiled and tested against expectations," the research paper concluded. "It will require a kind of paleontological crowd-sourcing, but the effort will be worth it."