T. rex’s short arms may have reduced the risk of bites during binge eating

T. rex is probably the most famous dinosaur of all, but for years a key question about its anatomy has lingered — why were its arms so short?

Now an American paleontologist has claimed that the fearsome creature has evolved stumpy arms to keep it out of the way when it gorges.

Short arms may have reduced the risk of being bitten by other hungry T. rex adults while devouring a carcass.

The tiny arms of the otherwise mighty Tyrannosaurus rex are one of paleontology’s greatest and enduring mysteries.

For example, a 45-foot T. rex might have a five-foot skull but arms only three feet long—equivalent to a 6-foot human with 5-inch arms.

T. rex and other tyrannosaurids first appeared in the late Jurassic and peaked in the late Cretaceous before becoming extinct around 65.5 million years ago.

The new study was led by paleontologist Kevin Padiana, a professor at the University of California, Berkeley, and curator at the UC Museum of Paleontology (UCMP).

Professor Padian noted that the tyrannosaurid ancestors had longer arms, so there must have been a reason they were reduced in both size and joint mobility.

The answer came to him after other paleontologists found evidence that some tyrannosaurids hunted in packs, not individually as depicted in many paintings and dioramas.

“What if several adult tyrannosaurs converged on a carcass?” he said. “You have a bunch of massive skulls with incredibly powerful jaws and teeth that tear and crush flesh and bone right next to you.

“What if your friend over there thinks you’re getting a little too close? They could warn you by cutting off your arm,’ he said.

“So it might be an advantage to reduce the front legs since you don’t use them for hunting anyway.”

When he taught a freshman seminar called The Age of Dinosaurs, Professor Padiana was always asked by students why T. rex’s arms were so ridiculously short, but the answer was always, “Nobody knows.”

When the great dinosaur hunter Barnum Brown discovered the first T. rex fossils in 1900, he thought the arms were too small to be part of the skeleton.

Back then, his colleague Henry Fairfield Osborn, who described and named T. rex, hypothesized that the short arms might have been “breast clamps” — limbs that hold the female in place during copulation.

This is analogous to the pelvic clasps of some sharks and rays, which are modified fins.

But Osborn provided no evidence, and Professor Padian has determined that the T. rex’s arms are too short to bypass another T. rex, and certainly too weak to exert any control over a partner.

For more than a century, other proposed explanations for the short arms have included waving for mate attraction or social signals, serving as an anchor to allow T. rex to get up off the ground, hold down prey, stab enemies, and even poke over a nocturnal sleeping Triceratops, like cow butts.

Also, some paleontologists have suggested that the arms have no function at all, so let’s ignore them.

In his paper, Professor Padiana proposes his new hypothesis – T. rex arms were shrunk to prevent accidental or deliberate amputation when a pack of T. rex fell on a carcass with their massive heads and bone-crushing teeth.

Perhaps in the few million years that the species has existed, the arms have shrunk to get out of the way when feeding in packs.

Juvenile T. rex in particular would have been wise to wait until the larger adults had finished eating.

This is similar to the largest extant species of lizard, the giant Komodo dragon lizard (Varanus komodoensis) from Indonesia.

This extant species hunts in groups, and when killing prey, the larger dragons converge on the carcass, leaving the remains for the smaller ones.

Professor Padiana has admitted that 66 million years after the last T. rex went extinct, any hypothesis, including his, will be difficult to support.

“Several important quarry sites excavated over the past 20 years keep adult and juvenile tyrannosaurs together,” he said.

“We can’t really assume that they lived together or even died together. All we know is that they were buried together.

“But if you find multiple sites with the same animals, that’s a stronger signal. And the possibility that other researchers have raised is that they hunted in groups.’

Further research could reanalyze the fossils of T. rex arms for bite marks.

‘Bite wounds on the skull and other parts of the skeleton are known to occur in tyrannosaurs and other carnivorous dinosaurs,’ Professor Padiana said.

“If fewer bite marks were found on the reduced limbs, this could be a sign that the reduction worked.”

The study was published in the journal Acta Palaeontologia Polonica.