A new species of tiny-armed dinosaur, measuring 36 feet long, has been identified in Argentina.
It is the third type of dinosaur to independently evolve to have tiny arms.
Experts still don’t know what the tiny arms were for, but they are more certain than ever that they weren’t a fluke.
A new species of giant carnivorous dinosaur that had disproportionately small arms has been discovered in Argentina.
The discovery of Meraxes gigas marks the third group of huge carnivorous dinosaurs to have evolved in this way.
The creature disappeared 20 million years before other known small-armed predators like T. rex, meaning the tiny arms must have evolved independently. What is not clear is why.
The discovery, published in Current Biology on Thursday, adds fuel to a debate raging among paleontologists about the function of tiny arms for giant predators, experts told Insider.
“This is a pattern that really repeats itself among giant carnivores: they’re shrinking their arms,” said Dave Hone, a paleontologist at Queen Mary University of London who was not involved in the study.
Scientists discovered the remains in 2012, but it took 10 years to excavate and analyze the huge skeleton, said Juan Ignacio Canale, an author of the study from the CONICET research institute in Buenos Aires, Argentina.
Canale and his colleagues were on a field study of the fossil-rich Huincul Formation in northern Patagonia when they came across a fossilized vertebra, a section of spinal column, as large as a human head.
They recognized it as a fossil of a type of carcharodontosaurid, the family of carnivorous, two-legged dinosaurs that lived throughout the Cretaceous period but became extinct between 80 and 85 million years ago.
This was about 20 million years before the mass extinction of dinosaurs that wiped out T. rex, according to Canale.
The dinosaur, named Meraxes gigas after a dragon from the “Game of Thrones” books, is a new species of carcharodontosaurid. This skeleton belonged to an adult dinosaur that was about 45 years old, 36 feet long, and weighed more than four tons when it died.
Tiny arms, long mystery
The discovery makes carcharodontosaurids the third group of dinosaurs known to have developed disproportionately small arms for their large size. The others are tyrannosaurids and abelisaurids.
In all three groups, evolution appears to have followed the same pattern: As the two-legged predators grew, their heads grew but their arms shrank.
It’s unlikely that three groups of dinosaurs would evolve like this for no reason, Hone and Canale said.
“Once is a novelty. Twice it’s: hey! Third time? Okay, this is happening over and over again,” Hone said.
Its arms may have ended up being smaller because predators learned to hunt using only its enlarged head, leaving its arms redundant.
“Things that don’t work tend to get reduced or lost,” Hone said.
But there are signs, also seen in Meraxas gigas, that the arms may still have served a purpose.
The arm bones tended to still be quite strong and had huge ligamentous attachments, suggesting that they were attached to strong muscles. The overall shape of the arms was consistent over time, Hone said.
“They were used to grab something, we don’t know what. Maybe not for predation. Having a skull of about five feet, these little arms don’t seem to be for that,” Canale said.
“But maybe for other activities.”
Tip shots? Grappling hooks for mating?
Paleontologists have come up with many hypotheses to explain how large predators like T. rex might use their tiny arms.
Some have suggested that the arms were used to help grip a partner during sex or to counteract their huge heads during attacks.
Others said that perhaps the arms helped the predator to get up from a fall, or to knock down triceratopses during the hunt (this is called the “leaning cow” hypothesis).
To Hone, none of these are particularly compelling.
“I am all for the possibility of a mechanical function in these shrunken arms. But I want a reason that will withstand even 10 seconds of thought and scrutiny and I have yet to see one,” he said.
“There are many mysteries in paleontology. This is one of them,” he said.
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