A new species of giant carnivorous dinosaur with disproportionate arms has been discovered in Argentina.
The creature died out 20 million years before other well-known small-armed predators like the T. rex, meaning the small arms must have evolved independently. What remains unclear is why.
“It’s a real pattern repeated in giant carnivores: they shrink their arms,” said Dave Hone, a paleontologist at Queen Mary University in London who was not involved in the article.
Scientists discovered the remains in 2012, but it took ten years to dig and analyze the huge skeleton, said study author Juan Ignacio Canale from the CONICET research institute in Buenos Aires, Argentina.
Canale and his colleagues were doing a field investigation in the fossil-rich Huincul Formation in northern Patagonia when they came across a fossilized vertebra, a section of the spine as big as a human head.
They recognized it as a fossil of a type of carcharodontosaurids, the family of two-legged carnivorous dinosaurs that lived throughout the Cretaceous period but died out 80 to 85 million years ago.
That was about 20 million years before the mass dinosaur extinction that wiped out the 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.
This discovery makes carcharodontosaurids the third group of dinosaurs known to have evolved disproportionately small arms for their large size. The others are tyrannosaurids and abelisaurids.
Three groups of dinosaurs are unlikely to evolve like this for no reason, Hone and Canale said.
“Once is a novelty. Twice is: huh! The third time? Okay, it happens over and over again,” Hone said.
It is possible that their arms became smaller because the predators learned to hunt using only their enlarged heads, leaving their arms redundant.
“Things that aren’t functional tend to get reduced or lost,” Hone said.
But there are signs that were also seen in Meraxas gigas – that the weapons may still have had some use.
The arm bones tended to still be quite strong and had huge ligamentous insertions, suggesting that they were attached to strong muscles. The general shape of the weapons 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 a meter and a half, those little arms don’t seem useful for that,” Canale said.
“But maybe for other activities.”
Rock the cow? Grapples for mating?
Some have suggested that the arms were used to help grab a mate during sex or to counterbalance their massive heads when attacking.
Others have said that the arms may have helped the predator to get up from a fall or to knock the triceratops over during the hunt (this is called the “cow tipping” hypothesis).
For Hone, none of them are particularly convincing.
“I’m all for the possibility of mechanical function in these shrunken arms. But I want a reason that stands up to even 10 seconds of thought and scrutiny and I haven’t seen one yet.” he declared.
“There are a lot of mysteries in paleontology. This is one of them,” he said.