In Argentina, a new species of giant carnivorous dinosaur was discovered that had disproportionately small arms.
The discovery of Meraxes gigas marks the third group of huge carnivorous dinosaurs that evolved in this way.
The creature disappeared 20 million years earlier than other small-armed predators known as T. rex, meaning that the tiny arms must have evolved independently. What is still unclear is why.
The discovery, published Thursday in Current Biology, adds fuel to the debate among paleontologists about the function of tiny arms for giant predators, experts told Insider.
“This is a true pattern repeated among giant carnivores: they are lowering their arms down,” said Dave Hone, a paleontologist at Queen Mary University in London who did not participate in the paper.
Paleontologist and author Peter Makovicky is shown next to the dinosaur’s femur. Courtesy of Juan Ignacio Canales
Giggage mirages
Scientists discovered the remains in 2012, but it has taken ten years to excavate and analyze the huge skeleton, said Juan Ignacio Canale, author of the study at the CONICET research institute in Buenos Aires, Argentina.
Canale and his colleagues were in a field survey of the fossil-rich Huincul Formation in northern Patagonia when they encountered a fossilized vertebra, a section of the spine, as large as a human head.
A map shows the location of the dinosaur. Courtesy of Juan Ignacio Canales
It was recognized as a fossil of a type of carcharodontosaurid, the family of two-legged carnivorous dinosaurs that lived throughout the Cretaceous period, but became extinct between 80 and 85 million years ago.
That was about 20 million years before the mass extinction of the dinosaurs that wiped out the T. rex, according to Canale.
The bones found in the Gera Meraxes excavation are shown blank. Channel and colleagues, Current biology (2022)
The dinosaur, named Giga Meraxes in honor of 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.
A visual redness of a gigantic Meraxes. Carlos Papolio
Tiny arms, long mystery
The discovery makes carcharodontosaurids the third group of dinosaurs known to have evolved arms disproportionately small for their large size. The others are tyrannosaurids and abelisaurids.
In all three groups, evolution seems to have followed the same pattern: as two-legged predators grew larger, their heads grew but their arms shrank.
Three groups of dinosaurs are unlikely to evolve like this for no reason, Hone and Canale said.
“Once is a novelty. Two is: eh! Third time? Okay, this happens over and over again,” Hone said.
It is possible that their arms ended up smaller because predators learned to hunt using only their enlarged head, leaving their arms redundant.
“Things that aren’t functional tend to shrink or get lost,” Hone said.
But there are signs – which were also seen in Meraxas gigas – that the arms could still have been used for something.
The bones of the arms tend to be still quite strong and have large ligament insertions, suggesting that they were attached to strong muscles. The overall shape of the weapons has been constant over time, Hone said.
At the site of the excavation was one of the backbones of the giant Meraxes. Courtesy of John Ignatius Channels
“They were used to grab something, we don’t know what. Maybe not for predation. Having a skull about a meter and a half long, those little arms don’t seem to serve them for that,” Canale said.
“But maybe for other activities.”
A fossilized phalanx of Meraxes gigas. Courtesy of John Ignatius Channels
Cow donation? Hooks for mating?
Paleontologists have put forward many hypotheses to explain how large predators such as T. rex could use their tiny arms.
Some have suggested that the arms were used to help grab a partner during sex, or to counteract their massive heads during attacks.
Others said that perhaps the arms helped the predator to rise after a fall, or to overthrow triceratopsis during hunting (this is called the “cow’s mouth” hypothesis).
For Hone, none of them are especially convincing.
“I am totally in favor of the possibility of a mechanical function in these reduced arms. But I want a reason that resists even 10 seconds of thought and scrutiny and I have not seen any yet,” he said.
“There are a lot of mysteries in paleontology. This is one of them,” he said.