A new species of giant carnivorous dinosaur that had disproportionately small arms was discovered in Argentina.
The discovery of Meraxes gigas marks the third group of huge carnivorous dinosaurs to have evolved in this way.
The creature went extinct 20 million years before other known small-armed predators like T. rex, meaning the small arms must have evolved independently. It remains unclear why.
The discovery, published in Current Biology on Thursday, adds fuel to the debate raging among paleontologists about the function of tiny hands in the giant carnivores, experts told Insider.
“It’s a real recurring pattern among giant carnivores: they curl their arms down,” said Dave Hone, a paleontologist at Queen Mary University of London who was not involved in the report.
Paleontologist and article author Peter Makowicki, shown next to the dinosaur’s femur. Courtesy of Juan Ignacio Canales
Meraxes gigas
Scientists discovered the remains in 2012, but it took ten years to excavate and analyze the huge skeleton, said Juan Ignacio Canale, a study author at the CONICET research institute in Buenos Aires, Argentina.
Canale and his colleagues were on a field survey of the fossil-rich Huincul Formation in northern Patagonia when they came across a fossilized vertebra – part of the spine – as large as a human head.
A map shows the location of the dinosaur. Courtesy of Juan Ignacio Canales
They recognized it as a fossil of a species of carcharodontosaurid, the family of bipedal carnivorous dinosaurs that lived during the Cretaceous period but went extinct 80 to 85 million years ago.
That was about 20 million years before the dinosaur mass extinction that wiped out the T. rex, according to Canale.
Bones found in the excavation of Meraxes gigas are shown in white. Canale and colleagues, Current Biology (2022)
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.
Visual reddening of Meraxes gigas. Carlos Papolio
Little hands, 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 appears to have followed the same pattern: As bipedal predators grew larger, their heads grew, but their hands shrank.
It’s unlikely that three groups of dinosaurs evolved this way for no reason, Hone and Canale said.
“Once is a novelty. Twice it is: ah! Third time? Well, it happens over and over again,” Hone said.
It is possible that their hands turned out to be smaller because predators learned to hunt using only their enlarged heads, leaving their hands redundant.
“Things that are dysfunctional tend to either be reduced or lost,” Hawn said.
But there are signs – which were also seen in Meraxas gigas – that the hands may have been useful for something.
The bones in the arms were still quite strong and had huge ligaments, suggesting they were attached to strong muscles. The overall shape of the hands was consistent over time, Hone said.
One of the vertebrae of Meraxes gigas seen at the excavation site. Courtesy of Juan Ignacio Canales
“They were used to catch something, we don’t know what. Maybe not for predation. Having a skull that’s about a foot and a half, those little hands don’t look like they’re going to be useful for that,” Canale said.
“But maybe for other activities.”
Fossilized phalanx of Meraxes gigas. Courtesy of Juan Ignacio Canales
Tipping a cow? Grappling hooks for mating?
Paleontologists have put forth many hypotheses to explain how large predators like T. rex could use their tiny hands.
Some suggest that the hands were used to help grip a partner during sex or to counter their massive heads during attacks.
Others said that perhaps the arms helped the predator to rise from a fall or to overturn triceratops during hunting (this is called the “cow flip” hypothesis).
For Hone, neither is particularly convincing.
“I fully support the possibility of a mechanical function in these scaled hands. But I want a reason that stands up to even 10 seconds of thought and scrutiny, and I haven’t seen one yet,” he said.
“There are many mysteries in paleontology. This is one of them,” he said.
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