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 tiny arms must have evolved independently. What remains unclear is why. The discovery, published in Current Biology on Thursday, adds fuel to the debate raging among paleontologists about the function of tiny arms for giant predators, experts told Insider. “This is a real recurring pattern among giant carnivores: They shrink the arms down,” said Dave Hawn, a paleontologist at Queen Mary University in London who was not involved in the work. Paleontologist and paper author Peter Makovicky is 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 massive skeleton, said study author Juan Ignacio Canale from the CONICET research institute in Buenos Aires, Argentina. Canale and his colleagues were field surveying the fossil-rich Huincul Formation in northern Patagonia when they came across a fossilized vertebra—a segment of the spine—as large as a human head. A map shows the location where the dinosaur. Courtesy of Juan Ignacio Canales They identified it as a fossil from a type of carcharodontosaurus, the family of bipedal carnivorous dinosaurs that lived throughout 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, per Canale. Bones found in the Meraxes gigas excavation are shown in white. Canale et al., 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.
A visual blush of a Meraxes cigar. Carlos Papolio
Short arms, long mystery
The discovery makes carcharodontosaurs the third group of dinosaurs known to have evolved arms disproportionately short for their large size. The others are tyrannosaurs and abelisaurs. In all three groups, evolution appears to have followed the same pattern: as bipedal predators grew, their heads grew but their hands shrank. It’s unlikely that three groups of dinosaurs evolved like this for no reason, Hone and Canale said. “Once is a novelty. Twice is: huh! Third time? OK, this happens over and over again,” Hawn said. It is possible that their hands ended up smaller because predators learned to hunt using only their enlarged heads, leaving their hands unnecessary. “Things that are non-functional tend to either diminish or be lost,” Hawn said. But there are signs — seen in Merax’s gigas — that the arms might still be good for something. The bones in the arms tended to be still quite strong and have massive ligamentous interference, suggesting that they were attached to strong muscles. The overall shape of the guns has been consistent over time, Hone said. One of the skeletons of Meraxes gigas was seen at the excavation site. Courtesy of John Ignatius Channels “They were used to grab something, we don’t know what. Maybe not for prey. Having a skull about six feet long, those little arms don’t seem to be useful for that,” Canale said. “But maybe for other activities.” A fossil phalanx of Meraxes gigas. Courtesy of John Ignatius Channels
Cow-tip? Mating hook?
Paleontologists have put forward many hypotheses to explain how large predators like T. rex could use their tiny hands. Some have suggested that the arms were used to help grip a mate during sex, or to balance their massive heads during attacks. Others said that perhaps the arms helped the predator to get up from a fall or to topple the triceratops during the hunt (this is called the “cow tipping” hypothesis). For Hone, none of this is particularly convincing. “I’m all for the possibility of a mechanical function in these reduced arms. But I want a reason that stands up to even 10 seconds of thought and consideration, and I haven’t seen one yet,” he said. “There are many mysteries in paleontology. This is one of them,” he said.
