Meraxes gigas — a new species of dinosaur was discovered with disproportionately short T. rex-like arms called Meraxes gigas. Tyrannosaurs (like the infamous T. rex) aren’t the only group of giant carnivorous dinosaurs with tiny arms. In fact, paleontologists have just discovered a new species of dinosaur with disproportionately short T. rex-like arms called Meraxes gigas. The findings, published in the journal Current Biology today (July 7), argued that T. rex and M. gigas evolved to have tiny arms completely independently and identified several possible functions for the short arms, such as mating or support movement. “The M. gigas fossil shows never-before-seen whole regions of the skeleton, such as arms and legs that helped us understand some evolutionary trends and the anatomy of Carcharodontosaurids – the group to which M. gigas belongs,” says Juan Channel. , head of the project at the Ernesto Bachmann Paleontological Museum in Neuquén, Argentina. First, to set the record straight, the authors say that T. rex did not get their short arms from M. Gigas or vice versa. Not only did M. gigas go extinct nearly 20 million years before T. rex became a species, but on the evolutionary tree, they are also very far apart. “There is no direct relationship between the two,” says Canale. Instead, Canale believes that having tiny hands somehow gave the two dinosaurs some kind of survival advantage. Meraxes gigas is a giant carnivorous dinosaur. Credit: Carlos Papolio “I’m convinced that those proportionally tiny arms had some kind of function. The skeleton shows large muscle insertions and fully developed pectoral girdles, so the hand had strong muscles,” says Canale. This shows that the arms did not shrink because they were useless to the dinosaurs. The more difficult question is what exactly the functions were. From previous studies, the research team found that for dinosaurs like M. gigas and T. rex, the bigger their heads got, the smaller their hands got. They were certainly not useful for hunting, as “actions related to hunting were probably performed by the head,” Canale argues. “I tend to think that their hands were used in other kinds of activities,” Canale says. From the fossil record, the team was able to paint a picture of the life of this M. gigas before it died. Living in what is now Argentina’s northern Patagonia region, the dinosaur was 45 years old, about 11 meters long and weighed more than four tons. And, he had a big family. “The group flourished and reached a peak of diversity just before it disappeared,” says Canale. “They may have used the arms for reproductive behavior, such as holding the female during mating or propping herself up after a break or fall,” adds Canale. Excavation site of Meraxis Gigas. Credit: Juan I Canale The team also discovered that the skull of M. gigas was decorated with ridges, grooves, bumps and small hornlets. “These ornaments appear late in development, when individuals have reached adulthood,” says Canale. The team believes the traits were probably used to attract potential mates. “Sexual selection is a powerful evolutionary force. But since we can’t directly observe their behavior, it’s impossible to be sure of that,” says Canale. “The fossil has a lot of new information and is in great shape,” says Canale. He looks forward to exploring other questions that the M. gigas fossil can help him answer. “We found the perfect spot on the first day of the search and M. gigas was found,” says Canale, “It was probably one of the most exciting spots of my career.” Reference: “New giant carnivorous dinosaur reveals convergent evolutionary trends in theropod arm reduction” by John I. Canale, Sebastian Apesteguia, Paul A. Gallina, Jonathan Mitchell, Nathan D. Smith, Thomas M. Cullen, Akiko Shinya, Alexander Haluza, Federico A. Gianechini, Peter J. Makovicky, 7 July 2022, Current Biology.DOI:10.1016/j.cub.2022.05.057 This work was supported by the United States National Science Foundation and the National Geographic Society.
