Gigera meraxes, a recently discovered dinosaur species with disproportionately short arms like the T. rex. Credit: Jorge A González
Giant meraxes: A new species of dinosaur has been discovered with disproportionately short arms like the T. rex called Meraxes gigas.
Tyrannosaurs (like the infamous T. rex) are not the only group of giant carnivorous dinosaurs with tiny arms. In fact, paleontologists have just discovered a new species of dinosaur with disproportionately short arms like the T. rex 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 potential functions for short arms, such as l ‘pairing or movement support.
“The fossil of M. gigas shows entire regions of the skeleton never seen before, such as the arms and legs that helped us understand some evolutionary trends and the anatomy of the Carcharodontosaurids, the group to which M. gigas belongs,” he says. Juan Canale. , the project leader at the Ernesto Bachmann Paleontological Museum in Neuquén, Argentina.
First, to clarify the record, the authors say that the T. rex did not obtain its short arms from M. gigas or vice versa. M. gigas not only became extinct almost 20 million years before T. rex became a species, but in the evolutionary tree they are also very distant. “There’s no direct relationship between the two,” Canale says. Rather, Canale believes that having small arms somehow provided the two dinosaurs some sort of survival advantage.
Meraxes gigas is a giant carnivorous dinosaur. Credit: Carlos Papolio
“I’m convinced those proportionately small arms had some kind of function. The skeleton shows large muscle insertions and fully developed pectoral fissures, so the arm had strong muscles,” says Canale. This shows that the arms did not shrink because they were useless to dinosaurs. The most difficult question is what exactly the functions were.
Based on previous studies, the research group established that for dinosaurs like M. gigas and T. rex, the larger their heads, the smaller their arms became. They were definitely not useful for hunting, as “predation-related actions were most likely performed by the head,” Canale argues.
“I tend to think his arms were used in other types of activities,” Canale says. From the fossil record, the team was able to paint a picture of the life of this M. gigas before he died. Living in the present-day region of northern Patagonia, Argentina, 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 shortly before it became extinct,” says Canale. “He may have used his arms for reproductive behavior, such as holding the female during mating or leaning up to get up after a pause or fall,” Canale adds.
Meraxes gigas excavation site. Credit: Juan I Canale
The team also found that M. gigas ’skull was decorated with crests, grooves, bumps, and small horns. “These ornaments appear late in development when individuals become adults,” says Canale. The group believes the features 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,” Canale says.
“The fossil has a lot of new information and is in excellent shape,” says Canale. He is looking forward to exploring other questions that the M. gigas fossil may help answer. “We found the perfect spot on the first day of research and found M. gigas,” says Canale, “It was probably one of the most exciting points of my career.”
Reference: “The new giant carnivorous dinosaur reveals converging evolutionary trends in the reduction of the arm of theropods” 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, July 7, 2022, Current Biology.DOI: 10.1016 / j.cub.2022.05.057
This work was supported by the National Science Foundation of the United States and the National Geographic Society.