Meraxes gigas, a newly discovered dinosaur species with disproportionately short arms just like T. rex. Credit: Jorge A Gonzalez
Meraxes gigas — A new species of dinosaur has been discovered with disproportionately short arms just like T. rex, called Meraxes gigas.
Tyrannosaurs (like the infamous T. rex) weren’t the only group of giant carnivorous dinosaurs with small arms. In fact, paleontologists have just discovered a new species of dinosaur with disproportionately short arms just like T. rex, called Meraxes gigas. The findings, published in the journal Current Biology today (July 7), argue that T. rex and M. gigas evolved to have small arms completely independently, and identify several potential functions for the short arms, such as mating or aiding locomotion.
“The M. gigas fossil shows never-before-seen complete sections of the skeleton, such as arms and legs, which helped us understand some evolutionary trends and the anatomy of Carcharodontosaurids, the group to which M. gigas belongs,” says Juan Canale, project leader at The Ernesto Bachmann Paleontological Museum in Neuquen, Argentina.
First, to get the truth straight, the authors say that T. rex did not get its short arms from M. gigas or vice versa. Not only did M. gigas go extinct almost 20 million years before T. rex became a species, but they are also very far apart on the evolutionary tree. “There is no direct connection between the two,” says Canale. Rather, Canale believes that having small hands somehow gave the two dinosaurs some sort of survival advantage.
Meraxes gigas is a giant carnivorous dinosaur. Credit: Carlos Papolio
“I’m convinced that those proportionally small hands had some function. The skeleton shows large muscle insertions and fully developed pectoral girdle, so the arm has strong muscles,” says Canale. This shows that arms did not shrivel because they were useless to dinosaurs. The more difficult question is what exactly the functions were.
From past 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 definitely not useful for hunting, as “actions related to predation were most likely performed by the head,” Canale argues.
“I tend to think that their weapons 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 shortly before it disappeared,” says Canale. “They may have used the arms for reproductive behavior, such as holding the female during mating or supporting themselves to stand up after a break or fall,” adds Canale.
Excavation site of Meraxes gigas. Credit: Juan I Canale
The team also found that the skull of M. gigas was decorated with crests, furrows, ridges and small horns. “These ornaments appear late in development as individuals become adults,” says Canale. The group believes the features were likely used to attract potential mates. “Sexual selection is a powerful evolutionary force. But given that we can’t directly observe their behavior, it’s impossible to be sure,” says Canale.
“The fossil contains a lot of new information and is in superb shape,” says Canale. He looks forward to exploring other questions that the M. gigas fossil can help answer. “We found the perfect spot on the first day of searching and M. gigas was found,” says Canale, “It was probably one of the most exciting points of my career.”
Reference: “New Giant Carnivorous Dinosaur Reveals Convergent Evolutionary Trends in Theropod Arm Reduction” by Juan I. Canale, Sebastian Apesteguia, Pablo A. Galina, Jonathan Mitchell, Nathan D. Smith, Thomas M. Cullen, Akiko Shinya, Alejandro Haluza , Federico A. Gianechini, Peter J. Makovicky, 7 Jul 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.
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