A new study published in the journal Current Biology concludes that the newly discovered species, named Meraxes gigas , has tiny arms, like those of the ferocious Tyrannosaurus rex . The researchers do not relate one dinosaur to another, but each one would have evolved separately to end up having those short limbs. They suggest that the “little arms” could have served them when mating or to support movement.
“The M. gigas fossil shows complete regions of the skeleton, never seen before, such as the 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, director of the project at the Ernesto Bachmann Paleontological Museum in Neuquén, Argentina.
The scientists write in their publication that neither T. rex got its short arms from M. gigas , nor M. gigas from T. rex . The reason is that the two species are very far apart on the evolutionary tree and also that M. gigas became extinct almost 20 million years before T. rex became a species. “There is no direct relationship between the two,” says Canale. Rather, the expert believes that the dwarf arms would give these two large reptiles some sort of survival advantage.
“I am convinced that those proportionally tiny arms had some kind of function. The skeleton shows large muscle attachments and fully developed pectoral girdles, so the arm had strong muscles ,” says Canale. This discovery would indicate that the arms were not made small because they were useless for animals. However, its functions are still unknown.
In previous research, the group concluded that in dinosaurs like M. gigas and T.rex , the larger the animal’s head, the smaller its arms . The little arms were not used for hunting since “actions related to predation were most likely carried out by the head,” argues Canale. “I am inclined to think that their arms were used for other types of activities. “It is possible that they used their arms for reproductive behaviors, such as holding the female during mating or supporting themselves to get back up after a rest or fall,” Add.
Scientists believe that this specimen of M. gigas lived in what is now the northern region of Argentine Patagonia and that it was 45 years old when it died. The reptile would measure about 11 meters long and weigh more than four tons . In addition, he was part of a large family. “The group flourished and reached a peak of diversity shortly before becoming extinct,” says Canale.
The head of M. gigas would have featured ridges, grooves, bumps, and small horns. “These ornamentations appear at a late stage of development, when individuals become adults,” says Canale. The group believes these traits were likely used to attract potential mates. “Sexual selection is a powerful evolutionary force. But since we can’t directly observe its behavior, it’s impossible to be sure,” says Canale.
“The fossil has a lot of new information and is in magnificent shape,” says Canale, who intends to explore it further to discover other questions about the dinosaur.
Referencia: Canale, J.I. et. al. 2022. New giant carnivorous dinosaur reveals convergent evolutionary trends in theropod arm reduction. Current Biology. DOI: 10.1016/j.cub.2022.05.057
