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Discovery of a new giant carnivorous dinosaur with tiny arms like the T. rex

Meraxes close up

Meraxes gigasa newly discovered species of dinosaur with disproportionately short arms, just like T. rex. 1 credit

Meraxes gigas — a new species of dinosaur has been discovered with disproportionately short arms, just like T. rex called the Meraxes gigas.

Tyrannosaurs (like the infamous T. rex) is not the only group of gigantic carnivorous dinosaurs with tiny arms. Indeed, paleontologists have just discovered a new species of dinosaur with disproportionately short arms just like T. rex called the Meraxes gigas. The results, published in the journal Current biology today (July 7), argued that T. rex and Mr gigas evolved to have completely independent tiny arms and identified several potential functions for short arms such as coupling or motion support.

“The Fossil of Mr gigas shows never-before-seen, complete skeletal regions, such as arms and legs that have helped us understand some evolutionary trends and the anatomy of Carcharodontosaurids – the group that Mr gigas belongs,” says Juan Canale, project manager at the Ernesto Bachmann Paleontological Museum in Neuquén, Argentina.

First, to set the record straight, the authors say that T. rex haven’t gotten their arms short of Mr gigas or vice versa. Not only Mr gigas disappeared nearly 20 million years before T. rex have become a species, but on the evolutionary tree they are also very far apart. “There is no direct relationship between the two,” says Canale. On the contrary, Canale believes that having tiny arms somehow provided the two dinosaurs with some sort of survival advantage.


Meraxes gigas is a giant carnivorous dinosaur. Credit: Carlos Papolio

“I am convinced that these proportionately tiny arms had some sort of function. The skeleton shows large muscle insertions and fully developed pectoral girdles, so the arm 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 were the functions.

From previous studies, the research group established that for dinosaurs like Mr gigas and T. rex, the larger their head, the smaller their arms became. They certainly weren’t useful for hunting, as “predation-related actions were most likely performed by the head,” says Canale.

“I tend to think their arms were used in other types of activities,” says Canale. From the fossil record, the team was able to paint a picture of the life of this Mr gigas before his death. Living in what is now the northern Patagonia region of 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 died out,” says Canale. “They may have used the arms for reproductive behavior, such as holding the female during mating or supporting themselves to get up after a break or a fall,” adds Canale.

Excavation site of giant carnivorous dinosaurs

Excavation site of Meraxes gigas. 1 credit

The team also discovered that the skull of Mr gigas was decorated with ridges, furrows, bumps and small hornets. “These ornaments appear late in development when individuals have become adults,” says Canale. The group believes the features were likely used to attract potential partners. “Sexual selection is a powerful evolutionary force. But since 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 condition,” says Canale. He is eager to explore questions other than the Mr gigas fossil can help him answer. “We found the perfect place on the first day of the search, and Mr gigas was found,” says Canale, “It was probably one of the most exciting moments of my career.

Reference: “A New Giant Carnivorous Dinosaur Reveals Converging Evolutionary Trends in Theropod Arm Reduction” by Juan I. Canale, Sebastián Apesteguía, Pablo A. Gallina, Jonathan Mitchell, Nathan D. Smith, Thomas M. Cullen, Akiko Shinya , Alejandro 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 US National Science Foundation and the National Geographic Society.

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