Newly discovered dinosaur had tiny arms like T. Rex

In a new study published by Cell Press, researchers have discovered Meraxes gigas, a new species of dinosaur with short arms just like T. rex. The study demonstrated that T. rex and M. gigas evolved to have tiny arms for functions like mating or movement support.

“The fossil of M. gigas shows never seen before, complete regions of the skeleton, like the arms and legs that helped us to understand some evolutionary trends and the anatomy of Carcharodontosaurids –the group that M. gigas belongs to,” explained  Juan Canale, the project lead at Ernesto Bachmann Paleontological Museum in Neuquén, Argentina.

First, the authors clarified that the T. rex did not get their short arms from M. gigas or vice versa. In fact, these species are very far apart on the evolutionary tree. For instance, M. gigas became extinct almost 20 million years before T. rex even became a species. 

“There is no direct relationship between both,” said Canale. Instead, it is believed that tiny arms may have been a physical advantage to the survival of each species.

“I’m convinced that those proportionally tiny arms had some sort of function. The skeleton shows large muscle insertions and fully developed pectoral girdles, so the arm had strong muscles,” said Canale. In other words, the arms did not shrink because they were useless to the dinosaurs. 

Understanding the functions of these tiny arms is more of a challenge. For dinosaurs like M. gigas and T. rex, the larger their heads were, the smaller their arms became. The study authors also determined that actions related to hunting were more likely performed by the head. 

Using a well preserved fossil, the research team painted a picture of the M. gigas before it died. Living  in the northern Patagonia region of Argentina, the dinosaur was 45 years old, about 11 meters long, and weighed more than four tons. And, it had a big family. 

“The group flourished and reached a peak of diversity shortly before it became extinct,” said Canale. “They may have used the arms for reproductive behavior such as holding the female during mating or support themselves to stand back up after a break or a fall.”

The skull of M. gigas was found to be decorated with crests, furrows, bumps and small hornlets. “Those ornamentations appear late in the development when the individuals became adults,” explained Canale. These features were likely used to attract potential mates. “Sexual selection is a powerful evolutionary force. But given that we cannot directly observe their behavior, it is impossible to be certain about this.”

The research team looks forward to exploring other questions that the M. gigas fossil can help answer. The work was supported by The National Science Foundation of the United States and the National Geographic Society. 

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By Katherine Bucko, Earth.com Staff Writer