Researchers report to have found a new species of dinosaur with unusually short arms, similar to those of T. rex, and they believe this study also explains why T. rex has such small arms.

Tyrannosaurs (including the renowned T. rex) were not the only huge carnivorous dinosaurs with tiny arms. The Meraxes gigas is a newly discovered species of dinosaur with disproportionally short arms that resembles T. rex.

The findings, which were published today in the journal Current Biology, claimed that T. rex and M. gigas acquired tiny arms independently, and proposed several probable roles for the short arms, such as mating or movement support.

The project leader at the Ernesto Bachmann Paleontological Museum in Neuquén, Argentina, Juan Canale, claims that 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.”

The authors clarify that T. rex did not acquire its short arms from M. gigas or vice versa. In addition to going extinct over 20 million years before T. rex evolved into a species, M. gigas and T. rex are also located relatively far apart on the evolutionary tree.

According to Canale, “there is no direct relationship between both”. Instead, Canale thinks that the two dinosaurs’ small arms helped them in some way to stay alive.

“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,” adds Canale.  This suggests that the arms did not get shorter as a result of the dinosaurs’ inability to make use of them. The more difficult question is what the functions were.

Based on research done in the past, the group found that dinosaurs like M. gigas and T. rex had smaller arms the bigger their heads were. As “actions related to predation were most likely performed by the head,” Canale argues, they were undoubtedly useless for hunting.

“I’m inclined to think their arms were used in other kinds of activities,” adds Canale.

The researchers was able to reconstruct this M. gigas’s existence before its demise from the fossil record. The dinosaur, which lived 45 million years ago in what is now Argentina’s northern Patagonia region, was 11 meters long, weighed more than four tons, and was 45 years old. It also had a sizable family.

According to Canale, “the group flourished and reached a peak of diversity shortly before became extinct. 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.”

It was also discovered that M. gigas has crests, furrows, bumps, and minute hornlets decorating its skull. “Those ornamentations appear late in the development when the individuals became adults,” says Canale. 

The group believes the features attracted 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,” explains the author.

“The fossil has a lot of novel information, and it is in superb shape.”

He is excited to find out how the M. gigas fossil can help him answer other questions.

“We found the perfect spot on the first day of searching, and M. gigas was found,” add the author, “It was probably one of the most exciting points of my career.”

Image Credit: Juan I Canale

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