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Huge new carnivorous dinosaur with tiny arms like T. rex discovered

Meraxes Close Up
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Meraxes Gigasa newly discovered species of dinosaur with disproportionately short arms t rex. Photo credit: Jorge A. González

Meraxes Gigas – a new species of dinosaur with disproportionately short arms was discovered just like that t rex called the Meraxes Gigas.

Tyrannosaurs (like the infamous t rex) isn’t the only group of gigantic carnivorous dinosaurs with tiny arms. In fact, paleontologists have just discovered a new species of dinosaur with disproportionately short arms t rex called the Meraxes Gigas. The results published in the journal Current Biology today (July 7), argued that t rex and M.Gigas designed to have tiny arms fully independently and identified several potential functions for the short arms, such as mating or movement support.

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

First, to be clear, the authors say t rex didn’t get their short arms off M.Gigas or the other way around. Not only M.Gigas became extinct almost 20 million years ago t rex become one species, but they are also very far apart on the evolutionary tree. “There is no direct relationship between the two,” says Canale. Rather, Canale believes that the tiny arms somehow gave the two dinosaurs a survival advantage.

Meraxe

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

“I am convinced that these proportionally tiny arms had some function. The skeleton shows large muscle attachments and fully developed pectoral girdles, so the arm had strong muscles,” says Canale. This shows that the arms didn’t shrunk because they were useless to the dinosaurs. The harder question is what exactly the features were.

From previous studies, the research group found that for dinosaurs like M.Gigas and t rex, the larger their heads were, the smaller their arms became. They were definitely not suited to hunting, since “predator-related actions were most likely performed by the head,” argues Canale.

“I’m inclined to believe that their guns were used for other activities,” says Canale. From the fossil record, the team was able to draw a picture of its life M.Gigas before it died. The dinosaur, living in what is now Argentina’s Northern Patagonia region, was 45 years old, about 11 meters long and weighed more than four tons. And it had a big family. “The group thrived and reached a peak in diversity just before their extinction,” says Canale. “They may have used the arms for reproductive purposes, such as holding the female during mating or supporting themselves to get up after a pause or a fall,” Canale adds.

Excavation site of a giant carnivorous dinosaur

excavation site of Meraxes Gigas. Photo credit: Juan I Canale

The team also found that the skull of M.Gigas was decorated with crests, furrows, bumps and small hornets. “These ornaments appear late in development as individuals mature,” says Canale. The group believes the traits were likely used to attract potential mates. “Sexual selection is a powerful evolutionary force. But since we can’t observe their behavior directly, it’s impossible to be sure about it,” says Canale.

“The fossil contains a lot of new information and is in excellent condition,” says Canale. He looks forward to pursuing other questions that the M.Gigas Fossil can help him with the answer. “We found the perfect place on the first day of searching and M.Gigas found,” says Canale, “that was probably one of the most exciting parts of my career.”

References: “New giant carnivorous dinosaur reveals convergent 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 United States National Science Foundation and the National Geographic Society.

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