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Justice for T. Rex: Scientists Unearth Another Giant Dinosaur With Tiny Arms

A depiction of the carnivorous M. gigas.
An artist's illustration of what M. gigas looked like, billions of years ago.
Carlos Papolio

What's happening

In Argentina, a crew of scientists excavated the fossil of a dinosaur with arms comparable to T. rex's miniature limbs.

Why it matters

This adds evidence to the theory that mini dinosaur limbs weren't useless. They may have had some sort of evolutionary advantage.

Despite its colossal skull, towering height and carnivorous appetite, Tyrannosaurus rex gets the unfortunate honor of being a punchline due to one hard-to-miss quirk. "Catch me if you can with your silly little arms, T. rex," says a 10-year-old watching a dino documentary somewhere, probably. 

But thanks to new research published Thursday in the journal Current Biology, we may finally have justice for T. rex's teeny tiny arms. Not only did scientists excavate the fossil of yet another massive, meat-eating dinosaur with mini-upper limbs -- called Meraxes gigas -- but they also consider it to be evidence that small dinosaur arms actually had an important survival advantage long ago.

The team holding a white plaster jacket, with part of the full fossil inside.

The study team transports the fossil in a plaster jacket. 

Apesteguía

"I'm convinced that those proportionally tiny arms had some sort of function," Juan Canale, project lead at Ernesto Bachmann Paleontological Museum in Argentina, said in a statement. "The skeleton shows large muscle insertions and fully developed pectoral girdles, so the [arms] had strong muscles. This means that the arms did not shrink because they were useless to the dinosaurs."

However, as Canale puts it, "the harder question is what exactly the functions were."

New dino specs

Upon analyzing the new fossil, located in the present-day northern Patagonia region of the southern tip of South America, the recovery team concocted a visual image of what M. gigas might have looked like billions of years ago. "The fossil has a lot of novel information, and it is in superb shape," Canale said.

To name a few of its traits, this enormous dinosaur was probably about 45 years old at its time of death, about 11 meters (36 feet) long and more than 4 tons. And it had a rather majestic facade.

An artist's illustration of the head of M. gigas, crests furrows and indentations seen.

This is what M. gigas' head probably looked like. 

Jorge A. Gonzalez

The researchers found fossilized proof that this iteration of M. gigas held a skull decorated with crests, furrows, bumps and small hornlets, which Canale suggests showed up as the reptile went through adolescence and solidified once it became an adult. Possibly, these markings were used to attract potential mates. 

"Sexual selection is a powerful evolutionary force," Canale said. "But given that we cannot directly observe their behavior, it is impossible to be certain about this."

A dusty Argentinian excavation site.

The excavation site in Argentina where the M. Gigas fossil was found.

Juan I. Canale

All things considered, M. gigas looks quite a bit like T. rex, even though it descends from a totally separate dinosaur classification called -- prepare for a mouthful -- Carcharodontosauridae. And this brings us to perhaps the most striking part of the team's finding. 

M. gigas and T. rex never, ever interacted with each other. We can be sure of that because M. gigas became extinct 20 million years before T. rex even walked the Earth. 

Thus, the two dinos must have developed their unique limbs independently, which underscores the theory that their miniature arms had a special purpose. Evolutionary advantages tend to turn up at different points in history because they have benefits that permeate species across timelines. 

Though it's still speculation, some ideas Canale has for the scaled-down arm trait are these dinosaurs "may have used the arms for reproductive behavior, such as holding the female during mating, or [to] support themselves to stand back up after a break or a fall." In other words, maybe it was easier for T. rex or M. gigas to embrace their mates with a petite upper frame or swiftly push themselves off the ground after face-planting. 

For strategic movements, bigger isn't always better. 

Going forward, the researchers hope to continue studying the skeleton of M. gigas to get to the bottom of the small arm saga. And even further down the line, the fossil could help solve many other outstanding paleontological mysteries. "We found the perfect spot on the first day of searching, and M. gigas was found," Canale said. "It was probably one of the most exciting points of my career."