Fossils

This big dino had tiny arms before T. rex made them cool

Short but burly forelimbs may have helped Meraxes gigas with mating

No, the massive predatory dinosaur in this artist’s rendering is not a T. rex. It’s Meraxes gigas. It, too, had a remarkably big head and tiny, muscular arms. What those arms were for remains a mystery.

Carlos Papolio (CC BY-SA)

By Carolyn Gramling

August 4, 2022 at 6:30 am

The tiny arms on Tyrannosaurus rex have launched a thousand sarcastic memes. I love you this much, goes one of them. And then there’s: Can you pass the salt? (Of course, it can’t.) But T. rex wasn’t the only dino to have such strangely short upper limbs. It wasn’t even the first. Another big-headed, short-armed carnivore stalked Earth tens of millions of years earlier. It was also a continent away, in what’s now Argentina.

Meet Meraxes gigas. Scientists whimsically named this newfound species for a dragon in George R.R. Martin’s A Song of Ice and Fire series. (Game of Thrones was the first book in that series). This new dino shows that tiny arms alongside giant heads evolved independently in different dinosaur lines. Indeed, M. gigas went extinct nearly 20 million years before T. rex walked on Earth.

This earlier dino rose to dominate its landscape between 100 million and 90 million years ago, notes Juan Canale. He’s a paleontologist in Buenos Aires. He works as part of Argentina’s CONICET research network. And though M. gigas looks a lot like T. rex, the earlier one was no tyrannosaur. It belonged to a distantly related group of less-well-known predatory theropods.

The M. gigas fossil skeleton that Canale and his colleagues studied appears to have been about 45 years old at the time it died. They estimate the animal had weighed more than four metric tons (4.4 U.S. short tons). Its formidable body spanned some 11 meters (36 feet). A host of crests and bumps and tiny hornlets topped its head. These ornaments likely evolved to help attract mates, Canale’s team suspects. They described the beast July 7 in Current Biology.

Why these dinosaurs had such tiny arms remains a mystery. They weren’t for hunting: Both T. rex and M. gigas used their massive heads to hunt prey. The arms may have shrunk so they were out of the way during group feeding frenzies.

But, Canale notes, M. gigas’ arms were surprisingly muscular. That suggests to him that they were more than just an inconvenience. One possibility is that the arms helped lift the animal up from a reclining position. Another is that they aided in mating — perhaps showing a mate some love.