Meraxes gigas dinosaur had a big body and tiny arms like T. Rex

A dinosaur discovered in Argentina has been identified as a new species. Meraxes gigas lived 20 million years before T. rex, but had a similarly large body, sharp teeth and proportionately small arms

By Corryn Wetzel

A relatively recently discovered dinosaur species, Meraxes gigas, had a giant body and small forelimbs like Tyrannosaurus rex. It appears that both species evolved their proportionately tiny arms independently.

The dinosaur was unearthed in northern Patagonia, Argentina, in 2012, and since then paleontologists have been carefully preparing and examining the skeleton. Luckily, the fossil was “very complete and exquisitely preserved”, says Juan Ignacio Canale at the Ernesto Bachmann Paleontological Museum in Villa El Chocón, Argentina, who was one of the researchers that found the fossil. They compared the bones with other known species before concluding it was clear that it represented a new species, he says.

Like T. rex, M. gigas had sharp teeth, possessed a large head and tail and walked on two powerful legs. “For a long time, we thought it was mostly tyrannosaurs that did the big head, long legs, small arms thing,” says James Napoli at the American Museum of Natural History in New York, who wasn’t involved in the research. This fossil reveals that another group of dinosaurs called carcharodontosaurs, which includes M. gigas, was also doing the same kind of evolutionary trend, he says.

The researchers estimate that M. gigas was 11 metres long and weighed around 4 tonnes. The species had a skull adorned with crests, bumps and small horns, and the individual that was recovered appears to have died at around 45 years of age, around twice the typical lifespan of T. rex.

M. gigas roamed the planet during the Late Cretaceous Epoch around 95 million years ago – roughly 20 million years before T. rex – and the dinosaurs are very distant from each other on the evolutionary tree. This indicates that small forelimbs on rather large dinosaurs evolved independently at least twice.

“Those proportionally tiny arms had some sort of function,” says Canale. From the fossil, the researchers could tell that the arm muscles of M. gigas were well developed, suggesting they were put to use. Canale thinks they may have used their arms to hold a partner during mating or to help right themselves after a fall.

Finding M. gigas serves as a reminder of how limited our understanding of dinosaurs is, says Napoli. “Every new, weird, unexpected dinosaur is really important in reminding us just how much is left to be discovered.”

Journal reference: Current Biology, DOI: http://dx.doi.org/10.1016/j.cub.2022.05.057