Researchers in Argentina have discovered what they say is the oldest-known giant dinosaur and it’s altering the way paleontologists view the evolution of dinosaurs as a whole.

A study recently published in Nature indicates the newly-discovered Ingentia prima is three times the size of the largest previously discovered dinosaur species from the Triassic period and would have weighed up to nine tonnes as an adult.

With this discovery, researchers are now rethinking the evolution of dinosaurs, as they previously believed giant dinosaurs didn’t exist until 25 million years later.

"Before this discovery, gigantism was considered to have emerged during the Jurassic period, approximately 180 million years ago, but Ingentia prima lived at the end of the Triassic, between 210 and 205 million years ago,” Cecilia Apaldetti, researcher at the University of San Juan and the study’s principal author, said in a news release.

For most dinosaurs, gigantism proved to be an evolutionary survival tool, especially among herbivores who could use their size as a form of defence against predators.

The researchers say the Ingentia prima, which loosely translates to “first giant,” was a four-legged herbivore that grew cyclically, meaning it would grow quickly for stretches of time and then stop growing for similar stretches, kind of like a tree.

Cyclic growth was common among dinosaurs of that era, but no other known species exceeded more than three metres in length and 1.8 tonnes in weight.

Researchers also found cavities in the bones of the Ingentia prima, which they say would help lighten the weight of the species and allow it to grow more easily.

"These pneumatic cavities indicate that this new species had highly developed air sacs and a very efficient breathing system, similar to what happens in modern birds, which also helped it to keep its body cool despite its large size,” said Apaldetti.

Despite its size, the Ingentia prima paled in comparison to the 64-tonne giants of the Cretaceous period, when giant dinosaurs ruled the Earth 65 million years ago.

"This finding gives a new look at the magnitude of the evolutionary explosion that the dinosaurs had," said Diego Pol, a paleontologist at the Egidio Feruglio Museum in Argentina. “This evolutionary explosion was so great that it explains the success the dinosaurs had during the rest of their era.”

Paleontologists made the discovery in Balde de Leyes, an area in western Argentina where other dinosaur species, as well as ancient turtles, iguanas and mice, were previously uncovered.