Archaeologists have discovered the fossil of a giant dinosaur that roamed Earth more than 200 million years ago, which may change our understanding of how these enormous creatures evolved to be so big.

The fossil found in Argentina belongs to a new herbivorous dinosaur species called Ingentia prima, which means "first giant" in Latin, that existed 237 to 201 million years ago. It’s thought to have measured a huge 10 meters (33 feet) long and 4 meters (14 feet) tall, weighing up to 10 tons. A study describing the finding is published in Nature Ecology and Evolution.

“Until now it was believed the first giants to inhabit the Earth originated during the Jurassic – about 180 million years ago,” said Dr Cecilia Apaldetti from Universidad Nacional de San Juan in Argentina, lead author on the study.

“But with this discovery, we can see the first steps toward gigantism occurred 30 million years before the giants dominated practically the entire planet.”

The new dinosaur was found along with the fossils of three Lessemsaurus sauropoides, in what was then southeast of the supercontinent Pangaea. I. prima is thought to be an ancestor of huge dinosaurs known as sauropods, such as Brachiosaurus and Diplodocus, which had long necks and tails just like this new discovery.

Scientists think I. prima grew to such a large size thanks to bird-like air sacs in its body, which kept it cool and supplied lots of oxygen. It also looks like it grew pretty quickly, according to growth rings found in the bones, but it’s somewhat surprising that it had curved legs rather than straight ones to support its weight, note the researchers.

I. prima is by no means the largest dinosaur ever found, with Brachiosaurus and Diplodocus themselves reaching 50 tonnes, and others perhaps being even larger. But its early appearance in the fossil record is hugely interesting. All four of these creatures belong to the lessemsaurids, and it’s thought they were the first dinosaurs to reach huge sizes of about 10 tonnes, about 215 million years ago.

“What is really unexpected is that the lessemsaurids achieved their huge bodies independently of the gigantic sauropods like Brontosaurus and Diplodocus, which did indeed evolve later during the Jurassic,” Dr Steve Brusatte from the University of Edinburgh told BBC News.

“The development of huge size wasn't just a one-off event for the sauropods, but rather different types of dinosaurs were able to become colossal, which speaks to just how incredible these animals were.”

That means that not all large dinosaurs grew in the same way, and it may be possible there are more giants awaiting discovery. There’s more than one way to make a giant dinosaur, it would seem.