The findings, presented in a published on Monday in the journal Nature Ecology and Evolution, introduces this dinosaur species as Ingentia Prime (First Immense) and is described as one of the first giant sauropods.
Ingentia prima was big - up to 32 feet (10 meters) long - but not almost as large as the massive titanosaurs that lived millions of years after it.
What is really unexpected is that the lessemsaurids achieved their huge bodies independently of the enormous sauropods like Brontosaurus and Diplodocus, which did indeed evolve later during the Jurassic.
Before this discovery, it was thought that gigantism developed during the Jurassic period, around 180 million years ago. But the lessemsaurids tell us that at least some dinosaurs were able to attain giant sizes during the latest part of the Triassic, before the extinction.
The creature has been dubbed "Ingentia prima" (Latin for "first giant"), and it's been classed as a sauropodomorph, the group that would later evolve into the very big sauropods. It is a member of a group called sauropodomorphs, meaning "lizard-foot form".
Sauropods - the largest land animals to ever appear on Earth - came from humble beginnings.
Like later sauropods, lessemsaurids had elongated necks and tails, as well as air sacs resembling those that are found in birds. It was about 10m long.
The dinosaur's bone fragments displayed cyclical and seasonal growth, with a different kind of tissue to other sauropods, which allowed it to grow very quickly. Together, these dinosaurs belong to a sauropod subgroup known as the lessemsaurids, which lived between 237 and 201 million years ago (relatively soon after dinosaurs first appeared) in what is now Argentina but was then the southeast corner of the supercontinent Pangaea.
The paleontologist Diego Pol at the Egidio Feruglio Museum in Argentina said that the discovery "gives a new look at the magnitude of the evolutionary explosion that the dinosaurs had".
And they suggest that there may be even bigger and stranger dinosaurs out there, which are yet to be discovered. "This strategy was not used again in the history of dinosaurs".