The authors say many of these adaptations were not specific to gigantism, instead allowing for greater mobility as I. prima moved across the Triassic landscape. "It was at least twice as large as the other herbivores of the time".
Scientists in Argentina have discovered the earliest-known "giant" dinosaur, revealing the evolution of gigantism began around 30 million years earlier than previously thought.
The creature changes our understanding of how this group of species became such an enormous size.
What is really unexpected is that the lessemsaurids achieved their huge bodies independently of the big sauropods like Brontosaurus and Diplodocus, which did indeed evolve later during the Jurassic. To turn into towering behemoths, it was believed the development of straight legs for support and continuous, rapid growth were essential.
As the researchers pointed out, during early-Triassic, when dinosaurs started to grow, there were many small reptiles which evolved in giant dinosaurs.
The new dinosaur species Ingentia prima and similar species are grouped together as "lessemsaurids".
Ingentia Prima would have lived in what is now Argentina, but was then the southeast corner of the supercontinent Pangaea. "That's the surprise", asserted Cecilia Apaldetti, a San Juan University researcher. Like their notorious descendants, they also had elongated necks and tails.
But the dinosaur's size isn't the only feature that impressed paleontologists.
The fossil was found in the San Juan Province, in north-west Argentina, during a field trip. As for the bones, they belonged to the neck, tail, front and back legs.
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. I. prima grew via a combination of very fast growth spurts and bird-like lungs able to supply large amounts of oxygen and keep an vast body cool, scientists report in Nature Ecology & Evolution.
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. Ingentia has been estimated to have weighed about 10 tons, which is huge for its time but makes it a relative lightweight next to some of its descendants like Patagotitan, the largest land animal to ever walk the Earth.
"It is a new way to get body size in an early moment in evolutionary history", said Dr Apaldetti.