The remains of a 200-million-year old species, baptized Ingenia prima, was discovered in the Balde de Leyes dig site in San Juan province, 1,100km (680 miles) west of the Argentine capital, Buenos Aires.
Ingentia was an early member of a dinosaur group called sauropods that later included Earth's biggest terrestrial creatures including the Patagonian behemoths Argentinosaurus, Dreadnoughtus and Patagotitan.
What is unexpected is Ingentia prima dates back to Triassic, 30 million years before the well-known enormous Diplodocus and Brachiosaurus, which are thought to be the earliest giant dinosaurs, said Dr. Steve Brusatte of the University of Edinburgh.
Cecilia Apaldetti of the University of San Juan said, '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'.
Unlike later sauropods, Ingentia's legs were not pillar-like and its neck was shorter.
Dr Apaldetti said the climate would have been warm, with periodic monsoons. producing an African savannah style landscape, with plenty of shrubs on which Ingentia prima would feed.
Gigantism proved a successful evolutionary survival strategy, especially for herbivorous animals, because size is a form of defense against predators.
Like their notorious descendants, they also had elongated necks and tails. The dinosaur had a bird-like respiratory system and air sacs in its bones that helped keep it cool.
'In addition, this kind of breathing implied the presence of cavities, or deep holes, in their bones - known as a pneumatic skeleton - that lightened the weight and would have favoured a large body size'.
I. prima exhibited features that allowed sauropods to become sauropods, such as an accelerated growth rate (as seen in its bone structure), and straight legs capable of supporting tremendous weight.
"Their expansion in the Late Triassic is evident from the appearance of many small, agile two legged types recorded throughout the world", she said. It belongs to a group of dinosaurs known as sauropods, which includes some of the largest land animals ever known to have existed.
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