Based on its huge thigh bones, it was 40m (130ft) long and 20m (65ft) tall.
Weighing in at 77 tonnes, it was as heavy as 14 African elephants, and seven tonnes heavier than the previous record holder, Argentinosaurus.
Scientists believe it is a new species of titanosaur – an enormous herbivore dating from the Late Cretaceous period.
A local farm worker first stumbled on the remains in the desert near La Flecha, about 250km (135 miles) west of Trelew, Patagonia.
The fossils were then excavated by a team of palaeontologists from the Museum of Palaeontology Egidio Feruglio, led by Dr Jose Luis Carballido and Dr Diego Pol.
They unearthed the partial skeletons of seven individuals – about 150 bones in total – all in “remarkable condition”.
A film crew from the BBC Natural History Unit was there to capture the moment the scientists realised exactly how big their discovery was.
By measuring the length and circumference of the largest femur (thigh bone), they calculated the animal weighed 77 tonnes.
“Given the size of these bones, which surpass any of the previously known giant animals, the new dinosaur is the largest animal known that walked on Earth,” the researchers told BBC News.
“Its length, from its head to the tip of its tail, was 40m.
“Standing with its neck up, it was about 20m high – equal to a seven-storey building.”
This giant herbivore lived in the forests of Patagonia between 95 and 100 million years ago, based on the age of the rocks in which its bones were found.
But despite its magnitude, it does not yet have a name.
“It will be named describing its magnificence and in honour to both the region and the farm owners who alerted us about the discovery,” the researchers said.
There have been many previous contenders for the title “world’s biggest dinosaur”.
The most recent pretender to the throne wasArgentinosaurus, a similar type of sauropod, also discovered in Patagonia.
Originally thought to weigh in at 100 tonnes, it was later revised down to about 70 tonnes – just under the 77 tonnes that this new sauropod is thought to have weighed.
The picture is muddied by the various complicated methods for estimating size and weight, based on skeletons that are usually incomplete.
Argentinosaurus was estimated from only a few bones. But the researchers here had dozens to work with, making them more confident that they really have found “the big one”.
Dr Paul Barrett, a dinosaur expert from London’s Natural History Museum, agreed the new species is “a genuinely big critter. But there are a number of similarly sized big sauropod thigh bones out there,” he cautioned.
“Without knowing more about this current find it’s difficult to be sure. One problem with assessing the weight of both Argentinosaurusand this new discovery is that they’re both based on very fragmentary specimens – no complete skeleton is known, which means the animal’s proportions and overall shape are conjectural.
“Moreover, several different methods exist for calculating dinosaur weight (some based on overall volume, some on various limb bone measurements) and these don’t always agree with each other, with large measures of uncertainty.
“So it’s interesting to hear another really huge sauropod has been discovered, but ideally we’d need much more material of these supersized animals to determine just how big they really got.”
Source: BBC NEWS