Fossils found in southern Argentina are believed to represent the most complete remains of what could be the biggest dinosaur to have roamed the earth.

American scientists are calling it Dreadnoughtus - after the huge battleships of more than a century ago.

They estimate it measured 26 metres from head to tail and weighed approximately 65 tonnes.

The word big does not do justice to a massive, long-necked dinosaur that shook the earth in Argentina about 77 million years ago.

Try colossal, enormous, gargantuan and stupendous - and you might come close to an accurate description of this behemoth, known to scientists as Dreadnoughtus schrani.

Scientists on Thursday announced the discovery in southern Patagonia of remarkably complete and well-preserved fossil remains of the dinosaur, which weighed 65 tonnes and measured 26 metres with a neck 11.3 metres and a tail 8.7 metres.

Palaeontologist Kenneth Lacovara of Drexel University in Philadelphia, who discovered the dinosaur and led the effort for its excavation and analysis, said the scientists calculated its weight on the basis of the bones in its upper arm and thigh.

The massive dinosaur was no wimp, weighing 65 tonnes. ( Ken Lacovara )

Dreadnoughtus weighed more than an adult sperm whale or a herd of African elephants. Tipping the scales at seven times as much as the dinosaur T. rex, it made the North American menace that also lived during the Cretaceous period look puny.

Dreadnoughtus had "the largest reliably calculable weight" of any known land animal - dinosaur or otherwise, Professor Lacovara said.

Another giant Argentine dinosaur, Argentinosaurus, might have been larger, he said, but its scant remains do not allow a reliable weight estimate.

Another group of scientists in May had cited Argentinosaurus, with an estimated weight of 90 tonnes as the largest dinosaur.

Huge dinosaur likely had nothing to fear

While strictly a vegetarian, Dreadnoughtus was no wimp.

With its size and a tail that could have clobbered any predator foolish enough to attack, it probably had nothing to fear from even the largest meat-eating dinosaurs and its name reflects that.

"We decided on Dreadnoughtus, meaning 'fearer of nothing', because when you're as big as this thing was, you're probably not afraid of too much," researcher Matt Lamanna of the Carnegie Museum of Natural History in Pittsburgh said.

"Not to mention we thought it was time a plant-eating dinosaur got a badass name. Those are usually reserved for the meat-eaters," said Mr Lamanna, a palaeontologist.

Professor Lacovara said the name also was a nod to the powerful battleships called dreadnoughts, dating from the turn of the last century, that were designed to be impervious to attack.

Dreadnoughtus probably spent its days munching massive quantities of plants to fuel its enormous body.

The plant-eating dinosaur ate massive quantities of plant matter. ( Ken Lacovara )

It was a member of a group of dinosaurs called titanosaurs that are thought to have been the largest dinosaurs that ever lived. Argentinosaurus was also a titanosaur.

Most titanosaurs are known only from fragmentary remains, but the scientists have found 45 percent of the skeleton of Dreadnoughtus, including most of the important bones.

Mr Lamanna called it "a treasure trove of information on one of the most successful, but least understood dinosaur groups of all."

The researchers found two specimens of Dreadnoughtus side by side, one larger than the other, but the scary thought is that they believe the larger one was not even fully grown.

"It appears that both individuals died and were buried rapidly after a river flooded and broke through its natural levee, turning the ground into a soupy mixture of sand, mud and water," Professor Lacovara said.

To attack a healthy adult Dreadnoughtus, a solitary predator "would have to have been suicidal," Mr Lamanna said.

"It's conceivable that a pack of these predators could take down a sick or old Dreadnoughtus, but a single carnivore versus a 'Dread' would be a drubbing."

The study appears in the nature journal Scientific Reports.

Reuters