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It stood two-stories high at the shoulder. Its tail was almost as long as a city bus. And it tipped the scales at some 65 tons — heavier than a fully loaded semi-truck.

Meet Dreadnoughtus schrani, one of the most massive land animals of all time.

Other enormous dinosaurs are known due to a few found fragments of bone, but the recently unearthed remains of Dreadnoughtus are so complete and beautifully preserved that they will provide unprecedented insight into how giant dinosaurs were related to each other, how fast they grew and how something so heavy lugged itself around. Indeed, never before has so many representative pieces of a super-large dinosaur been found.

"It's a really important discovery," says paleontologist Michael D'Emic of Stony Brook University, who is not associated with the new find, reported in this week's edition of the journal Scientific Reports. "The glimpses we have of those giant, giant dinosaurs are pretty fleeting. (Dreadnoughtus) will serve as a key to understanding all those other, more fragmentary specimens."

The completeness of the Dreadnoughtus remains, discovered in Argentina in 2005 and unearthed between 2005 and 2009, allowed scientists to estimate its weight with unusual precision. At 65 tons, Dreadnoughtus was heavier than many models of the Boeing 737. It's the largest dinosaur whose weight can be accurately calculated, says discoverer Kenneth Lacovara of Drexel University, who named the beast after the vast 20th-century battleships called dreadnoughts.

But Lacovara allows there's a "good chance" that a dinosaur known as Argentinosaurus was heavier than Dreadnoughtus. Paleontologist Roger Benson of the University of Oxford, who was not affiliated with the new find, also puts Dreadnoughtus behind Argentinosaurus, which may have weighed as much as 90 tons.

Whether it was the largest or only the second-largest dinosaur, Dreadnoughtus was unimaginably imposing. Though a plant-eater, it came equipped for battle with a muscular tail it could have used as a weapon and large claws on its back feet. Its size would've made it the king of the forests where it lived in the late Cretaceous period.

"If this thing just leaned on a T. Rex, it would probably kill it," Lacovara says. Walking up to it "would be like approaching a living building. … It would be a pretty overwhelming moment, and not advisable."

Though it could laugh off all comers, even Dreadnoughtus was not immune to the power of a river in full flood. Some 80 million years ago, a raging torrent swept away two Dreadnoughtuses — perhaps already dead — before dumping them on a bed of quicksand-like sediment. In a stroke of luck for scientists, the mire swallowed the animals whole.

Thanks to the dinosaurs' entombment, Lacovara and his team recovered some 70% of the bones Dreadnoughtus had below its head, the researchers report in Scientific Reports. Until now, no more than 27% of any giant dinosaur's bone types had been found. Argentinosaurus, for example, is known from a half-dozen vertebrae, a leg bone and a few scraps of hipbone, Lacovara says.

The new find will serve as a Rosetta Stone to unlocking the mysteries of other enormous dinosaurs, D'Emic says. For example, did these animals need to survive for a century to get so big, or would a growth spurt of a few decades suffice? (Analysis of the fossil suggests it was not yet full-sized.)

"This is a truly huge animal," Lacovara says. "It kind of staggers the imagination."

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