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Scientists crown heavyweight champ of dinosaurs

Scientists unveiled body weight estimates for an astounding 426 different dinosaur species, using a formula based on the thickness of their leg bones to crown the biggest of them all.

WASHINGTON, May 6 (Reuters) - This guy could really throw his weight around.

Scientists on Tuesday unveiled body weight estimates for an astounding 426 different dinosaur species using a formula based on the thickness of their leg bones, crowning the truly immense long-necked Argentinosaurus as the biggest of them all.

That plant-eating dinosaur weighed a earth-shaking 90 tons when it lived about 90 million years ago in Argentina. It is the largest known land creature in the planet's history.

"Argentinosaurus, that's the champion," Oxford University paleontologist Roger Benson, who led the study, said in a telephone interview. "It's colossal."

In their dinosaur "weigh-in', the scientists included birds, which arose roughly 150 million years ago within a group of feathered dinosaurs called maniraptorans. A sparrow-sized bird called Qiliania that lived about 120 million years ago in China earned the distinction of being the smallest dinosaur, weighing a mere 15 grams.

Benson noted that Argentinosaurus was about 6 million times the weight of Qiliania, and that both still fit within the dinosaur family. "That seems amazing to me," added Benson, whose study was published in the scientific journal PLOS Biology.

The largest meat-eating dinosaur was Tyrannosaurus rex, which weighed 7 tons and is also the largest known land predator of all time. The T. rex edged out another super predator that some scientists had once figured was bigger based on the length of its skull, Giganotosaurus, which lived alongside Argentinosaurus in ancient South America.

The study estimated Giganotosaurus at about 6 tons, pretty darned big, but just a bit shy of dethroning T. rex.

Dinosaurs had a remarkable run on Earth. They first appeared about 228 million years ago during the Triassic period, achieved stunning dimensions during the ensuing Jurassic Period and then disappeared at the end of the Cretaceous Period about 65 million years ago. All but the birds, that is.

The mass extinction at the end of the Cretaceous, caused by an asteroid that hit Mexico, doomed most creatures but some birds survived. Benson said this study underscores the reasons that birds made it while their bigger dinosaur brethren did not.

Other groups of dinosaurs such as long-necked sauropods like Argentinosaurus, the tank-like ankylosaurs, the duck-billed hadrosaurs, the spike-tailed stegosaurs and the meat-eating tyrannosaurs were essentially locked into a certain ecological niche. But birds filled all kinds of ecological niches with their widely diverse body sizes and "occupations".

Flying birds lived in all kinds of different habitats, both inland and coastal, and came in a wide range of sizes. But there also were large, ostrich-like flightless birds like Gargantuavis and flightless diving birds like Hesperornis.

"It might be that they were simply much more ecologically diverse and that could have helped them survive an extinction," said Benson, who also noted that smaller creatures did a better job surviving the asteroid impact at the end of the Cretaceous.

Paleontologist David Evans of Canada's Royal Ontario Museum said dinosaur body size evolved relatively quickly early on in their time on Earth as they invaded new ecological niches, but then slowed down among most lineages. The exception was the maniraptoran lineage that led to birds, Evans added.

More than 1,000 species of dinosaurs have been identified but many are known from only fragmentary fossil remains.

This study estimated the weight of every dinosaur whose remains are complete enough to contain the bones needed for the study's formula, which is based on the relationship between the robustness of the limbs and the weight of the animal, the researchers said.

(Reporting by Will Dunham; editing by Peter Galloway)