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 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.
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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)
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