Harris says Gansus shares many skeletal features with modern birds, including the knobby knees characteristic of underwater swimmers like loons and grebes.
Moreover, he says, the preserved skin of the webbed feet shows the same microscopic structure seen in aquatic birds today.
"It was unexpected to find a bird this advanced in rocks this old," Harris said. "It tells us that the anatomical features we use to characterize modern birds evolved very quickly."
According to the researchers, Gansus is the oldest clearly established member of the subclass Ornithurae, the group most closely related to modern birds.
The Gansus fossils are only 10 to 15 million years younger than the "feathered dinosaurs" discovered a decade ago at Liaoning, in western China. (See feathered-dinosaur pictures.)
Most fossil birds dating so far back belong to a different evolutionary lineage called opposite birds. The name stems from the fact that bones in their shoulders and feet fit together opposite from the way seen in birds today.
Opposite birds made up the dominant bird group of the Cretaceous Period (145.5 to 65.5 million years ago). They disappeared along with the dinosaurs when that period ended, leaving no modern descendants.
The study authors say the fossil bed that yielded Gansus may be the earliest Cretaceous site dominated by ornithurans rather than opposite birds.
Experts differ in their assessments of how much light the Gansus fossils shed on the origins of modern-day bird groups such as ducks and other waterfowl.
The authors of the Science paper argue that, since Gansus and other ancestral species were water specialists, modern birds probably originated in an aquatic environment.
Their theory is that aquatic ornithurans like Gansus first evolved from earlier, land-based species early in the Cretaceous.
These water-based ornithurans gave rise to modern birds, which quickly spread back into nonaquatic habitats as the once dominant opposite birds declined.
But paleontologist Julia Clarke, of North Carolina State University in Raleigh, disagrees.
She says Gansus and other early fossils don't necessarily imply such a back-and-forth evolutionary shift between habitat types.
Rather, Clarke says, the findings illustrate that there was a wide range of bird types during the period that preceded the emergence of truly modern birds.
"The new findings contribute importantly to our understanding of the ecological diversity present in these close cousins of our existing birds," Clarke said. "They speak to the evolution of shape and form."
While the Gansus discoveries seem likely to fuel debate among paleontologists, experts agree that the excavation site may have even more to offer.
Luis Chiappe is a co-author of the Science paper and director of the Dinosaur Institute at the Natural History Museum of Los Angeles County, California.
Chiappe says the potential of this and other fossil localities in Gansu Province is enormous.
"I expect that 'feathered dinosaurs' and other key fossils for understanding vertebrate evolution will be unearthed from this site in the near future," Chiappe said.
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