Exquisite Chinese fossils support the idea that the ancestors of modern birds may have lived on water.
Five 110-million-year-old specimens of the grebe-like Gansus yumenensis are described in the journal Science.
The detail in their preservation, such as the bone structure and even foot webbing, indicates the animals were well adapted to an aquatic existence.
Scientists say Gansus is the oldest known member of the group that includes modern birds.
They believe this makes its story a critical one in understanding the evolution of avian species.
"Every bird living today, from ostriches... to bald eagles, probably evolved from a Gansus-like ancestor," Matthew Lamanna, of Carnegie Natural History Museum in Pittsburgh, US, told a news conference on Thursday.
Gansus yumenensis was first described from a fossil leg found in 1983.
The new finds, however, give scientists an almost complete view of the animal. All they lack now is an example of a skull.
The specimens come from a quarry near the town of Changma, in China's Gansu Province, about 2,000km (1,200 miles) west of Beijing.
Co-author Jerald Harris, of Dixie State College of Utah, said the animal was very modern in its appearance.
"If you took most of the bones in its body, including famous pieces like the breastbone and the wishbone, and put them next to those of a modern bird, you'd have a lot of difficulty telling them apart," he told the BBC Radio 4's Leading Edge programme.
"Gansus would probably have looked very much like a grebe or a diver, or certain kinds of ducks. It had webbed feet and it had fairly powerful legs. We can tell that from looking at the bones in the knee area. This tells us it was a very well-adapted diving or swimming-type bird."
Water to land
According to Harris, these adaptations all demonstrate how the Gansus branch of the family tree, the structurally modern birds called ornithuromorphs, split from the enantiornitheans (or "opposite birds").
Enantiornitheans were among the famous feathered fossils found in northeastern China during the 1990s.
The analysis implies that the line that would become modern birds left the land and became adapted to life on the water and then, at a later date, came back onto land.
All scientists are missing now is an example of a Gansus skull
"At the same time that Gansus was around, the types of birds you have found living on land, perching in trees, belonged to a second branch of the bird evolutionary tree that went extinct at the end of the Cretaceous," he explained.
"This is interesting because that means the lineage to modern birds had to either out-compete these birds - which we call opposite birds - for the niches of living on land, or wait for them to go extinct and then take over those niches; to come back out of the water and go back to living again on land."