Today's birds could not fly with such feathers, the team said.
Studies suggest primitive feathers first evolved in flightless dinosaurs that generated heat internally and so would have benefited from the insulation that down can provide.
Feathers later evolved for use in flight, the theory holds, although experts debate whether birds' immediate ancestors were tree-dwelling, gliding dinosaurs or terrestrial dinos that ran at high speeds and eventually lifted off the ground.
Either way, the amber-encased feathers show for the first time the transition from downy filaments toward an aerodynamic, planar shape that enabled flight, Perrichot said.
"This most critical step in the evolution of feathers" was suggested by evolutionary theories but had never previously been seen in either modern or fossil feathers, he said.
Team member Néraudeau added that this missing link has been "an argument for creationists and others to reject the theropod-birds lineage and to argue in favor of different origins for theropod feathers and bird feathers."
"First Good Look"
Nick Longrich, of the University of Calgary in Canada, said "this could be our first good look at a dinosaur body feather."
The bird-fossil expert, who was not involved in the study, noted that the newfound feathers are around 50 million years younger than the first known flying bird, Archaeopteryx, which lived about 150 million years ago.
(See a picture of an Archaeopteryx fossil that shows the imprints of feathers.)
"Obviously this animal [the feathers came from] isn't directly ancestral to anything except later dinosaurs, but it's quite likely that we are seeing aspects of the ancestral [feather structure]," Longrich said in an email.
"So the animal isn't transitional' but it may preserve a transitional structure."
It's also possible that the simplified structure of the feathers isn't so primitive, he added.
Modern flightless birds such as ostriches and emus have highly simplified feathers, he said.
More samples from the fossil record are needed to settle the issue, so "hopefully this study will cause more people to look for dinosaur feathers in amber," Longrich added.
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES