The turkey-sized creatures probably lived in the trees about 130 million years ago and used their wings and long tail to glide from branch-to-branch.

It gives us one more piece of evidence as to how flight got started Dr Angela Milner, Natural History Museum

The six specimens belong to a new dinosaur family that may have played a key role in the evolution of birds.

They were unearthed in Liaoning, China - a place that has yielded many spectacular fossils of bird-like dinosaurs.

The find is exciting because it backs an older theory that birds evolved from gliding four-winged tree-dwellers.

	:	In Pictures
The 130-million-year-old four-winged dino

The idea was first raised in 1915 but until now there has been no fossil evidence to support it.

According to the Chinese team that found the specimens - led by Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing - this may be the first confirmation.

"Although the dinosaurian hypothesis of bird origins is widely accepted, debate remains about how the ancestor of birds first learned to fly," the scientists report in the journal Nature.

"Here we provide new evidence suggesting that basal dromaeosaurid dinosaurs [a sub-group of theropod dinosaurs] were four-winged animals and probably could glide, representing an intermediate stage towards the active, flapping-flight stage."

The suggestion is that dinosaurs went through a gliding stage before learning to fly with two fore-limbs, says Dr Angela Milner of London's Natural History Museum.

Rival camps

Scientists will now want to look again at the fossils of the earliest bird, Archaeopteryx, to see if there are any vestiges of hind-wings.

"The importance of this find is that it gives us one more piece of evidence as to how flight got started," Dr Milner told the BBC.

The creature was about 1 metre long (Photo: Xu Xing)

"This is clearly a small dinosaur, but equipped for climbing and with well developed feathers on its arms which have flight capability and assisted by gliding feathers on its hind legs."

The question of how flight arose has caused much heated debate. Many dinosaur experts belong to a rival camp - that birds evolved from ground-living dinosaurs with powerful hind-limbs adapted for running.

The discovery of the four-winged fossils - cousins of the previously described Microraptor - is unlikely to be the end of the story.