The stunning remains of a "four-winged" dinosaur have confirmed that birds owe their ancestry to two-footed dinosaurs that lived millions of years ago, the world's most famous fossil-hunter said.
Xing Xu of the Chinese Academy of Science in Beijing is staking the claim thanks to an astonishingly-preserved fossil of a bird-like dinosaur called Anchiornis huxleyi.
Until now, A. huxleyi was thought to be a primitive bird. It was presumed to have been a near-contemporary of Archaeopteryx, the first recognised bird, which flew around 150 million years ago.
But these opinions were based on an incomplete fossil.
The new, nearly-complete specimen gives a different picture, suggesting that A. huxleyi is millions of years older than Archaeopteryx and has both dinosaur and avian features.
It is the long-sought evidence that proves birds descended from theropod dinosaurs, argues Xu.
His team, whose work was announced late Thursday by the British journal Nature, describe a dinosaur with long feathers covering its arms, tail as well as its feet.
This is an arrangement that Xu says is "four-winged", although no guarantee that the creature had aerial ability. In contrast, its elongated lower legs suggest it was a good runner.
Some evolutionary biologists have suggested that a four-winged condition played a role in the origin of flight, but the idea is opposed by others.
The plumage attachment is especially important because it shows how bird-like dinosaurs developed skeletal and other features enabling them to have feathers, the paper says.
Scientists have long argued about the evolutionary line taken by birds.
Some have said bird-like dinosaurs appear too late in the fossil record to be the true ancestors of birds, an argument known as the "temporal paradox."
The debate has raged for years mainly because the fossil evidence is so rare or fragmented.
The new evidence comes from in Daxishan, in Jianchang county in northeastern China.
It was found in rock dated to the early part of the Late Jurassic, between 151 and 161 million years ago, which means it is clearly older than Archaeopteryx.
Rather than be considered a bird, A. huxleyi is a late member of the Troodontidae, a category of dinosaurs closely related to avians, the study argues.
"This new find refutes the temporal paradox' and provides significant information on the temporal framework of theropod divergence," it says.