Fossils excavated in Liaoning province have challenged the theory that the Archaeopteryx is indeed the earliest known bird.

Research of the fossils led by Pascal Godefroit and his team from the Royal Belgian Institute of Natural Sciences in Brussels revealed that the dinosaur-bird Aurornis xui, like the Archaeopteryx, existed nearly 150 million years ago.

Godefroit told British-based Daily Mail, "This new comprehensive phylogeny, or evolutionary development, shows that point of origin avialans, or dinosaur-birds, were already diversified in northern China during the Middle-Late Jurassic".

The study, published in the journal Nature, showed that the Aurornis xui was 50 centimeters in length and had tiny, triangular teeth used for hunting and defense, similar to other non-avialan dinosaurs.

Godefroit said the results support a "single origin of powered flight", which suggests that all dinosaurs that became birds took flight around the same time, and eventually lost hunting and defensive features, such as teeth, during adaptation.

Hu Dongyu, a professor at Shenyang Normal University and third author of the research article published in Nature, pointed out that fossils of some dinosaurs, particularly the Microraptor, show that the legs initially had long feathers, which suggests they assisted wings in flight.

"But the long feathers on the legs degenerated as the wings became strong enough to flap," Hu said.

Hu said this is not the first time that fossil evidence has showed the evolution path from dinosaur to bird.

One fossil, found in Liaoning in 2009 and studied by scientists led by Xu Xing from the Institute of Vertebrate Paleontology and Paleoanthropology under the Chinese Academy of Sciences in Beijing, showed that the earliest dinosaur-bird was the Anchiornis huxleyi, a flying dinosaur that lived 160 million years ago.

Another fossil studied by Xu and his colleagues, also believed to be a dinosaur-bird, was named Xiaotingia zhengi.

"Aurornis xui, Anchiornis huxleyi and Xiaotingia zhengi, are similar dinosaur-bird fossils," Hu said. "The initial question was whether these dinosaur-birds existed before the Archaeopteryx, but as we studied further, the age became irrelevant. And the bone of contention became whether all these dinosaur-birds, including the Archaeopteryx, are dinosaurs or birds?"

Hu added that earlier research suggested that all dinosaur-birds were dinosaurs, not birds. However, recent study suggests the opposite.

Scientists from the Institute of Vertebrate Paleontology and Paleoanthropology used computer software to record the features of dinosaur-birds and create an evolution tree.

"The evolution tree, like an actual tree, shows branches that grow from the trunk," Xu said. "There is a point when birds diverge from the dinosaur, but different databases show a different diverge point".

The name of Aurornis xui was given to mark Xu's contribution to the study of the evolution of dinosaur and the origin of the bird.

However, Xu does not agree that Aurornis xui, Anchiornis huxleyi, Xiaotingia zhengi and Archaeopteryx should be classified as birds.

"My analysis shows these dinosaur-birds are Deinonychosaurs rather than birds," Xu said.

Deinonychosaurs are another classification of dinosaur closely related to birds.