The discovery of a fossil of Archaeopteryx in 1861 was one of the greatest fossil discoveries ever. The fossil showed the earliest known (at the time) bird and also a transitionary stage in the evolution of a branch of reptiles into birds, helping illustrate Darwin's theory of evolution too. Now finally an earlier stage in that evolution has been found.
Aurornis xui predates Archaeopteryx by 10 million years in the Jurassic period (about 150 million years ago). It shares many of the other early bird's features such as a long tail but is considered more primitive and the most primitive avian yet discovered. The fossil has preserved the outline of feathers along the body and tail though Aurornis was unlikely to have been able to fly.
Some scientists are not convinced Aurornis should be classified as a "bird" but rather was still a reptile but all agree it provides an important step in the evolution of the birds. Interestingly the discovery of Aurornis helps re-establish Archaeopteryx in the bird family tree after some scientists relegated it to a class of winged dinosaurs.