A primitive confuciusornithid bird from China and its implications for early avian flight. 2008. Zhang, F., et al. Science in China Series D-Earth Sciences 51: 625-639.
From the press release:
The new bird, Eoconfuciusornis is the oldest known confuciusornithid, a group unique to China. It therefore represents an early stage in bird evolution. Confuciusornithids lived from 120-131 million years ago and include the oldest birds with horny beaks.
The fossil deposits of Liaoning, called collectively the Jehol Group, span much of the Early Cretaceous. Eoconfuciusornis falls between Archaeopteryx (150 million years) and the bulk of the Liaoning birds (125 million years), and it shows an intermediate stage in the improvement of bird flying capabilities.
Most bird fossils come from the Jiufotang and Yixian groups, dated about 125-120 million years, but the Dabeigou Formation where Eoconfuciusornis was found is 131 million years, so giving a span of 11 million years of evolution of this particular bird group.