The classification of "A. xui" as a bird is somewhat contentious, however, due to the various differing definitions of the word "bird". Recent discoveries "[emphasize] how grey the dividing line is between birds and [non-avian] dinosaurs", says Paul Barrett of the Natural History Museum, in London. "There's such a gradation in features between them that it's very difficult to tell them apart ... ["Aurornis xui"] is certainly an older member of the bird lineage than "Archaeopteryx", and it's fair to call it a very primitive bird. But what you call a bird comes down to what you call a bird, and a lot of definitions depend on "Archaeopteryx"." Bird evolution specialist Lawrence Witmer called the new analysis compelling, but said it remains difficult to distinguish birds from birdlike dinosaurs: "All of these little feathered species running and flapping around ... were all very similar."