Are birds descended from dinosaurs? Most paleontologists think so. A minority, however, bump dinosaurs and their descendants completely off the bird family tree, suggesting instead that avian evolution was well under way tens of millions of years before Archaeopteryx--a presumed ancestral bird--and its kin appeared on the scene, and perhaps even before the age of dinosaurs.
That view--largely scoffed at by the dinosaur-descent majority-- gained new credibility recently with the discovery of a fossil bird, called Liaoningornis, in northeastern China. The sparrow-size bird, which was analyzed by ornithologist Alan Feduccia of the University of North Carolina at Chapel Hill and his colleagues, may be the oldest example of a modern- type, or ornithurine, bird. Liaoningornis, for instance, possesses a keeled sternum for the attachment of flight muscles, a feature found in all modern birds but not in dinosaurs or Archaeopteryx.
Yet radiometric dating of the deposits in which Liaoningornis was found suggests that the bird is 137 million to 142 million years old--just a tad younger than 150-million-year-old Archaeopteryx, and nearly 20 million years older than the oldest previously discovered ornithurines. Furthermore, the fossil was found in the same deposit as another fossil bird, called Confuciusornis, which shares a number of primitive characteristics with Archaeopteryx and other ancient birds.
That dating, it must be said, is already being challenged by Canadian researchers, who date the fossil at just 121 million years old, young enough for Liaoningornis to have evolved from Archaeopteryx. But if the dates hold up, says Feduccia, it would mean that two distinct lineages of birds already existed at the time of Archaeopteryx. We have an early dichotomy of birds, with one lineage producing Archaeopteryx and birds like Confuciusornis, says Feduccia, and the other lineage leading to the ornithurine birds, like Liaoningornis.
The common ancestor to both branches, Feduccia says, must have lived tens of millions of years earlier--and was therefore not likely a dinosaur. When you have fully developed forms like Confuciusornis and Liaoningornis, you can bet that the divergence goes back quite a bit further. That means there was a much earlier period of avian history that preceded Archaeopteryx, which we know nothing about.