Ever since the first skeleton of Archaeopteryx was discovered in 1861, the feathered dinosaur has been considered the oldest bird. During the last several decades, however, scientists have found that many "bird" features, such as feathers, first appeared among theropod dinosaurs. What defines a bird rather than a non-avian feathered dinosaur has become a much more complex issue. There is no better example of this than the recently-described Anchiornis huxleyi.
Earlier this year an international team of scientists described Anchiornis in the journal Chinese Science Bulletin. While the incomplete skeleton of the animal shared many traits in common with dinosaurs, the scientists hypothesized that the 155-million-year-old Anchiornis was the closest fossil relative of early birds like Archaeopteryx. Since the time that paper went to press, however, an even better specimen of Anchiornis has been discovered, one that caused the scientists to revise what they had thought about the dinosaur.
In next week's issue of Nature, paleontologists Hu Dongyu, Hou Lianhai, Zhang Lijun, and Xing Xu present a revised take on Anchiornis based upon the new material. The fossil used for the first study consisted of a somewhat jumbled skeleton, not unlike the first skeletal specimen of Archaeopteryx, but a new exceptional fossil preserves almost the entire animal. It even includes feather impressions. Yet when the paleontologists studied this new fossil they discovered that their original study had placed Anchiornis too close to birds. Instead they found that Anchiornis was a feathered troodontid dinosaur, still a relatively close relative of the ancestors of the first birds but now a few steps removed. Despite the headline of a New Scientist story about the new study, Anchiornis was not the "earliest bird" but a non-avian feathered dinosaur that possessed some bird characteristics. Ed. Note -- New Scientist has since corrected their headline.
Interestingly, though, the new fossil showed that Anchiornis is the third non-avian dinosaur known to possess long feathers on its hindlimbs. This feature was either inherited from a feathered common ancestor of dinosaurs like Anchiornis and Microraptor or evolved independently more than once. Given that Anchiornis is also at least five million years older than Archaeopteryx, it is definitive evidence that long feathers and other avian characteristics evolved in dinosaurs before the first birds took to the air.
Anchiornis and other feathered dinosaurs have shown beyond dispute that birds evolved from small theropod dinosaurs, but the precise relationships between feathered dinosaurs and their avian relatives are still being worked out. There are so many fossils turning up at such a fast rate that the evolutionary tree of feathered dinosaurs is constantly undergoing a lot of changes. Given the strong resemblance of Anchiornis to Archaeopteryx, one may even wonder if the celebrated "first bird" might eventually be shuffled into a different position in the feathered dinosaur family tree. That will be a matter for future studies to work out, and our new view of Anchiornis suggests that there are many more fossils of feathered dinosaurs and early birds out there waiting to be discovered.