A new feathered fossil, Aurornis – introduced in today’s Nature – has the potential to resolve a debate about bird evolution that’s had evolutionary biologists in a bit of a flap in recent years.
The origin of birds from bipedal carnivorous dinosaurs is a modern-day poster-child for evolution, with a series of feathered dinobirds bridging the gap between dinosaurs and birds.
Many of the most iconic dinosaurs, such as Velociraptor, are now known to have sported bird-like traits such as feathers, wishbones and light hollow bones. But certain finer points of this family tree remain debated, with vigorous discussion over the exact position of Archaeopteryx – a magpie-sized creature from limestone quarries in Bavaria, southern Germany.
Aurornis, the new feathered fossil, further closes the increasingly small gaps between dinosaurs and birds – and helps nail down the exact position of Archaeopteryx within the avian evolutionary tree.
Since its discovery more than 150 years ago, Archaeopteryx has been widely accepted as a primordial bird, with avian features such as wings and feathers but also some very primitive reptile-like features such as teeth and a long tail.
This assumption was challenged in 2011 when scientists drew up a new evolutionary tree of dinosaurs (and their avian descendants) which separated Archaeopteryx well away from other birds.
This tree suggested that true bird flight (powered by flapping wing-like arms) evolved twice within the “dinobirds” – once in Archaeopteryx, and a second time in modern birds.
Such convergent evolution is widespread. Among vertebrates, true flight has definitely evolved separately multiple times (in pterosaurs and bats as well as birds), so the idea of having it evolve one extra time wasn’t too far fetched.
Furthermore, many of the dinobirds already had some degree of gliding ability, though many (such as Microraptor) employed a bizarre technique involving leaping with feathered arms and legs.
But other researchers, using different methods to infer evolutionary trees, maintained that Archaeopteryx probably sat at the base of birds, implying a single shared origin of bird flight.
Enter Aurornis
Aurornis, a chicken-sized creature from Jurassic deposits in China, is every palaeontologist’s dream. The single exquisite fossil preserves virtually every bone in its natural position, including the tips of the tail, fingers and toes.
Its anatomy demonstrates that it is one of the most bird-like dinosaurs yet discovered: in fact, it looks rather much like Archaeopteryx, but with shorter arms and hands, suggesting it had less developed wings.
But the nature of the flight feathers (if any) remains a mystery, as only a few patches of downy feathers are preserved. Nevertheless, Aurornis almost totally closes the gap between dinosaurs and birds.
When the team re-analysed the evolutionary tree of dinosaurs and birds, adding Aurornis and examining more than twice as many anatomical traits as previous analyses, they found that Aurornis sat just below birds, with Archaeopteryx at the base of all birds.
Thus, the 160-million year old Aurornis and the 150-million-year-old Archaeopteryx provide excellent “before” and “after” snapshots of the evolution of bird flight. Furthermore, bird flight only evolved once, in an Archaeopteryx-like ancestor which gave rise ultimately to all other birds.
There is now overwhelming evidence that birds evolved from bipedal, carnivorous dinosaurs, thanks largely to the wealth of feathered transitional fossils from China, which began emerging around 1998.
Fine volcanic ash instantly preserved these fossils with such fidelity that even their colour patterns can often be determined, based on pigment spots (melanosomes) in the feathers.
The full name of the new fossil (Aurornis xui) is both appropriate and slightly cheeky. It is named in honour of Xu Xing, the leading palaeontologist who has described many of these exceptional fossils, and aired the idea Archaeopteryx might not be a true bird (a proposal contradicted by the new fossil and analysis).
But there are many other important feathered fossils sitting in museums still to be described from China, and even more in the ground waiting to be discovered. Future work will undoubtedly further refine the smaller branches of the evolutionary tree between dinosaurs to birds.