Until China's feathered dinosaurs were discovered, the only fossil evidence for feather evolution came from archaeopteryx – and that wasn't very informative as its feathers were essentially modern. The Chinese fossils have changed all that. Although younger than archaeopteryx, the haul includes dinosaurs sporting what are presumed to be all stages of feather evolution, from simple filaments to fully formed flight feathers.
Last year Xu and his colleagues described the simplest type of "protofeather" – stiff hair-like filaments on the body of a therizinosaur, Beipiaosaurus (Proceedings of the National Academy of Sciences, vol 106, p 832). Feathers gradually became more complex, turning into the veined "pennaceous" feathers suitable for flight (Integrative Zoology, vol 1, p 4).
Exactly when feathers first appeared is not clear. Xu and his colleagues think that protofeathers pre-date the dinosaurs, first appearing on an archosaur ancestor 245 million years ago. All of the evidence points to feathers arising long before flight. "The first feathers probably evolved for display," says Xu. He believes they were too stiff and too unevenly distributed to serve as insulation. Others don't rule out the role of insulation. "Feathers could easily have evolved for both insulation and display. Very few structures in nature do just one thing," says Turner.
Two main scenarios have been put forward to explain how dinosaurs became airborne. The "ground up" hypothesis holds that small and speedy theropods with long, feathered arms took to the air with running jumps. The other, more widely favoured scenario is "trees down", with flight evolving among tiny feathered dinosaurs that took to the trees to escape predators or find food.
The recent discovery of Microraptor supports this view. This 75-centimetre-long dromaeosaur had flight feathers on both its legs and arms, suggesting that it could not have run along the ground. Instead, it probably used four "wings" to glide or weakly flap from tree to tree (Nature, vol 421, p 335). Although Microraptor is not on the bird lineage, Xu believes it preserves a four-winged gliding stage that birds went through on their way to powered flight.
Long leg feathers have also been found in other bird-like dinosaurs, notably Anchiornis (a troodontid) and the enigmatic Pedopenna, which may be a bird (Naturwissenschaften, vol 92, p 173). Xu's claim is controversial, however. "Long leg feathers may have been unconnected to the evolution of flight and I do not believe that birds went through a four-winged phase," says Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County.
Whether any other feathered dinosaurs evolved powered flight isn't clear, but once birds had mastered it they soared ahead. An enormous range of habitats, in the air, over water, in trees and on islands, became available and new species proliferated.
Author: James O'Donoghue, a writer based in Essex, UK.