Originally, single filaments may well have been for display, the dinosaur equivalent of a peacock's iridescent plumage. Vivid evidence for that theory appeared when scientists unveiled the true colors of 125-million-year-old feathers. Bird feathers and reptile scales contain melanosomes—tiny sacs holding varieties of the pigment melanin. Many paleontologists suspected that dinosaur feathers also contained melanosomes. In Mike Benton's laboratory at the University of Bristol, IVPP's Zhang Fucheng spent more than a year searching for melanosomes in photographs of bird and dinosaur fossils taken with an electron microscope. Zhang's diligence paid off in 2009 when he pinpointed melanosomes in Confuciusornis that contained eumelanin, which gives feathers a gray or black tinge, and pheomelanin, which gives them a chestnut to reddish-brown color. The animal's feathers had patches of white, black and orange-brown coloring.
Sinosauropteryx was even more stunning. Zhang found that the filaments running down its back and tail must have made the dinosaur look like an orange-and-white-striped barber pole. Such a vibrant pattern suggests that "feathers first arose as agents for color display," Benton says.
Early feathers could have served other purposes. Hollow filaments may have dissipated heat, much as the frills of some modern lizards do today. Other paleontologists speculate feathers first evolved to retain heat. A telling example comes from fossils of Oviraptor—a theropod unearthed in Mongolia that lived around 75 million years ago—squatting over egg-filled nests. Oviraptors tucked their legs into the center of the clutch and hugged the periphery with their long forelimbs—a posture bearing an uncanny resemblance to brooding birds keeping their eggs warm. Dinosaurs related to Oviraptor were covered with pennaceous feathers, suggesting that Oviraptor was as well. "Sitting on a nest like that only made sense if it had feathers" to gently insulate its young, says Sues.
Feathers did, of course, eventually become an instrument of flight. Some paleontologists envision a scenario in which dinosaurs used feathers to help them occupy trees for the first time. "Because dinosaurs had hinged ankles, they could not rotate their feet and they couldn't climb well. Maybe feathers helped them scramble up tree trunks," Carrano says. Baby birds of primarily ground-dwelling species like turkeys use their wings in this way. Feathers may have become increasingly aerodynamic over millions of years, eventually allowing dinosaurs to glide from tree to tree. Individuals able to perform such a feat might have been able to reach new food sources or better escape predators—and pass the trait on to subsequent generations.
One of the most beguiling specimens to emerge from Liaoning's shale beds is Microraptor, which Xu discovered in 2003. The bantamweight beast was a foot or two long and tipped the scales at a mere two pounds. Microraptor, from the Dromaeosaur family, was not an ancestor of birds, but it was also unlike any previously discovered feathered dinosaur. Xu calls it a "four-winged" dinosaur because it had long, pennaceous feathers on its arms and legs. Because of its fused breastbone and asymmetrical feathers, says Xu, Microraptor surely could glide from tree to tree, and it may even have been better at flying under its own power than Archaeopteryx was.
Last year, Xu discovered another species of four-winged dinosaur, also at Liaoning. Besides showing that four-winged flight was not a fluke, the new species, Anchiornis huxleyi, named in honor of Thomas Henry Huxley, is the earliest known feathered dinosaur. It came from Jurassic lakebed deposits 155 million to 160 million years old. The find eliminated the final objection to the evolutionary link between birds and dinosaurs. For years, skeptics had raised the so-called temporal paradox: there were no feathered dinosaurs older than Archaeopteryx, so birds could not have arisen from dinosaurs. Now that argument was blown away: Anchiornis is millions of years older than Archaeopteryx.
Four-winged dinosaurs were ultimately a dead branch on the tree of life; they disappear from the fossil record around 80 million years ago. Their demise left only one dinosaur lineage capable of flight: birds.
Just when did dinosaurs evolve into birds? Hard to say. "Deep in evolutionary history, it is extremely difficult to draw the line between birds and dinosaurs," says Xu. Aside from minor differences in the shape of neck vertebrae and the relative length of the arms, early birds and their Maniraptoran kin, such as Velociraptor, look very much alike.
"If Archaeopteryx were discovered today, I don't think you would call it a bird. You would call it a feathered dinosaur," says Carrano. It's still called the first bird, but more for historic reasons than because it is the oldest or best embodiment of birdlike traits.
On the other hand, Confuciusornis, which possessed the first beak and earliest pygostyle, or fused tail vertebrae that supported feathers, truly looks like a bird. "It passes the sniff test," Carrano says.