Feathers are far more intricate than reptiles' scales or mammals' hair -- they're the most complex integumentary structure -- the outer covering of an animal -- known. There are also lots of different feather shapes, from streamlined flight feathers on an eagle's wing to the billowy, decorative plumes of an ostrich's tail. But in spite of their complexity and diversity, most feathers have a similar structure. They're made of a central, hollow tube, called a rachis, and branching structures called barbs. Flight feathers have shorter barbs on one side of the rachis, creating an aerodynamic shape that gives birds lift.
While some fossil specimens show the impression of well-defined, modern feathers, not all of the plumage is so complex. Some have the impressions of fine, branching filaments. For the most part, these filaments have one of two basic arrangements. Several can meet at a common base, or they can branch off a central filament in a pattern reminiscent of a rachis and barbs. Often, paleontologists are reluctant to call the simplest arrangements feathers, instead referring to them as integumentary structures. In some specimens, critics contend that the filaments aren't feathers at all but are impressions of frilly body structures that decayed as the bones fossilized.
Dinosaur feathers also didn't necessarily create lift. Some species may have developed downy feathers to provide extra insulation. This could explain why most of the feathered fossils discovered so far are from relatively small dinosaurs -- a gigantic dinosaur like Apatosaurus wouldn't need the extra protection. Beipiaosaurus inexpectus, the biggest feathered dinosaur found in the Liaoning fossil beds so far, was only about 2.2 meters (7 feet) long [source: Norell and Xing]. The presence of quill knobs, or bumps that connect feathers to bones, in the forearms of a Velociraptor also suggests that this notorious predator had feathers on at least part of its body. Velociraptor was slightly smaller than Beipiaosaurus.
Although dinosaurs may not have grown feathers solely for the purpose of flight, it does appear that some eventually used their feathers to glide. For example, Microraptor gui made headlines in 2003 for being a four-winged dinosaur -- one of its fossil specimens has feather impressions around both its front and back legs. Some researchers argue that Microraptor gui used the claws at the end of its fore and hind legs, or its front and back wings, to climb trees, and then it used all four limbs to glide down. This interpretation may support the theory that birds learned to fly by gliding down from trees rather the theory that flight started from the ground up.
However, there's still debate about Microraptor gui and whether its back legs were used for flying at all. It may not have been able to move its hind legs into a position that would be useful for gliding. Instead, it may have glided on the lift from its forelegs while using the feathers on its back legs to stay warm.
In some ways, feathered dinosaur fossils have raised more questions about feathers and flight than they have answered, and much of the evidence is open to interpretation. However, it's likely that fossil beds full of fine-grained paper shale, like the ones in Liaoning Province, will continue to offer researchers new clues on the origins of feathers and flight.