Fossil birds sport a new kind of feather.

Two fossil specimens of a primitive, starling-size bird that lived about 125 million years ago have tail feathers that, its discoverers say, could reveal how feathers originated.

The new species, which the Chinese paleontologists named Protopteryx fengningensis, is the most primitive known among the birds called enantiornithines. This group at one time included the majority of birds but became extinct by the end of the Cretaceous period 65 million years ago.

Described in the Dec. 8 SCIENCE, the new bird sported three types of feathers, says Zhonghe Zhou, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing and coauthor of the report. Two of the types have appeared in other fossils: the downy feathers on Protopteryx's head and body and the flight feathers on its wings. However, the bird's central tail feathers are of a kind that hasn't been found in ancient birds, the researchers say.

These long feathers appear to have been elongated, scale-like structures. Unlike feathers on modern birds, they weren't differentiated into individual barbs that extended from a central shaft.

This design, Zhou says, is either a holdover from ancestors or a reversion to a previous form from a modern-style feather. In either case, the researchers contend, the plumes may provide insight into the evolution of modern feathers.

Zhou and his coauthor Fucheng Zhang, also of the institute in Beijing, suggest that modern feathers evolved from scales that first became elongated and then developed a central shaft. Later, vanes that extend from the shaft differentiated into barbs. In succeeding generations of animals, these barbs evolved smaller structures, called barbules and barbicels, that hook neighboring barbs together.

Other scientists don't agree, however. "This suggestion is much too speculative," says Robert R. Reisz, a paleontologist at the University of Toronto in Mississauga, Ontario. "We just don't know."

It's equally conceivable that Protopteryx's tail feathers arose to take on a form that was totally different from more ancestral feathers, he notes.

Moreover, Reisz says, it's unlikely that feathers--being very specialized structures--evolved from elongated scales, which are equally specialized. It's more likely that early feathers arose from small scales that first acquired a featherlike structure and then became increasingly longer in subsequent generations of animals, he says.

The key to the evolution of feathers is yet to be found, Reisz says, and it likely won't be discerned from the fossils of relatively advanced birds such as Protopteryx. Instead, he maintains, the answers lie in the undiscovered fossils of the predecessors of Archaeopteryx, the first known bird. Although Archaeopteryx lived about 15 million years before Protopteryx, it had feathers almost identical to those of existing birds.

Richard O. Prum, an ornithologist at the University of Kansas in Lawrence, agrees. "[Protopteryx] is too high up the evolutionary tree to tell us much of anything about the origins of feathers," he says.

Nevertheless, Prum calls the Protopteryx "a fascinating new find." He says it suggests that the origins of specialized feathers used for display--and therefore complex social behavior between individual birds in a species--may stretch back to the earliest phases of avian evolution.