Evolution's fast track toward slow flight.

Moving slowly can take a toll on anything that flies. A plane traveling at low speed generates little lift and has a hard time staying in the air. Birds, however, evolved a solution to this aerodynamic impasse extremely early in their evolutionary history, according to evidence gleaned from a 115-million-year-old fossil discovered in Spain.

During slow flight, birds make use of a special structure called a bastard wing, or alula, which consists of several feathers attached to the first digit of the wing bones. By moving that digit, a bird can separate the feathers of the alula from the rest of the wing, creating a slot that helps channel air over the flight feathers. The improved airflow enhances lift and prevents the bird from stalling during takeoff and landing.

Because feathers rarely fossilize, scientists have had trouble tracing the evolution of this important flight adaptation. The Spanish discovery, however, features remarkably well preserved alula feathers in their original position, report Jose L. Sanz of the Universidad Autonoma of Madrid, Luis M. Chiappe of the American Museum of Natural History in New York, and their colleagues.

The paleontologists named the new species Eoalulavis, because this goldfinch-sized bird shows the earliest evidence of an alula. They describe the fossil in the Aug. 1 Nature.

"It's a significant find, and it helps to bridge the gap between Archaeopteryx and more modern birds," comments Lawrence M. Witmer, an evolutionary biologist at Ohio University in Athens.

Archaeopteryx, the earliest known bird, lived about 150 million years ago, during the late Jurassic period, and apparently lacked an alula. The Spanish discovery "suggests that some portions of the flight apparatus appeared quite early after Archaeopteryx," says Witmer.

With its long legs, long tail, claws, and teeth, Archaeopteryx looked like the small, bipedal dinosaurs from which birds evolved, according to a theory popular among paleontologists. Although Archaeopteryx had feathers and could fly, researchers think this early bird was an awkward aerialist at best.

Eoalulavis falls between Archaeopteryx and today's birds. "What's particularly interesting is that it retains many of the primitive dinosaurian features that are absent in modern birds, but it has wing and shoulder [adaptations] that are very avian," says Chiappe.

Until recently, the early history of birds has remained obscure, with few fossils filling the interval between Archaeopteryx and the late Cretaceous.

But discoveries in Argentina, China, Korea, Mongolia, Madagascar, and Spain have given paleontologists a wealth of new fossils from this critical period.

Eoalulavis is the third bird species to come out of Spain's Las Hoyas deposits-the remains of a large lake surrounded by tropical forests.

Paleontologists have deduced the diet of Eoalulavis from the fragments of crustaceans found in its abdominal region. "We imagine these birds fed in the water or close to the water," says Chiappe.