A new "missing link" in the evolution of birds has been found. Paleontologists have discovered the fossil remains of birds whose skeletons show anatomic features intermediate between Archaeopteryx, the oldest known fossil bird, and modern birds.

Archaeopteryx, a pigeon-sized, toothed animal that lived in dinosaur times, has a skeleton that looks much like that of a tiny dinosaur, but the impressions of feathers can be seen in the hardened sediment that encloses the fossil. As a result it is generally regarded as representing an intermediate stage in the evolution of birds from certain dinosaurs.

The Archaeopteryx skeleton lacks the modifications that are known to make the forelimbs function as wings, and there is debate as to whether it could have flown. The newly discovered fossils, which date from a period after that of Archaeopteryx, show several more birdlike features such as a well-developed wing and shoulder skeleton, both obvious adaptations for flight that are far more developed than in Archaeopteryx. The pelvis and hind limbs, however, retain a distinctly reptilian cast.

The new fossils were found in Spain by J.L. Sanz of Madrid and colleagues from Spain and Argentina. Archeopteryx was found in the late 19th century in Germany. Sanz's report was in last week's Nature.