In 1989 a fossil hunter scouring the limestone formations of the south-central Pyrenees in Spain found the mineralized remains of an unusual ancient bird. Now in the hands of paleontologists, those fossil bones are heating up an already vigorous evolutionary debate: Are birds the direct evolutionary descendants of dinosaurs? Is "the lost world" not lost at all, but hidden all around us under a cloak of feathers?

Seeking answers to those questions, Jose L. Sanz of the Autonomous University of Madrid analyzed the limestone relic to understand its relation to modern birds. In a paper published in the June 6, 1997, issue of Science, Sanz and his colleagues describe the primitive bird as an intriguing blend of the old and the new.

Like the famed Archaeopteryx, generally recognized as the first true bird, the new fossil has teeth in its jaw. Like modern birds, it has well-articulated wings that probably could have supported sophisticated flapping motions; although the specimen is 135 million years old, its flying abilities were probably significantly superior to those of Archaeopteryx, which lived just 10 million years earlier. And yet some aspects of the skull--in particular, the bone formations behind the eyes--closely resemble those of theropods (small, bipedal dinosaurs), not birds.

Sanz's analysis turned up another surprise: the fossil bird appears to be a hatchling, the most ancient one yet found. The surface pattern of its bones looks much like those of today's hatchlings. Based on this and other evidence, the researchers surmise that 135 million years ago, the early growth patterns of birds had already been set. The hatchling's many birdlike qualities, in sharp contrast to its primitive, dinosaurlike head, provide "additional data solidifying the notion that modern birds are short-tailed, feathered descendants of theropod dinosaurs," Sanz and his colleagues write.

This analysis comes on the heels of the announcement of another "missing link" between dinosaurs and birds. Fernando E. Novas of the Argentine Museum of Natural Science in Buenos Aires and Pablo F. Puerta of the Paleontological Museum in Trelew, Argentina, recently announced the discovery of a new species whose morphology suggests an intermediate step on the evolutionary path toward birds. Novas and Puerta describe the small creature, which stood perhaps one meter tall, in the May 22 issue of Nature. They give it the provocative name Unenlagia, meaning "half bird" in the language of the Mapuche Indians.

In most ways, Unenlagia was clearly a dinosaur--there is no evidence that it had feathers, for instance. But it bears a closer resemblance to a bird than any other dinosaur yet studied, the scientists claim. They point in particular to the bone structure of the creature's arms, which could be folded in much the way that birds tuck in their feathered wings. Unenlagia could also elevate its forearms in a manner similar to the upstroke that birds use for flight. >Lawrence M. Witmer of Ohio University in Athens, Ohio, describes the mix of features "a true mosaic, begging the question of where to draw the line between what is, or is not, a 'bird.' "

Although Unenlagia could not fly, its fossil remains contain some fascinating clues about how flight might have evolved. Birds did not start out as gliders like pterosaurs, Novas and Puerta believe. Rather they began as flappers whose beating wings co-opted motions that originally evolved to perform some other functions. Speculating on what those functions might have been the authors propose that Unenlagia "used its arms not only in predation but probably also in maintenance of balance and to control its body attitude while running and leaping." Interestingly, Sanz's hatchling fossil seems to tell the same story. Its advanced, flapping wings already show close kinship with modern fliers. Other aspects of bird anatomy, in comparison, took many millions of years to fall into place.

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