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As luck would have it, the first complete fossil of Archaeopteryx was discovered in 1862, only two years after Charles Darwin published The Origin of Species. For believers in the new theory of evolution, this was a major find: With its birdlike beak, pigeon-sized body and feathers, Archaeopteryx was a prime candidate for the "missing link" between dinosaurs and birds. (By the way, the fossil site most famous for its Archaeopteryx remains is Solnhofen in Germany.) See 10 Facts About Archaeopteryx and a gallery of Archaeopteryx pictures
Although Archaeopteryx is widely considered to be the first creature that's more bird than dinosaur, it's important not to lose sight of its distinctly reptilian characteristics, including a long, bony tail, a flat breastbone, and (most unbirdlike of all) its three claws jutting out from the middle of each wing. It's not even certain that Archaeopteryx could fly for extended periods of time, though it was almost surely capable of short jaunts. (One specimen of Archaeopteryx was recently assigned to its own genus, Wellnhoferia, on the basis of small anatomical differences, though not all paleontologists are convinced.) Was Archaeopteryx a Bird or a Dinosaur? discusses where Archaeopteryx falls on the bird-dinosaur spectrum, and How Was Archaeopteryx Discovered? discusses the various fossil specimens of Archaeopteryx.
Recently, a new study has concluded that the inner ear of Archaeopteryx was similar to that of modern-day emus. Although emus aren't known for their particularly sharp hearing, the fact remains that Archaeopteryx's hearing ability seems to have been more on the bird than on the reptile end of the evolutionary spectrum. On the other hand, another recent study concludes that Archaeopteryx's feathers weren't nearly strong enough to support powered flight, which would place it smack down in the dinosaur camp again! One more piece of the puzzle is the recent discovery of Xiaotingia, a very Archaeopteryx-like critter that preceded its more famous relative by five million years--and has been classified as a feathered theropod dinosaur rather than a bird.