Archaeopteryx

When the first complete Archaeopteryx skeleton was described in 1861 — just two years after Darwin published On the Origin of Species — it caused a sensation. Here was a creature with the wings and feathers of a bird but the teeth, clawed fingers, and long bony tail of a dinosaur. It was the missing link made tangible.

Archaeopteryx was found in the Solnhofen limestone of Bavaria, Germany — a formation deposited in a shallow, warm lagoon. The extremely fine-grained sediment preserved extraordinary detail: not just bones, but the impressions of every individual feather. The quality of preservation is unmatched in the fossil record.

Whether Archaeopteryx could truly fly is still debated. Its wings were asymmetrical — like modern flying birds, not gliding ones — suggesting powered flight was possible. But it lacked the keeled sternum that anchors flight muscles in modern birds, suggesting it couldn't fly as efficiently. The current thinking is that it was a capable, if not elegant, flier.

Archaeopteryx has been removed from and returned to the base of the bird family tree several times as new feathered dinosaur discoveries have complicated the picture. It remains the most important transitional fossil ever found — a window into the moment reptiles became birds.