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Missing Links and Found Links

In and out of the water, transitional forms from the fossil record illuminate the nuts and bolts of evolution

Pat Shipman

Though missing links are often talked about, it's the found ones that hold a special place in my heart. Found links are fossils that illustrate major transitions during evolutionary history. More than that, such creatures offer unexpected glimpses of the never-predictable twists and turns taken by evolution. Their discovery and surprise bring sheer fun to paleontology and biology.

I have always loved the iconic Archaeopteryx, a beautiful fossil recognized in 1860 that unmistakably combines features of two major groups of animals: birds and reptiles. The exquisite feathered wings of Archaeopteryx bear most unbirdlike claws; its birdlike skull contains an avian brain but carries sharp reptilian teeth, not a beak; and its feathered tail is underlain by a long bony tail typical of a small dinosaur, not a bird. Still, the feathers and wings on these 150-million-year-old fossils qualify Archaeopteryx for the title of First Bird.

Archaeopteryx is a found link in another sense, because the anatomy of this extraordinary species reveals how creatures evolved from propelling themselves along solid substrates, such as the ground or tree limbs, to moving through the air. It was a difficult transition. Archaeopteryx fascinates me in part because its anatomy is not that of a skillful, modern bird, yet it competed with contemporary pterodactyls, which flew using different anatomical structures. I often wonder why birds survived and those wonderful pterodactyls went extinct.

At the time of its discovery, Archaeopteryx was hailed by the anatomist Thomas Henry Huxley as stony proof of evolutionary theory. Decades later, Archaeopteryx was trumped by an extraordinary plethora of feathered dinosaurs—some nonflying—that tell different stories about the evolution of avian features.

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