MARGINALIA
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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