Free Why Evolution Is True by Jerry A. Coyne Page A

feet in height! It’s not hard to envision the next evolutionary step: the flapping of colugolike limbs to produce true flight, as we see in bats. But we no longer have to only imagine this step: we now have the fossils that clearly show how flying birds evolved.
    Since the nineteenth century, the similarity between the skeletons of birds and some dinosaurs led paleontologists to theorize that they had a common ancestor—in particular, the theropods: agile, carnivorous dinosaurs that walked on two legs. Around 200 million years ago, the fossil record shows plenty of theropods but nothing that looks even vaguely birdlike. By 70 million years ago, we see fossils of birds that look fairly modern. If evolution is true, then we should expect to see the reptile-bird transition in rocks between 70 and 200 million years old.
    And there they are. The first link between birds and reptiles was actually known to Darwin, who, curiously, mentioned it only briefly in later editions of The Origin, and then only as an oddity. It is perhaps the most famous of all transitional forms: the crow-sized Archaeopteryx lithographica, discovered in a limestone quarry in Germany in 1860. (The name Archaeopteryx means “ancient wing,” and “lithographica” comes from the Solnhofen limestone, fine-grained enough to make lithographic plates and preserve the impressions of soft feathers.) Archaeopteryx has just the combination of traits one would expect to find in a transitional form. And its age, about 145 million years, places it where we would expect.
    Archaeopteryx is really more reptile than bird. Its skeleton is almost identical to that of some theropod dinosaurs. In fact, some biologists who didn’t look at the Archaeopteryx fossils closely enough missed the feathers, and misclassified the beasts as theropods. (Figure 9 shows this similarity between the two types.) The reptilian features include a jaw with teeth, a long bony tail, claws, separate fingers on the wing (in modern birds these bones are fused, as you can see by inspecting a gnawed chicken wing), and a neck attached to its skull from behind (as in dinosaurs) instead of from below (as in modern birds). The birdlike traits number just two: large feathers and an opposable big toe, probably used for perching. It still isn’t clear whether this creature, though fully feathered, could fly. But its asymmetrical feathers-one side of each feather is larger than the other—suggest that it could. Asymmetrical feathers, like airplane wings, create the “airfoil” shape necessary for aerodynamic flight. But even if it could fly, Archaeopteryx is mainly dinosaurian. It is also what evolutionists call a “mosaic.” Rather than having every feature appear halfway between those of birds and reptiles, Archaeopteryx has a few bits that are very birdlike, while most bits are very reptilian.
    After the discovery of Archaeopteryx, no other reptile-bird intermediates were found for many years, leaving a gaping hole between modern birds and their ancestors. Then, in the mid-1990s, a spate of astonishing discoveries from China began to fill in the gap. These fossils, found in lake sediments that preserve the impressions of soft parts, represent a veritable parade of feathered theropod dinosaurs. 8 Some of them have very small filamentous structures covering the whole body—probably early feathers. One is the remarkable Sinornithosaurus millenii (Sinornithosaurus means “Chinese bird-lizard”), whose whole body was covered with long, thin feathers—feathers so small that they couldn’t possibly have helped it fly (figure 10a) And its claws, teeth, and long, bony tail clearly show that this creature was far from being a modern bird. 9 Other dinosaurs show medium-sized feathers on their heads and forelimbs. Still others have large feathers on the forelimbs and tail, much like modern birds. The most striking of all is Microraptorgui, the “four-winged dinosaur.” Unlike any modern bird, this