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

bizarre, thirty-inch-long creature had fully feathered arms and legs (figure 10b), which when stretched out were probably used for gliding. 10

    FIGURE 9 . Skeletons of a modern bird (chicken), a transitional form (Archaeopteryx), and a small, bipedal, carnivorous theropod dinosaur ( Compsognathus ), similar to one of Archaeopteryx’s ancestors. Archaeopteryx has a few features like those of modern birds (feathers and an opposable big toe), but its skeleton is very similar to that of the dinosaur, including teeth, a reptilian pelvis, and a long bony tail. Archaeopteryx was about the size of a raven, Compsognathus slightly larger.

    FIGURE 10A . The feathered dinosaur Sinornithosaurus millenii, original fossil from China (about 125 million years old), and artist’s reconstruction. The fossil clearly shows the impression of filamentous feathers, especially on the head and forelimbs (arrows).

    FIGURE 10B . The bizarre “four-winged” dinosaur Microraptor gui, which had long feathers on both its fore- and hindlimbs. These feathers (arrows) are clearly visible in the fossil, about 120 million years old. It’s not clear whether this animal could fly or only glide, but the rear “wings” almost certainly helped it land, as shown in the drawing.
     
    Theropod dinosaurs didn’t just have primitive birdlike features, it seems: they even behaved in birdlike ways. The American paleontologist Mark Norell and his team described two fossils showing ancient behavior—and if ever fossils could be called “touching,” these are they. One is a small feathered dinosaur sleeping with its head tucked under its folded, winglike forearm—exactly as modern birds sleep (figure 11). The animal, given the scientific name Mei long (Chinese for “soundly sleeping dragon”), must have died while slumbering. The other fossil is a female theropod who met her end while sitting on her nest of eggs, showing brooding behavior similar to that of birds.
    All the nonflying feathered dinosaur fossils date between 135 and 110 million years ago—later than the 145-million-year-old Archaeopteryx. That means that they could not be Archaeopteryx’s direct ancestors, but they could have been its cousins. Feathered dinosaurs probably continued to exist after one of their kin gave rise to birds. We should, then, be able to find even older feathered dinosaurs that were the ancestors of Archaeopteryx. The problem is that feathers are preserved only in special sediments—the fine-grained silt of quiet environments like lake beds or lagoons. And these conditions are very rare. But we can make another testable evolutionary prediction: someday we’ll find fossils of feathered dinosaurs that are older than Archaeopteryx. 11 12
    We’re not sure whether Archaeopteryx is the one single species that gave rise to all modern birds. It seems unlikely that it was the “missing link.” But regardless, it’s one of a long string of fossils (some found by the intrepid Paul Sereno) that clearly document the appearance of modern birds. As these fossils get younger, we see the reptilian tail shrinking, the teeth disappearing, the claws fusing together, and the appearance of a large breastbone to anchor the flight muscles.

    FIGURE 11 . Fossil behavior: the feathered theropod dinosaur Mei long (top) fossilized in a birdlike roosting position, sleeping with its head tucked under its forelimb. Middle: a reconstruction of Mei long from the fossil. Bottom: a modern bird (juvenile house sparrow) sleeping in the same position.
     
    Put together, the fossils show that the basic skeletal plan of birds, and those essential feathers, evolved before birds could fly. There were many feathered dinosaurs, and their feathers are clearly related to those of modern birds. But if feathers didn’t arise as adaptations for flying, what on earth were they for? Again, we don’t know. They could have been used for ornamentation or display—perhaps to attract mates. It seems more likely,