bild 3 lecture 3

bild 3 lecture 3 - A single missing link is not enough to...

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A single “missing link” is not enough to fill in an evolutionary story. How for example did feathers evolve? It seems unlikely that they evolved in such a way as to permit immediate flight, because flight feathers are large and complex and are unlikely to have appeared at once. An artist’s reconstruction of the early Cretaceous, 125 million years ago, in what is now northeastern China. In the remarkable Liaoning formation, which is rich in superbly preserved fossils of both mammals and reptiles, a wide variety of feathered dinosaurs and early birds have been found. They include the large birdlike dinosaur Sinosauropteryx , with primitive downy feathers, the feathered dinosaur Protarchaeopteryx on the right, shown displaying its plumage, and the
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primitive bird Confuciusornis on the branch above. In fossils found in these beds, all the stages of feather evolution have been found. It is now clear from both molecular and fossil evidence that feathers evolved in a series of stages, that they were brand-new structures and not simply modified reptilian scales, and that they must have had a variety of functions.
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roles in feather development, just as they do in other bone and skin structures. These genes produce positional signaling molecules that affect cell differentiation, resulting in a variety of different patterns according to how far the cells are from the source of the signaling. The pattern of expression of these genes in the follicle collar (the sonic hedghog pattern is shown in the figure) varies in collars that will give rise to different types of feather. Five stages of feather development can be distinguished, and fossils of these feathers have been found that represent all five stages. Perhaps most remarkably, the most advanced flight-type feather of stage 5 has been found in many dinosaurs, including a fossil reptile from Liaoning, Microraptor gui , that was clearly dinosaur-like and not a bird, and that had in effect four wings! If it was a flyer, it must have been an extremely clumsy one. It seem more likely to have been able to glide for short distances. Thus, all the stages of feather evolution actually took place before the evolution of bird-like
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This note was uploaded on 11/05/2008 for the course BILD BILD 3 taught by Professor Woodruff during the Spring '08 term at UCSD.

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bild 3 lecture 3 - A single missing link is not enough to...

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