National Geographic : 2011 Feb
• theropods with threadlike feathers alive today: baby birds. All the feathers on a developing chick begin as bristles rising up from its skin; only later do they split open into more complex shapes. In the bird embryo these bristles erupt from tiny patches of skin cells called placodes. A ring of fast-growing cells on the top of the placode builds a cylindrical wall that becomes a bristle. Reptiles have placodes too. But in a reptile embryo each placode switches on genes that cause only the skin cells on the back edge of the placode to grow, eventually forming scales. In the late 1990s Richard Prum of Yale University and Alan Brush of the University of Connecticut developed the idea that the transition from scales to feathers might have depended on a simple switch in the wiring of the genetic commands inside placodes, causing their cells to grow vertically through the skin rather than horizon- tally. In other words, feathers were not merely a variation on a theme: ey were using the same genetic instruments to play a whole new kind of music. Once the rst laments had evolved, only minor modi cations would have been required to produce increasingly elaborate feathers. Until recently it was thought that feath- ers rst appeared in an early member of the lineage of theropods that leads to birds. In 2009, however, Chinese sci- entists announced the discovery of a bristly-backed creature, Tianyulong, on the ornithischian branch of the dinosaur family tree---about as distant a relative of theropods as a dinosaur can be. is raised the astonishing possibility that the ancestor of all dinosaurs had hairlike feathers and that some species lost them later in evolution. e origin of feathers could be pushed back further still if the "fuzz" found on some pterosaurs is con rmed to be feathers, since these ying reptiles share an even older ancestor with dinosaurs. ere's an even more astonishing possibility. e closest living relatives of birds, dinosaurs, and pterosaurs are alligators and crocodilians. Although these scaly beasts obviously do not have feathers today, the discovery of the same from trees. Other paleontologists argued that birds did not evolve from dinosaurs---rather, their similarities derived from a shared com- mon ancestor deeper in the past. In 1996 Chinese paleontologists delivered startling support for Ostrom's hypothesis. It was the fossil of a small, short-armed 125-million- year-old theropod, Sinosauropteryx, which had one extraordinary feature: a layer of thin, hollow laments covering its back and tail. At last there was evidence of truly primitive feathers---found on a ground-running theropod. In short, the origin of feathers may have had nothing to do with the origin of ight. Soon paleontologists were nding hundreds of feathered theropods. With so many fossils to compare, they began piecing together a more detailed history of the feather. First came simple laments. Later, di erent lineages of theropods evolved various kinds of feathers, some re- sembling the u y down on birds today, some having symmetrically arranged barbs. Other theropods sported long, sti ribbons or broad laments, unlike the feathers on any living birds. e long, hollow laments on theropods posed a puzzle. If they were early feathers, how had they evolved from at scales? Fortunately, there are If feathers did not evolve first for flight, what was their purpose? Perhaps for insulation or, as recent research suggests, to get the attention of the opposite sex. Carl Zimmer wrote about carnivorous plants in the March 2010 issue of National Geographic. Robert Clark is a frequent contributor. Xing Lida's dinosaur books are best sellers in China.