考研英語作文:恐龍化石有疑問 生物學家考羽毛進化過程

         A beautifully preserved fossil from southern Germany raises questions about how feathers evolved from dinosaurs to birds, two paleontologists argue in a study published Thursday.
　　The 150 million-year-old fossil is a juvenile carnivorous dinosaur about 2 1/2 feet long that scientists named Juravenator, for the Jura mountains where it was found.
　　It would have looked similar in life to the fleet-footed predators that menaced a young girl on the beach during the opening scene of "The Lost World," the second Jurassic Park movie.
　　The fossil’s exceptionally well-preserved bone structure clearly puts it among feathered kin on the dinosaur family tree. Because all of its close relatives are feathered, paleontologists would expect Juravenator to follow suit.
　　But a small patch of skin on the creature’s tail shows no sign of feathers. And the skin also doesn’t have the follicles that are typical of feathered dinosaurs, said Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County. He and Ursula B. Gohlich of the University of Munich describe the fossil in Thursday’s issue of the journal Nature.
　　"It has a typical scaly dinosaurian skin," Chiappe said.
　　The paleontologists believe Juravenator’s closest known relative may have been a fully feathered dinosaur from China, Sinosauropterix.
　　There are a number of possible explanations for Juravenator’s nakedness. Feathers could have been lost on the evolutionary line leading to Juravenator after arising in an ancestor to both it and its feathered relatives. Or feathers could have evolved more than once in dinosaurs, cropping up in sister species at different times and places. It is also possible that this particular fossil of Juravenator, which appears to be a juvenile, only grew feathers as an adult or lost its feathers for part of the year.
　　But there is another possibility as well, said Mark Norell, curator of paleontology at the American Museum of Natural History: It is entirely possible that Juravenator did have feathers, but they simply failed to fossilize.
　　"Feathers are really just difficult things to preserve," Norell said.
　　To support his hypothesis he pointed out that several fossils of the oldest known bird, archaeopteryx, lack feathers.
　　Whether or not the new specimen raises interesting questions about how feathers — and thus birds — evolved, most experts do not see it as a challenge to the widely accepted view that modern birds are descended from dinosaurs.