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生物学家考羽毛进化过程

2006-07-28 15:29

  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.

  恐龙化石有疑问 古生物学家思索羽毛进化过程从体形庞大的恐龙到身材娇小的鸟类,动物的羽毛是如何进化的?在德国南部发现的一具保存完好的恐龙化石日前引起了古生物学家们对动物羽毛进化过程的思考。

  古生物学家聚焦恐龙羽毛

  据美联社3月15日报道,这具有1.5亿年历史的化石是一只大约2.5英尺长的幼年食肉恐龙留下来的,因为发现化石的地点位于侏罗山,因此科学家们称它为Juravenator.

  这具化石保存有异常完好的骨骼结构,因此可以将其明确地归入到恐龙家族中长有羽毛的一类。由于它所有的近亲都长着羽毛,所以考古学家认为Juravenator也是如此。

  但考古学家们发现,Juravenator尾部残留的一小块皮肤却丝毫显示不出它曾长有羽毛的迹象。美国洛杉矶县自然历史博物馆恐龙研究院负责人刘易斯·齐亚比说,这块皮肤上也没有长羽毛的恐龙所拥有的典型的羽囊。齐亚比说,“这是典型的有鳞恐龙的皮肤”。齐亚比和德国慕尼黑大学的研究人员厄休拉·高立克在周四(3月16日)出版的《自然》杂志上阐述了他们的上述观点。

  这些古生物学家们相信,已知的与Juravenator关系最为紧密的亲属可能要数中国的一种全身长有羽毛的恐龙——中华龙鸟。

  恐龙为何无羽的多种可能

  对于Juravenator为何没有羽毛,可能的解释有很多。其中之一认为,在通向Juravenator的进化道路上,羽毛可能消失了。或者,羽毛在恐龙身上不是首次进化,这种进化突然发生在不同时间和地点的相似种类的恐龙身上。还可能因为,这是Juravenator幼年时期的化石,而它只有在成年后才会长出羽毛。

  美国自然历史博物馆的古生物学家马克·诺莱尔还提出了另外一种说法,即完全有可能Juravenator的确长有羽毛,但是这些羽毛却没能形成化石。诺莱尔说:“羽毛确实是很难保存下来的东西。”为了证明这种观点,他指出,已知的最早的鸟类——始祖鸟的一些化石也没有羽毛。

  报道说,无论这个新发现的化石是否引起人们,对羽毛及鸟类进化过程的新质疑,但大多专家认为,它不会对人们已普遍接受的现代鸟类是由恐龙进化而来的这个观点构成挑战。

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