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。

　　A beautifully preserved fossil from southern Germanyraisesquestions about how feathers evolved from dinosaurs to birds,twopaleontologists argue in a study published Thursday.

　　The 150 million-year-old fossil is a juvenile carnivorousdinosaurabout 2 1/2 feet long that scientists named Juravenator,for theJura mountains where it was found.

　　It would have looked similar in life to the fleet-footedpredatorsthat menaced a young girl on the beach during the openingscene ofThe Lost World, the second Jurassic Park movie.

　　The fossils exceptionally well-preserved bone structureclearlyputs it among feathered kin on the dinosaur family tree.Becauseall of its close relatives are feathered, paleontologistswouldexpect Juravenator to follow suit.

　　But a small patch of skin on the creatures tail shows no signoffeathers. And the skin also doesnt have the follicles thataretypical of feathered dinosaurs, said Luis Chiappe, director oftheDinosaur Institute at the Natural History Museum of LosAngelesCounty. He and Ursula B. Gohlich of the University ofMunichdescribe the fossil in Thursdays issue of the journalNature.

　　It has a typical scaly dinosaurian skin, Chiappe said.

　　The paleontologists believe Juravenators closest knownrelativemay have been a fully feathered dinosaur fromChina,Sinosauropterix.

　　There are a number of possible explanations forJuravenatorsnakedness. Feathers could have been lost on theevolutionary lineleading to Juravenator after arising in anancestor to both it andits feathered relatives. Or feathers couldhave evolved more thanonce in dinosaurs, cropping up in sisterspecies at different timesand places. It is also possible that thisparticular fossil ofJuravenator, which appears to be a juvenile,only grew feathers asan adult or lost its feathers for part of theyear.

　　But there is another possibility as well, said MarkNorell,curator of paleontology at the American Museum of NaturalHistory:It is entirely possible that Juravenator did have feathers,butthey simply failed to fossilize.

　　Feathers are really just difficult things to preserve,Norellsaid.

　　To support his hypothesis he pointed out that several fossilsofthe oldest known bird, archaeopteryx, lack feathers.

　　Whether or not the new specimen raises interesting questionsabouthow feathers — and thus birds — evolved, most experts do notsee itas a challenge to the widely accepted view that modern birdsaredescended from dinosaurs.