树懒颈椎骨因何独特
文章来源:未知 文章作者:meng 发布时间:2010-10-19 03:15 字体: [ ]  进入论坛
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By examining the development of bones in the vertebral column(脊柱) , limbs, and ribcage(胸腔) , scientists at the University of Cambridge have discovered how sloths1(树懒) evolved their unique neck skeleton. From mice to giraffes(长颈鹿) , mammals are remarkable2 in that all but a handful of their 5000 species have exactly seven vertebrae in the neck. Among the few that deviate3 from(偏离,脱离) this number are three-toed sloths, which may have up to ten ribless4 vertebrae in the neck.

Traditionally, vertebrae above the shoulders that lack ribs5 are known as cervical(颈的) or neck vertebrae. Animals such as birds and lizards6 show great variety in the number of vertebrae(椎骨) in their neck. For example, a swan may have twice as many as a songbird.

Mammals, on the other hand, are much more conservative. A giraffe has the same number of neck vertebrae as a human, mouse, elephant, or armadillo(犰狳) ; all have exactly seven. Sloths are an exception, with up to 10 vertebrae in their neck.

In order to discover if patterns of bone formation in these strange animals give any clues to their divergent(相异的,分歧的) vertebral anatomy7(解剖) , scientists based at the University of Cambridge have investigated the development of the skeleton in mammals, focusing on the vertebral column in sloths.

The scientists found that in all mammals except for sloths, bone formation always took place earlier in the body of first few vertebrae of the ribcage than in the neck. The only exception was among three-toed sloths, which show early bone-formation in the bodies of their distal, ribless neck vertebrae, before those of the ribcage.

However, by observing the position of bone-formation within the vertebral column, the investigators8 made a startling discovery: all mammals, including sloths, show early development of the body of the eighth vertebra down from the head, whether or not it is part of the neck.

In other words, the bottom neck vertebrae of sloths show a similar sequence of development as the top ribcage vertebrae of other mammals, both of which start at eight vertebrae down from the head. This shows that the bottom "neck" vertebrae of sloths are developmentally the same as ribcage vertebrae of other mammals, but lack ribs.

Dr Robert Asher, of the Department of Zoology9 at the University of Cambridge, said: "The remarkable conservatism of the mammalian neck is apparent even in those few species that superficially seem to be exceptions, like sloths. Even though they've got eight to ten ribless vertebrae above the shoulders, unlike the seven of giraffes, humans, and nearly every other species of mammal, those extra few are actually ribcage vertebrae masquerading as(化装成,冒充成) neck vertebrae."

These new results support the interpretation10 that the limb girdles and at least part of the ribcage derive11 from different embryonic12 tissues than the vertebrae, and that during the course of evolution, they have moved in concert with each other relative to the vertebral column. In three-toed sloths, the position of the shoulders, pelvis(骨盆) , and ribcage are linked with one another, and compared to their common ancestor shared with other mammals, have shifted down the vertebral column to make the neck longer.



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1 sloths 99bb49e2cc8aa5774736e771d9f65efa     
懒散( sloth的名词复数 ); 懒惰; 树獭; (经济)停滞。
参考例句:
  • Mummies of pleistocene ground sloths, with original skin, hair, tendons and claws have been found. 还发现了保存原有皮肤,毛发,腱和爪的更新世时期地面树懒的木乃伊。
  • He was inspired by fossils of armadillos and sloths. 犰狳和树懒化石让他获得了灵感。
2 remarkable 8Vbx6     
adj.显著的,异常的,非凡的,值得注意的
参考例句:
  • She has made remarkable headway in her writing skills.她在写作技巧方面有了长足进步。
  • These cars are remarkable for the quietness of their engines.这些汽车因发动机没有噪音而不同凡响。
3 deviate kl9zv     
v.(from)背离,偏离
参考例句:
  • Don't deviate from major issues.不要偏离主要问题。
  • I will never deviate from what I believe to be right.我绝不背离我自信正确的道路。
4 ribless c6a56f38e12d1cede035ea687e27a318     
无肋骨的
参考例句:
5 ribs 24fc137444401001077773555802b280     
n.肋骨( rib的名词复数 );(船或屋顶等的)肋拱;肋骨状的东西;(织物的)凸条花纹
参考例句:
  • He suffered cracked ribs and bruising. 他断了肋骨还有挫伤。
  • Make a small incision below the ribs. 在肋骨下方切开一个小口。
6 lizards 9e3fa64f20794483b9c33d06297dcbfb     
n.蜥蜴( lizard的名词复数 )
参考例句:
  • Nothing lives in Pompeii except crickets and beetles and lizards. 在庞培城里除了蟋蟀、甲壳虫和蜥蜴外,没有别的生物。 来自辞典例句
  • Can lizards reproduce their tails? 蜥蜴的尾巴断了以后能再生吗? 来自辞典例句
7 anatomy Cwgzh     
n.解剖学,解剖;功能,结构,组织
参考例句:
  • He found out a great deal about the anatomy of animals.在动物解剖学方面,他有过许多发现。
  • The hurricane's anatomy was powerful and complex.对飓风的剖析是一项庞大而复杂的工作。
8 investigators e970f9140785518a87fc81641b7c89f7     
n.调查者,审查者( investigator的名词复数 )
参考例句:
  • This memo could be the smoking gun that investigators have been looking for. 这份备忘录可能是调查人员一直在寻找的证据。
  • The team consisted of six investigators and two secretaries. 这个团队由六个调查人员和两个秘书组成。 来自《简明英汉词典》
9 zoology efJwZ     
n.动物学,生态
参考例句:
  • I would like to brush up my zoology.我想重新温习一下动物学。
  • The library didn't stock zoology textbooks.这家图书馆没有动物学教科书。
10 interpretation P5jxQ     
n.解释,说明,描述;艺术处理
参考例句:
  • His statement admits of one interpretation only.他的话只有一种解释。
  • Analysis and interpretation is a very personal thing.分析与说明是个很主观的事情。
11 derive hmLzH     
v.取得;导出;引申;来自;源自;出自
参考例句:
  • We derive our sustenance from the land.我们从土地获取食物。
  • We shall derive much benefit from reading good novels.我们将从优秀小说中获得很大好处。
12 embryonic 58EyK     
adj.胚胎的
参考例句:
  • It is still in an embryonic stage.它还处于萌芽阶段。
  • The plan,as yet,only exists in embryonic form.这个计划迄今为止还只是在酝酿之中。
TAG标签: neck sloths vertebrae
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