Official 27 Set 5

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Sauropod

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What does the professor mainly discuss?
  • A. Comparisons between land animals and ocean-going animals of the Mesozoic era

  • B. Comparisons between sauropods and modern animals

  • C. Possible reasons why sauropods became extinct

  • D. New theories about the climate of the Mesozoic era

显示答案 正确答案: B

我的笔记 编辑笔记

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    NARRATOR:Listen to part of a lecture in a zoology class.

    FEMALE PROFESSOR:Your reading for today touched on dinosaur fossils from the Mesozoic era... which ended about 65 million years ago.Today, we'll be discussing the sauropods.

    I think our discussion of sauropods will illustrate what we can learn by comparing the fossil record to modern animals.By "fossils," we mean traces of prehistoric animals-such as bones, which become mineralized...or impressions of bones or organs that're left in stone...

    Now, sauropods were among the largest animals to exist, ever.They were larger than blue whales, which are the largest animals alive today; they weighed up to 100 tons-20 times as much as elephants.Also, they were an extremely successful kind of dinosaur: there's evidence of sauropods in the fossil record for an unusually long time-over 100 million years.So: Why were sauropods so successful?

    Biologically speaking, sauropods shouldn't have been successful.Large animals-like... elephants, say... they require much more food and energy... and have fewer offspring than smaller animals... this makes maintaining a population harder.The largest animals today don't live on land, but in the ocean, where food's easier to find.A blue whale, for instance, can eat up to 8,000 pounds of food a day... and they give birth only once every few years.We also know that body heat-that... uh, well... large animals can't easily get rid of excess body heat... but for an ocean-going whale, that's not a problem; for a 100-ton land animal, it can be.

    For years we've assumed it was the abundant plant life of the Mesozoic that allowed these giants to thrive.However, we now know that, since oxygen levels were much lower in the Mesozoic than we'd assumed, there was much less plant life for sauropods to eat than we'd thought.So now... well, we're looking at other-we... we're-we're trying to understand the biology of sauropods, comparing their fossils to the anatomy of modern animals, to get a better idea of how they lived.

    What we've found is that sauropods were experts at conserving energy.They had enormous stomach capacity-the ability to digest food over a long period... converting it to energy at a slower pace-"saving it" for later.For animals with small stomachs, it takes lots of energy to constantly look for food and then digest it... with larger stomachs and slower digestion, you don't need as much energy... Joseph?

    MALE STUDENT:[respectful, but unconvinced] Does... mm, do scientists actually know about sauropods, from looking-I mean, how much can we actually learn, looking at some ancient bones...compared to all we can learn from modern animals?... and comparisons between animals that lived millions of years apart-w-well, it just seems...more like guessing.

    FEMALE PROFESSOR:There's always some guesswork when studying extinct animals; but that's exactly what leads to discoveries:A hypothesis-a type of "guess"-is made...we "test" the hypothesis, by looking for evidence to support it... then some questions are answered, which may lead to new questions...

    For example: Let's look at one of these comparisons...We know sauropods couldn't chew food- their skulls show they had no chewing muscles.Lots of modern animals-like birds and reptiles-also can't chew food...they need to swallow it whole.But modern animals have an interesting aid for digesting food:They swallow stones... stones that're used to help grind up the food... before it's actually digested in the stomach.These stones are called "gastroliths."

    Gastroliths make food easier to digest, essentially smashing food up, just as we do when we chew.Over time, gastroliths inside the animal are ground down and become smooth and rounded...Now, sauropod fossils are commonly found with smoothed stones... for years, we thought these were gastroliths-they looked just like gastroliths, and were found in the area of the sauropods' stomachs.

    A recent study measured the gastroliths in modern animals-in ostriches...and the study showed that ostriches need to ingest about 1% of their total body weight in gastroliths.But we've been able to determine that the stones found with sauropods total much less proportionally... less than a tenth of 1% of their body weight.

    So now, we're not quite sure what these sauropod stones were used for.It could be they were accidentally ingested as the sauropods foraged for food- that they served no real purpose.Other researchers speculate that sauropods ingested these stones as a source of some of the minerals they needed, such as calcium.

    So, the original hypothesis- that the stones found with sauropods were gastroliths- even though it hasn't been supported, has helped us to make new hypotheses, which may eventually lead to the answer.

  • 旁白:听一段动物学课程。

    教授:你们今天的阅读材料讲的是中生代时期的恐龙化石,中生代在六千五百万年前结束。今天我们要讲的是蜥脚类动物。

    我认为,我们对蜥脚类动物的讨论能告诉我们,将化石和现代动物对比,我们能学到什么。化石是指史前生物的遗迹,比如已经矿化的骨头,或者骨头、器官在石头上留下的印记。

    有史以来,在存在过的动物中,蜥脚类动物是体型最大的。它们比蓝鲸还大,而蓝鲸是现存动物中最大的。蜥脚类动物体重能达到100吨,是大象体重的二十倍。并且它们作为恐龙而言是很成功的。有化石证据显示,蜥脚类动物的存在时间长得有些不寻常,超过了一亿年。为什么蜥脚类动物这么成功?

    从生物学角度来看,蜥脚类动物不应该这么成功。大型动物(如大象)比起体型更小的动物而言,需要更多的食物和能量,而后代更少,这就使维持一定的种群数量变得更加困难。现在,体型最大的动物不生活在陆地上,而是生活在更容易觅食的海洋里。举个例子,一只蓝鲸一天能吃掉八千磅的食物,而且它们每隔几年才繁衍一次。我们还知道体温……嗯...大型动物不容易散去多余体温。对于在海里生活的鲸鱼来说,这不是问题;但对于一百吨的陆生生物而言,就是个问题了。

    多年来,我们都以为,是中生代时期的丰富植物让这些巨型动物繁荣起来。但我们现在知道了,由于中生代的氧气含量比我们所以为的要低得多,在那时候并没有我们想象中那么多的植物供蜥脚类动物食用。现在,我们在看其它……我们正在努力弄明白蜥脚类动物的生物性质,把它们的化石和现代动物的解剖结构作比较,以更好地理解它们是怎样生存的。

    我们发现,蜥脚类动物很擅长储能。它们的胃容量非常大,能够长时间消化食物,以缓慢的速度把食物转化成能量,供以后利用。对胃容量小的动物来说,它们要不断觅食并消化食物......有了更大的胃且消化得更慢,你就不需要这么多能量了。约瑟夫?

    学生:科学家们了解蜥脚类动物的方式是……我是说,和我们能从现代动物身上学到的知识相比,我们究竟能从那些古老的骨头中学到什么?把相隔了几百万年的动物放在一起对比,这看起来更像是...靠猜的。

    教授:在研究已经绝种的动物时,总会有些猜测成分的;但这也正因为如此导致了重大发现。一个假说,一种猜测...我们通过寻找证据支持来“检验”这个假说,有些问题解决了,可能会引发新的问题。

    举个例子,看看其中一组比较。我们知道蜥脚类动物无法咀嚼食物——它们的头骨显示,它们不具有咀嚼肌。很多现代动物,像鸟类和爬行动物,也不能咀嚼食物...它们得整个吞下去。但现代动物有个有趣的帮手,能帮它们消化食物。它们会吞下石头,这些石头有助于把食物磨碎,在食物真正进入胃部被消化以前。这些石头称为胃石。

    胃石让食物更容易被消化,本质上就是把食物弄碎,和我们咀嚼食物一样。久而久之,动物体内的胃石也被磨碎了,变得圆滑。蜥脚类动物的化石通常伴有平滑的石头。多年来我们都以为这些是胃石---它们看起来就像是胃石,也是在蜥脚类动物的胃部找到的。

    最近一个研究中,人们测量了现代动物体内的胃石——鸵鸟体内的胃石...研究表明,鸵鸟需要吞下的胃石重量达到体重的百分之一。但我们确定了,蜥脚类动物内发现的石头总重量所占比例要低得多,低于体重的千分之一。

    所以现在我们不肯定这些蜥脚类动物的石头到底有什么用。有可能蜥脚类动物只是把它们错当成食物吞下了,这些石头其实并无用处。另外一些科学家猜测,蜥脚类动物吞下这些动物以摄取一些所需矿物质,如钙质。

    那么,即使最初的假说---即和蜥脚类动物一起发现的石头是胃石---没有证据支持,它也帮助我们建立了新的假说,可能最后会得出答案。

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    题型分析:主旨题

    原文定位Professor: Your reading for today touched on dinosaur fossils from the Mesozoic era, which ended about 65 million years ago. Today we will be discussing the sauropods. I think our discussion of sauropods will illustrate what we can learn by comparing the fossil record to modern animals. By fossils, we mean traces of prehistoric animals such as bones, which become mineralized, or impressions of bones or organs that are left in stone.

    选项分析

    文章一开始就明确了文中接下来说的事情是sauropods 并且表明了用和modern animal compare的方法。

    B选项与文意关键信息完全一致,是对全文主旨最准确的概括

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