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英['lɪs(ə)n] 美['lɪsn]
vi. 听,倾听;听从,听信
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TPO-02-The Origins of Cetaceans

It should be obvious that cetaceans-whales, porpoises, and dolphins-are mammals. They breathe through lungs, not through gills, and give birth to live young. Their streamlined bodies, the absence of hind legs, and the presence of a fluke1 and blowhole2 cannot disguise their affinities with land dwelling mammals.However, unlike the cases of sea otters and pinnipeds (seals, sea lions, and walruses, whose limbs are functional both on land and at sea), it is not easy to envision what the first whales looked like. Extinct but already fully marine cetaceans are known from the fossil record. How was the gap between a walking mammal and a swimming whale bridged? Missing until recently were fossils clearly intermediate, or transitional, between land mammals and cetaceans.


Very exciting discoveries have finally allowed scientists to reconstruct the most likely origins of cetaceans. In 1979, a team looking for fossils in northern Pakistan found what proved to be the oldest fossil whale. The fossil was officially named Pakicetus in honor of the country where the discovery was made. Pakicetus was found embedded in rocks formed from river deposits that were 52 million years old. The river that formed these deposits was actually not far from an ancient ocean known as the Tethys Sea.

The fossil consists of a complete skull of an archaeocyte, an extinct group of ancestors of modern cetaceans. Although limited to a skull, the Pakicetus fossil provides precious details on the origins of cetaceans. The skull is cetacean-like but its jawbones lack the enlarged space that is filled with fat or oil and used for receiving underwater sound in modern whales. Pakicetus probably detected sound through the ear opening as in land mammals. The skull also lacks a blowhole, another cetacean adaptation for diving. Other features, however, show experts that Pakicetus is a transitional form between a group of extinct flesh- eating mammals, the mesonychids, and cetaceans. It has been suggested that Pakicetus fed on fish in shallow water and was not yet adapted for life in the open ocean. It probably bred and gave birth on land.

Another major discovery was made in Egypt in 1989. Several skeletons of another early whale, Basilosaurus, were found in sediments left by the Tethys Sea and now exposed in the Sahara desert. This whale lived around 40 million years ago, 12 million years after Pakicetus. Many incomplete skeletons were found but they included, for the first time in an archaeocyte, a complete hind leg that features a foot with three tiny toes. Such legs would have been far too small to have supported the 50-foot-long Basilosaurus on land. Basilosaurus was undoubtedly a fully marine whale with possibly nonfunctional, or vestigial, hind legs.

An even more exciting find was reported in 1994, also from Pakistan. The now extinct whale Ambulocetus natans ("the walking whale that swam") lived in the Tethys Sea 49 million years ago. It lived around 3 million years after Pakicetus but 9 million before Basilosaurus. The fossil luckily includes a good portion of the hind legs. The legs were strong and ended in long feet very much like those of a modern pinniped. The legs were certainly functional both on land and at sea. The whale retained a tail and lacked a fluke, the major means of locomotion in modern cetaceans. The structure of the backbone shows, however, that Ambulocetus swam like modern whales by moving the rear portion of its body up and down, even though a fluke was missing. The large hind legs were used for propulsion in water. On land, where it probably bred and gave birth, Ambulocetus may have moved around very much like a modern sea lion. It was undoubtedly a whale that linked life on land with life at sea.

1. Fluke: the two parts that constitute the large triangular tail of a whale

2. Blowhole: a hole in the top of the head used for breathing

应该很明显,鲸类鲸,海豚和海豚是哺乳动物。他们通过肺呼吸,而不是通过鳃,生下活的年轻人。他们流线型的身体,没有后腿,还有侥幸1和气孔2,都不能掩饰他们与哺乳动物的亲缘关系。然而,与海獭和鳍(海豹,海狮和海象,它们的四肢是功能性的无论在陆地上还是在海上),想象出第一批鲸鱼的样子并不容易。从化石记录中已知灭绝但已完全海洋鲸类动物。行走的哺乳动物和游泳鲸鱼之间的差距是如何弥合的?直到最近,还没有发现陆地哺乳动物和鲸目动物之间的中间化石或过渡化石。

 

非常令人兴奋的发现终于让科学家们重建了鲸类最可能的起源。 1979年,巴基斯坦北部一个寻找化石的团队发现了最古老的化石鲸鱼。化石被正式命名为Pakicetus以纪念发现的国家。 Pakicetus被发现嵌入在5200万年前河流沉积物形成的岩石中。形成这些矿床的河流实际上离特提斯海这个古老的海洋不远。

 

化石包括一个完整的古细胞头骨,一个已经灭绝的现代鲸类动物祖先组。尽管仅限于一个头骨,Pakicetus化石提供了关于鲸类动物起源的宝贵细节。头骨像鲸类似的,但是它的下颚骨缺乏充满脂肪或油的扩大的空间,并且用于在现代鲸鱼中接收水下声音。 Pakicetus很可能像在陆地上的哺乳动物一样,通过耳朵听到声音。头骨也缺乏一个气孔,另一个鲸鱼适应潜水。然而,其他的一些特征表明,专家们认为,帕克西图特(Pakicetus)是一群已经灭绝的食肉哺乳动物,mesonychids和鲸目动物之间的过渡形式。有人建议,Pakicetus在浅水中喂鱼,还没有适应在公海上的生活。它可能是孕育而生的。

 

另一个重大发现是1989年在埃及发现的。在特提斯海留下的沉积物中发现了另一只早期鲸鱼(Basilosaurus)的几个骸骨,现在在撒哈拉沙漠中露出。这只鲸鱼大约在四千万年前,也就是在帕基西卡之后一千二百万年。发现了许多不完整的骨骼,但是它们首次在古细胞中包括一个完整的后腿,其特征是脚有三个小脚趾。这样的腿本来就太小了,不足以支撑陆地上50英尺长的罗勒龙。 Basilosaurus毫无疑问是一种完全海洋鲸鱼,可能是无功能的,或者是退化的后腿。

 

据报道,1994年还有一起来自巴基斯坦的令人兴奋的发现。 4900万年前,现在已经灭绝的鲸鱼Ambulocetus natans(“游泳的行走的鲸鱼”)生活在特提斯海。它在Pakicetus之后大约300万年,但在Basilosaurus之前是900万年。幸运的是化石包括了后腿的一大部分。腿很结实,长得很像现代的鳍。腿在陆地和海上当然都是有用的。鲸保留了尾巴,缺乏侥幸,现代鲸目动物的主要运动方式。然而骨干的结构表明,即使失去了一只侥幸,Ambulocetus也会像现代鲸鱼一样游走身体的后部。大后腿用于在水中推进。在可能孕育和分娩的土地上,Ambulocetus可能像现代海狮一样移动。毫无疑问,这是一条将陆上生命与海上生命联系在一起的鲸鱼。

 

1.福禄克:构成鲸鱼大三角形尾巴的两个部分

2.气孔:用于呼吸的头顶部有一个孔

中文翻译

In paragraph 1, what does the author say about the presence of a blowhole in cetaceans?

  • It should be obvious that cetaceans-whales, porpoises, and dolphins-are mammals. They breathe through lungs, not through gills, and give birth to live young.  A It clearly indicates that cetaceans are mammals. B It cannot conceal the fact that cetaceans are mammals. C It is the main difference between cetaceans and land-dwelling mammals. D It cannot yield clues about the origins of cetaceans.
正确答案: B

网友解析

  • 用户头像

    恩恩耶  发表于 2018-07-24 15:27:22

    正确答案:B
    题目解析:
    根据blowhole 查找原文,找到:the presence of a fluke1 and blowhole2 cannot disguise their affinities with land dwelling mammals.选项B正确,是对该句的同义改写。 A错误,反了。 C错误,没有提到。 D错误,没有提到。

  • 用户头像

    joannazz  发表于 2017-12-07 13:39:17

    根据题干blowhole in cetaceans定位到Their streamlined bodies, the absence of hind legs, and the presence of a fluke and blowhole cannot disguise their affinities with land dwelling mammals.意为这些特征不能掩盖它们与mammals的姻亲关系,即B选项它们也是mammal。A选项太肯定了,与原文表达不一致。CD选项未提及。

  • 用户头像

    joannazz  发表于 2017-12-06 16:56:20

    事实信息题: 根据题干blowhole in cetaceans定位到Their streamlined bodies, the absence of hind legs, and the presence of a fluke and blowhole cannot disguise their affinities with land dwelling mammals.意为这些特征不能掩盖它们与mammals的姻亲关系,即B选项它们也是mammal。A选项太肯定了,与原文表达不一致。CD选项未提及。

  • 用户头像

    lalla  发表于 2017-12-05 13:42:02

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