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 namedPakicetusin honor of the country where the discovery was made.Pakicetuswas 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, thePakicetusfossil 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.Pakicetusprobably 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 thatPakicetusis a transitional form between a group of extinct flesh-eating mammals, the mesonychids, and cetaceans. It has been suggested thatPakicetusfed 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 afterPakicetus.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-longBasilosauruson land.Basilosauruswas 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 whaleAmbulocetus natans("the walking whale that swam") lived in the Tethys Sea 49 million years ago. It lived around 3 million years afterPakicetusbut 9 million beforeBasilosaurus.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, thatAmbulocetusswam 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,Ambulocetusmay 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
【Paragraph 1】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.
1. In paragraph 1, what does the author say about the presence of a blowhole in cetaceans?
○It clearly indicates that cetaceans are mammals.
○It cannot conceal the fact that cetaceans are mammals.
○It is the main difference between cetaceans and land-dwelling mammals.
○It cannot yield clues about the origins of cetaceans.
2. Which of the following can be inferred from paragraph 1 about early sea otters?
○It is not difficult to imagine what they looked like.
○There were great numbers of them.
○They lived in the sea only.
○They did not leave many fossil remains.
【Paragraph 3】The fossil consists of a complete skull of an archaeocyte, an extinct group of ancestors of modern cetaceans. Although limited to a skull, thePakicetusfossil 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.Pakicetusprobably 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 thatPakicetusis a transitional form between a group of extinct flesh-eating mammals, the mesonychids, and cetaceans. It has been suggested thatPakicetusfed on
fish in shallow water and was not yet adapted for life in the open ocean.It probably bred and gave birth on land.
3. The word “precious”in the passage is closest in meaning to
○exact
○scarce
○valuable
○initial
4.Pakicetusand modern cetaceans have similar
○hearing structures
○adaptations for diving
○skull shapes
○breeding locations
5. The word “It” in the passage refers to
○Pakicetus
○fish
○life
○ocean
【Paragraph 4】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 afterPakicetus.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-longBasilosauruson land.Basilosauruswas undoubtedly a fully marine whale with possibly nonfunctional, or vestigial, hind legs.
6. The word “exposed”in the passage is closest in meaning to
○explained
○visible
○identified
○located
7. The hind leg ofBasilosauruswas a significant find because it showed thatBasilosaurus
○lived later thanAmbulocetus natans
○lived at the same time asPakicetus
○was able to swim well
○could not have walked on land
8. It can be inferred thatBasilosaurusbred and gave birth in which of the following locations
○On land
○Both on land and at sea
○In shallow water
○In a marine environment
【Paragraph 5】An even more exciting find was reported in 1994, also from Pakistan. The now extinct whaleAmbulocetus natans("the walking whale that swam") lived in the Tethys Sea 49
million years ago. It lived around 3 million years afterPakicetusbut 9 million beforeBasilosaurus.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,Ambulocetusmay have moved around very much like a modern sea lion. It was undoubtedly a whale that linked life on land with life at sea.
9. Why does the author use the word “luckily” in mentioning that theAmbulocetus natansfossil included hind legs?
○Fossil legs of early whales are a rare find.
○The legs provided important information about the evolution of cetaceans.
○The discovery allowed scientists to reconstruct a complete skeleton of the whale.
○Until that time, only the front legs of early whales had been discovered.
10. Which of the sentences below best expresses the essential information in the highlighted sentencein the passage? Incorrect choices change the meaning in important ways or leave out essential information.
○Even thoughAmbulocetusswam by moving its body up and down, it did not have a backbone.
○The backbone ofAmbulocetus, which allowed it to swim, provides evidence of its missing fluke.
○AlthoughAmbulocetushad no fluke, its backbone structure shows that it swam like modern whales.
○By moving the rear parts of their bodies up and down,modern whales swim in a different way from the wayAmbulocetusswam.
11. The word “propulsion”in the passage is closest in meaning to
○staying afloat
○changing direction
○decreasing weight
○moving forward
Extinct but already fully marine cetaceans are known fromthe fossil record.■How was the gap between a walking mammal and aswimming 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 namedPakicetusin honor of the country where the discovery was made.Pakicetuswas 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.
12. Look at the four squares [■] that indicate where the following sentence can be added to the passage.
This is a question that has puzzled scientists for ages.
Where would the sentence best fit?
13. 【Directions】An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage.This question is worth 2 points.
This passage discusses fossils that help to explain the
likely origins of cetaceans—whales, porpoises, and dolphins.
●
●
●
Answer Choices
○Recent discoveries of fossils have helped to show the link between land mammals and cetaceans.
○The discovery ofAmbulocetusnatansprovided evidence for a whale that lived both on land and at sea.
○The skeleton ofBasilosauruswas found in what had been the Tethys Sea, an area rich in fossil evidence.
○Pakicetusis the oldest fossil whale yet to be found.
○Fossils thought to be transitional forms between walking mammals and swimming whales were found.
○Ambulocetus' hind legs were used for propulsion in the water.
参考答案：
1. ○2
This is a Factual Information question asking for specific information that can be found in paragraph 1. Choice 2 is the best answer. It is essentially a rephrasing of the statement in paragraph 1 that blowholes cannot disguise cetaceans’affinities with other mammals. The other three choices are refuted, either directly or indirectly, by that paragraph.
2.○1
This is an Inference question asking for information that can be inferred from paragraph 1 . Choice 1 is the best answer because paragraph 1 says that sea otters are unlike early mammals whose appearances are not easy to imagine. By inference, then, the early appearance of sea otters must be easy (or not difficult) to imagine.
3.○3
This is a Vocabulary question. The word being tested is precious. It is highlighted in the passage. The correct answer is choice 3, "valuable." Anything that is precious is very important and therefore valuable.
4.○3
This is a Factual Information question asking for specific information that can be found in the passage. Choice 3 is the best answer. Paragraph 3 describes the differences and similarities betweenPakicetusand modern cetaceans. Sentence 3 of that paragraph states that their skulls are similar. The other three choices describe differences, not similarities.
5.○1
This is a Reference question. The word being tested is It. That word is highlighted in the passage. This is a simple pronoun referent item. Choice I , "Pakicetus" is the correct answer. The word It here refers to a creature that probably bred and gave birth on land.Pakicetus is the only one of the choices to which this could apply.
6.○2
This is a Vocabulary question. The word being tested is exposed. It is highlighted in the passage. The correct answer is choice 2, "visible." Exposed means "uncovered." A skeleton that is uncovered can be seen. Visible means "can be seen."
7.○4
This is a Factual Information question asking for specific information that can be found in the passage. Choice 4 is the best answer because it is the only detail about the skeleton ofBasilosaurusmentioned in paragraph 4, meaning that it is significant. Choice 1 is true, but it is not discussed in the detail that choice 4 is, and does not represent the significance of the discovery. Choice 3 is not mentioned, and choice 2 is not :mentioned.
8.○4
This is an Inference question asking for a conclusion that can be drawn from the entire passage. Choice 4 is the best answer based on the last sentence of paragraph 4, which describesBasilosaurusas a fully marine whale. That implies that everything it did, including breeding and giving birth, could have been done only in a marine environment.
9.○2
This is an Inference question asking for a conclusion that can be drawn from the passage.Paragraph 5 explains that this discovery provided important information to scientists that they might not have been able to obtain without it. Therefore, you can infer that the discovery was a "lucky" one. The passage offers no support for the other choices. Therefore, choice 2 is the best answer.
10. ○3
This is a Sentence Simplification question. As with all of these items, a single sentence in the passage is highlighted: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.Choice 3 is the best answer because it contains all of the essential information in the highlighted sentence. Choice 1 is not true becauseAmbulocetusdid have a backbone. Choice 2 is not true because the sentence says that the backbone showed how theAmbulocetusswam, not that it was missing a fluke. Choice 4 is untrue because the sentence states thatAmbulocetusand modern whales swam in the same way.
11. ○4
This is a Vocabulary question. The word being tested is propulsion. It is highlighted in the passage. Choice 4, "moving forward" is the best answer because it means the action of propelling. The whale in the sentence used its hind legs to push itself forward in the water.
12. ○2
This is an Insert Text question. You can see the four black squares in paragraphs 1 and 2 that represent the possible answer choices here.
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 sentence provided is"This is a question that has puzzled scientists for ages."The best place to insert it is at square 2. The sentence that precedes square 2 is in the form of a rhetorical question and the inserted sentence explicitly provides a response to it. None of the other sentences preceding squares is a question, so the inserted sentence cannot logically follow any one of them.
13. ○1, 2, 5
This is a Prose Summary question. It is completed correctly below.The correct choices are 1, 2, and 5. Choices 3, 4, and 6 are therefore incorrect.
Correct Choices
【Choice 1】“Recent discoveries of fossils have helped to show the link between land mammals and cetaceans,”is correct because it represents the major idea of the entire passage. The bulk of the passage consists of a discussion of the major discoveries (Pakicetus, Basilosaurus, and Ambulocetus) that show this link.
【Choice 2】“The discovery of Ambulocetus natans provided evidence for a whale that lived both on land and at sea,”is correct because it is one of the major discoveries cited in the passage in support of the passage’s main point, that land mammals and cetaceans are related.
【Choice 5】“Fossils thought to be transitional forms between walking mammals and swimming whales were found,”is correct because like choice 1, this is a statement of the passage’s major theme as stated in paragraph 1: these fossils were “clearly intermediate, or transitional between land mammals and cetaceans.”The remainder of the passage discusses these discoveries.
Incorrect Choices
【Choice 3】“The skeleton of Basilosaurus was found in what had been the Tethys Sea, an area rich in fossil evidence,”is true, but it is a minor detail and therefore incorrect.
【Choice 4】“Pakicetus is the oldest fossil whale yet to be found,”is true, but it is a minor detail and therefore incorrect.
【Choice 6】“Ambulocetus’hind legs were used for propulsion in the water,”is true, but it is a minor detail and therefore incorrect.
参考译文：鲸类的起源
众所周知，鲸类动物是哺乳动物,如鲸鱼、鼠海豚和海豚。它们用肺呼吸，而不是鳃，属于胎生。鲸类动物呈流线型的身体，后腿的消失，尾片和气孔的出现，这些特征都不能掩饰它们和陆生哺乳动物的相似之处。然而，想知道世上第一只鲸长什么样并非易事，不像还原海獭及鳍足类动物（四肢水陆两用如海豹，海狮，海象）的原貌那么简单。一些完全水生的鲸类动物虽然已经灭绝，但仍可通过化石来对它们进行考察。陆栖哺乳动物和海洋鲸类之间有何联系？近期发现的化石已经可以很清晰地帮助人们了解这个问题，以及他们之间的过渡关系。
科学家们通过一些令人振奋的发现重现了鲸类动物几近真实的起源。1979年，在巴基斯坦北部，一个寻找化石的考察队发掘到了最古老的鲸鱼化石。这块化石被官方命名为Pakicetus，以纪念人们发现它的地方。这块化石是在一条河的沉积岩中发现的，这条河有5200万年的历史，离古地中海不远。
Pakicetus包括一个完整原始动物的头盖骨，它的主人是现代鲸类的祖先。尽管只是个头盖骨，但它却提供了研究原始鲸类动物起源的珍贵信息。这个头盖骨和鲸类动物的很像，但它的下颌骨和现代鲸类略有不同，现代鲸类动物的下颌骨中含有额外的空间储存脂肪或者油脂来吸收水下的声音。Pakicetus的主人可能会像陆生哺乳动物那样通过张开的耳朵来探测声音。另外，这个头盖骨没有呼吸孔，而鲸类动物有，这便是鲸类动物为了适应水生环境的另一种适应性表现。然而，专家认为Pakicetus的其它特征表明它们是已灭绝的食肉哺乳动物（中兽科动物）和鲸类动物的过渡型。有人认为Pakicetus靠吃浅水的鱼类为生，未能适应在辽阔的大海里生活。它们很有可能在陆地进行生育繁殖。
1989年，在埃及有了另一个重大发现。人们在古地中海残留的沉积物中发现了另一类早期鲸鱼Basilosaurus的一些骨骸，这些骨骸如今暴露在撒哈拉大沙漠上。Basilosaurus生活在大约4000万年前，比Pakicetus鲸鱼晚了1200万年。尽管发现的这些骨骼并不完整，但这是专家们第一次在原始动物身上发现完整的后肢，它有三个小脚趾作为的足部特征。可这些后肢还太小，远无法支撑50英尺长的Basilosaurus在陆地行走。因此，Basilosaurus必定是完全水生的鲸鱼，它们的后肢已经不起任何作用，或者说已经退化。
1994年，巴基斯坦报道了一个更令人兴奋的发现。目前已经灭绝的鲸鱼Ambulocetus
natans（可以步行的鲸类）4900万年前曾在古地中海生活过。比Pakicetus晚大约300万年，比Basilosaurus早900万年左右。幸运的是，被发现的Ambulocetus natans保留着完整的后肢。它的后肢很强壮，底部有长足，非常像现在的鳍足类动物。这些后肢使得他们既能在陆地行走又能在海里游行。虽然Ambulocetus natans保留了尾巴，但它们缺少现代水生鲸类动物用于行动的主要身体部位——尾片。不过，从Ambulocetus的脊椎结构上可以看出，即使缺少尾片，它们也能像现代鲸鱼那样通过身体背部的上下摆动来游走。大的后肢通常被当作是水中前行的发动机。在它们可能交配繁殖的陆地上，Ambulocetus行动起来非常像现代海狮。毫无疑问，鲸鱼是连接着陆地生命和海洋生命的物种。
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