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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

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

1. "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 fluke3 and blowhole4 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.

託福TPO2Part2閱讀題目

Directions: Mark your answer by filling in the oval next to your choice.

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, 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.

3. The word precious in the passage is closest in meaning to

Exact

Scarce

Valuable

Initial

4. Pakicetus and 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 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.

6. The word exposed in the passage is closest in meaning to

Explained

Visible

Identified

Located

7. The hind leg of Basilosaurus was a significant find because it showed that Basilosaurus

Lived later than Ambulocetus natans

Lived at the same time as Pakicetus

Was able to swim well

Could not have walked on land

8. It can be inferred that Basilosaurus bred 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 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

9. Why does the author use the word luckily in mentioning that the Ambulocetus natans fossil 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 sentence in the passage?

Incorrect choices change the meaning in important ways or leave out essential information.

Even though Ambulocetus swam by moving its body up and down, it did not have a backbone.

The backbone of Ambulocetus, which allowed it to swim, provides evidence of its missing fluke.

Although Ambulocetus had 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 way Ambulocetus swam.

11. The word propulsion in the passage is closest in meaning to

Staying afloat

Changing direction

Decreasing weight

Moving forward

Paragraph 1: 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.

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?

13-14. 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

1. Recent discoveries of fossils have helped to show the link between land mammals and cetaceans.

2. The discovery of Ambulocetus natans provided evidence for a whale that lived both on land and at sea.

3. The skeleton of Basilosaurus was found in what had been the Tethys Sea, an area rich in fossil evidence.

4. Pakicetus is the oldest fossil whale yet to be found.

5. Fossils thought to be transitional forms between walking mammals and swimming whales were found.

6. Ambulocetus' hind legs were used for propulsion in the water

　託福TPO2Part2閱讀答案

參考答案：

1、2

2、1

3、3

4、3

5、1

6、2

7、4

8、4

9、2

10. 3

11. 4

12. 2

13-14. 1 2 5

託福TPO2Part2閱讀原文翻譯

沙漠已經佔據了地球表面積約四分之一，而且最近幾十年正以驚人的速度擴張。沙漠化是指類似沙漠的環境漫延到原本並非沙漠的區域。據估計，地球表面另外四分之一的地方正面臨沙漠化威脅。

沙漠化主要通過以下過程實現：首先自然植被不斷減少，隨後風力和雨水加速了土壤的腐蝕。有的時候鬆散的土壤全部被風颳走，留下石質化的表層;其它情況下細小的沙粒可能會被吹走，而正常沙粒大小的砂子不斷堆積，從而形成移動的沙丘或者沙脊。

即便是在保留了土壤表層的區域，植被減少也已成爲土壤大量吸取地下水的能力下降的典型因素。雨水對鬆散土壤的衝擊會把細小的粘土顆粒衝到土壤空隙中，封閉了土壤並降低土地表層水的滲透率。地表對水的吸收急劇減少，大量水資源流失，因此土壤的腐蝕率也隨即增加。地表吸收水分的能力進一步弱化使得土壤越發乾燥，導致植被的進一步流失，於是便形成了土壤沙漠化的惡性循環。

在一些地方，沙漠面積的擴大很大程度上歸因於乾燥的氣候條件。在過去的幾千年裏，不斷增加的溫室效應使得一些地方乾旱問題愈發嚴重。倘若空氣污染帶來的溫室效應繼續惡化，沙漠化進程會在未來數十年內加速實現。

然而，可以肯定的是，大部分地區沙漠化主要都是由於人類活動造成，而非自然條件導致。沙漠邊緣的半乾旱土地所處的生態平衡環境非常脆弱，環境壓力持續增加，而這些半乾旱區域適應環境壓力的能力極其有限。人口數量的增加使得人們不斷向土地施壓，依其提供食物和燃料。在溼潤的季節裏，土地興許能夠應付這些壓力。但是在乾旱的季節裏，在沙漠周邊的土地上，存在着這樣一個十分普遍的現象：人類對土地施加的壓力遠遠超過了土地自身減壓的能力，因此最終形成了沙漠。

導致沙漠化的主要因素有四個：過度種植，過度放牧，過分砍伐，過度灌溉。由於人口密度增加，人們對糧食作物的種植已經擴展到日益乾燥的區域進行。這些區域很有可能經常會發生乾旱，所以農作物種植失敗是很正常的事情。大多數農作物的種植需要事先移除天然植被，而農作物欠收後又會留下大面積荒地，非常容易被風力和雨水侵蝕。

在半乾旱地區，草坪是主要的天然植被，家畜飼養是當地的一項主要經濟活動。在一個地區過量飼養家畜會導致植被覆蓋面積減少，土地被大量踐踏和碾碎。通常，隨之而來的就是土地硬化和加速侵蝕。

在很多國家木材是用來做飯和加熱的最主要燃料。人口增加帶來的壓力促使人們大量砍伐木材，導致許多城市和鄉村周圍大面積樹木和灌木減少。同時人們大量使用烘乾的動物排泄物作爲替代燃料同樣對土壤不利，因爲這些珍貴的土壤成分調節劑和植物營養資源將不會再回歸至土壤當中。

造成土地沙漠化的最後一個主要人爲因素在於人類過度灌溉導致土壤的鹽鹼化。灌溉多餘的水滲透到地下水位。假如沒有排水系統的存在，那麼地下水位上升，把溶解的鹽分帶到土壤表面。水分蒸發後，鹽分留在了表面，形成白色的地殼層，這一地殼層阻止了空氣和水接觸地底下的土壤。