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
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
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
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答案
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.
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."
眾所周知，鯨類動(dòng)物是哺乳動(dòng)物,如鯨魚、鼠海豚和海豚。它們用肺呼吸，而不是鰓，屬于胎生。鯨類動(dòng)物呈流線型的身體，后腿的消失，尾片和氣孔的出現(xiàn)，這些特征都不能掩飾它們和陸生哺乳動(dòng)物的相似之處。然而，想知道世上第一只鯨長(zhǎng)什么樣并非易事，不像還原海獺及鰭足類動(dòng)物（四肢水陸兩用如海豹，海獅，海象）的原貌那么簡(jiǎn)單。一些完全水生的鯨類動(dòng)物雖然已經(jīng)滅絕，但仍可通過(guò)化石來(lái)對(duì)它們進(jìn)行考察。陸棲哺乳動(dòng)物和海洋鯨類之間有何聯(lián)系？近期發(fā)現(xiàn)的化石已經(jīng)可以很清晰地幫助人們了解這個(gè)問(wèn)題，以及他們之間的過(guò)渡關(guān)系。
科學(xué)家們通過(guò)一些令人振奮的發(fā)現(xiàn)重現(xiàn)了鯨類動(dòng)物幾近真實(shí)的起源。1979年，在巴基斯坦北部，一個(gè)尋找化石的考察隊(duì)發(fā)掘到了最古老的鯨魚化石。這塊化石被官方命名為 Pakicetus，以紀(jì)念人們發(fā)現(xiàn)它的地方。這塊化石是在一條河的沉積巖中發(fā)現(xiàn)的，這條河有5200萬(wàn)年的歷史，離古地中海不遠(yuǎn)
Pakicetus包括一個(gè)完整原始動(dòng)物的頭蓋骨，它的主人是現(xiàn)代鯨類的祖先。盡管只是個(gè)頭蓋骨，但它卻提供了研究原始鯨類動(dòng)物起源的珍貴信息。這個(gè)頭蓋骨和鯨類動(dòng)物的很像，但它的下頜骨和現(xiàn)代鯨類略有不同，現(xiàn)代鯨類動(dòng)物的下頜骨中含有額外的空間儲(chǔ)存脂肪或者油脂來(lái)吸收水下的聲音。Pakicetus的主人可能會(huì)像陸生哺乳動(dòng)物那樣通過(guò)張開(kāi)的耳朵來(lái)探測(cè)聲音。另外，這個(gè)頭蓋骨沒(méi)有呼吸孔，而鯨類動(dòng)物有，這便是鯨類動(dòng)物為了適應(yīng)水生環(huán)境的另一種適應(yīng)性表現(xiàn)。然而，專家認(rèn)為Pakicetus的其它特征表明它們是已滅絕的食肉哺乳動(dòng)物（中獸科動(dòng)物）和鯨類動(dòng)物的過(guò)渡型。有人認(rèn)為Pakicetus靠吃淺水的魚類為生，未能適應(yīng)在遼闊的大海里生活。它們很有可能在陸地進(jìn)行生育繁殖。
1989年，在埃及有了另一個(gè)重大發(fā)現(xiàn)。人們?cè)诠诺刂泻埩舻某练e物中發(fā)現(xiàn)了另一類早期鯨魚Basilosaurus的一些骨骸，這些骨骸如今暴露在撒哈拉大沙漠上。Basilosaurus生活在大約4000萬(wàn)年前，比Pakicetus鯨魚晚了1200萬(wàn)年。盡管發(fā)現(xiàn)的這些骨骼并不完整，但這是專家們第一次在原始動(dòng)物身上發(fā)現(xiàn)完整的后肢，它有三個(gè)小腳趾作為的足部特征。可這些后肢還太小，遠(yuǎn)無(wú)法支撐50英尺長(zhǎng)的Basilosaurus在陸地行走。因此，Basilosaurus必定是完全水生的鯨魚，它們的后肢已經(jīng)不起任何作用，或者說(shuō)已經(jīng)退化
1994年，巴基斯坦報(bào)道了一個(gè)更令人興奮的發(fā)現(xiàn)。目前已經(jīng)滅絕的鯨魚Ambulocetus natans（可以步行的鯨類）4900萬(wàn)年前曾在古地中海生活過(guò)。比Pakicetus晚大約 300萬(wàn)年，比Basilosaurus早 900萬(wàn)年左右。幸運(yùn)的是，被發(fā)現(xiàn)的Ambulocetus natans保留著完整的后肢。它的后肢很強(qiáng)壯，底部有長(zhǎng)足，非常像現(xiàn)在的鰭足類動(dòng)物。這些后肢使得他們既能在陸地行走又能在海里游行。雖然Ambulocetus natans保留了尾巴，但它們?nèi)鄙佻F(xiàn)代水生鯨類動(dòng)物用于行動(dòng)的主要身體部位——尾片。不過(guò)，從Ambulocetus的脊椎結(jié)構(gòu)上可以看出，即使缺少尾片，它們也能像現(xiàn)代鯨魚那樣通過(guò)身體背部的上下擺動(dòng)來(lái)游走。大的后肢通常被當(dāng)作是水中前行的發(fā)動(dòng)機(jī)。在它們可能交配繁殖的陸地上，Ambulocetus行動(dòng)起來(lái)非常像現(xiàn)代海獅。毫無(wú)疑問(wèn)，鯨魚是連接著陸地生命和海洋生命的物種。
