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.
In paragraph 1, what does the author say about the presence of a blowhole in cetaceans?
解析：这是一个事实信息问题，要求提供可以在第1段中找到的具体信息。选择B是最好的答案。
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 Pakicifus，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。
Pakicifus 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。Pakicifus 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 Pakicifus 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 Pakicifus 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 Pakicifus 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。
Which of the following can be inferred from paragraph 1 about early sea otters?
这是一个要求从第1段推断出的信息的推理问题。选择A是最好的答案，因为第1段说海獭不像早期哺乳动物，它们的外表不容易想象。由此推断，海獭的早期出现很容易（或不难）想象。
The word precious in the passage is closest in meaning to
这是一个事实信息问题，它询问在文章中可以找到的特定信息。选择C是最好的答案。第3段描述了Pakigabes和现代鲸类动物的区别和相似之处。那一段的第3句说他们的头骨是相似的。其他三个选择描述差异，而不是相似性。
Pakicetus and modern cetaceans have similar
这是一个事实信息问题，它询问在文章中可以找到的特定信息。选择3是最好的答案。第3段描述了Pakigabes和现代鲸类动物的区别和相似之处。那一段的第3句说他们的头骨是相似的。其他三个选择描述差异，而不是相似性。
The word it in the passage refers to
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.
The word exposed in the passage is closest in meaning to
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."
The hind leg of Basilosaurus was a significant find because it showed that Basilosaurus
这是一个事实信息问题，它询问在文章中可以找到的特定信息。选择D是最好的答案，因为它是关于第4段中提到的蜥蜴龙骨架的唯一细节，意思是它很重要。选择A是真的，但是没有详细讨论选择D是什么，并且不表示发现的意义。选择C没有提到，选择B不是。
It can be inferred that Basilosaurus bred and gave birth in which of the following locations
这是一个推理问题，要求能够从整个段落中得出结论。选择D是基于第4段最后一句话的最好答案，该段将龙须鲸描述为全海洋鲸。这意味着它所做的一切，包括繁殖和生育，都只能在海洋环境中进行。
Why does the author use the word luckily in mentioning that the Ambulocetus natans fossil included hind legs?
这是一个推理问题，要求能够从文章中得出结论。第5段解释说，这一发现为科学家提供了重要的信息，如果没有它，他们可能无法获得。因此，你可以推断这一发现是一个“幸运”的发现。这篇文章没有提供其他选择的支持。因此，选择B是最好的答案。
Which of the sentences below best expresses the essential information in the highlighted sentence in the passage?
这是一个句子简化问题。和所有这些项目一样，文章中只强调了一句话：然而，骨干的结构表明，浮游鲸像现代鲸鱼一样通过上下移动其身体的后部来游动，即使没有侥幸。
选择C是最好的答案，因为它包含突出显示的句子中的所有基本信息。选择A是不正确的，因为ActoOcdots确实有一个脊椎。选项B不是真的，因为句子说骨干表明了浮游动物是如何游动的，而不是说它错过了一个侥幸。选择D是不真实的，因为句子陈述了浮游鲸和现代鲸鱼以同样的方式游泳。
The word propulsion in the passage is closest in meaning to