The Origins of Cetaceans (OG Practice Set 1, TPO2)
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 fluke and blowhole 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.

答案：该题是修辞目的题，问作者为什么要提到blowhole这个“细节”，也就是为什么要有这个例子出现。

正确选项-B。从'blowhole"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.”这个任务并不难。托福的发问段是直接摆在你面前的。接下来认识disguise是伪装、隐藏的意思的话，可以知道B选项的conceal就是disguise的同义转述。如果不认识，则采取排除法：首先A选项首先排除，该选项对应的是原文中的首句，但是由于是“目的”题，目的一定在前提的后面，不可能在段落第一句出答案。接着选项C不可能，该选项对应文中的“How was the gap between a walking mammal and a swimming whale bridged?”，但该句是一个疑问句，所以陆生哺乳动物和海生动物之间的不同还没被发现，起码文章还没有提及，因此排除。最后，D选项的错误在于，作者不可能说了一堆例证（blowhole/ hind legs/presence of a fluke）结果最后又说这些例子给不出什么结论，并且，鲸类起源为什么难以发现，本段还没有涉及，所以D选项属于跟第一段关系不密切的选项，排除。

■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.