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OFFICIAL2 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.

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The Origins of Cetaceans
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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.



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.

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.

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

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

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

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

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

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

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正确答案:ABE
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整篇文章是典型的时间叙述加比较结构。手段提出问题:缺少对C进化过程的了解。2345段按时间顺序介绍最近对该进化过程的最新发现了解。其中23段介绍的是P,大约52M前,4段介绍B,大约40M前,5段介绍A,大约49M前。 给出句是对整篇文章的内容概括,正确选项应该是3个最新发现,概括各发现的主要特征、价值。 A正确,是对整篇文章内容概括,对应首段末句。 B正确,是对23段的概括。 C不对,过于细节,且后半句Tethys Sea, an area rich in fossil evidence文章原文没有提到。 D不对,过于细节,排除。 E正确,是对5段的概括。 F不对,原文写: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.F本身是对的,但由于是因为属于minor idea不所以选

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