TF阅读真题第468篇TheOriginsofCetaceans
It should be obvious that cetaceans—whales, porpois, and dolphins—are mammals. They breathe through lungs, not through gills, and give birth to live young. Their streamlined bodies, the abnce of hind legs, and the prence of a fluke1 and blowhole2 cannot disgui their affinities with land dwelling mammals. However, unlike the cas of a otters and pinnipeds (als, a lions, and walrus, who limbs are functional both on land and at a), 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
饥荒怎么抓兔子
周易原文>看图说话的技巧deposits that were 52 million years old. The river that formed the 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 今天我当家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 ud 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 diments left by the Tethys Sea and now expod in
the Sahara dert. 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 tho of a modern pinniped. The legs were certainly functional both on land and at a. 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 leg少年狂歌曲
s were ud for propulsion in water. On land, where it probably bred and gave birth, Ambulocetus may have moved around very much like a modern a lion. It was undoubtedly a whale that linked life on land with life at a
1. Fluke: the two parts that constitute the large triangular tail of a whale
2. Blowhole: a hole in the top of the head ud for breathing
【Paragraph 1】It should be obvious that cetaceans—whales, porpois, and dolphins—are mammals. They breathe through lungs, not through gills, and give birth to live young. Their streamlined bodies, the abnce of hind legs, and the prence of a fluke1 and blowhole2 cannot disgui their affinities with land dwelling mammals. However, unlike the cas of a otters and pinnipeds (als, a lions, and walrus, who limbs are functional both on land and at a), 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 cet
格凸河aceans.
1. In paragraph 1, what does the author say about the prence 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.怀远三绝
○It cannot yield clues about the origins of cetaceans.
2. Which of the following can be inferred from paragraph 1 about early a otters?
○It is not difficult to imagine what they looked like.
○There were great numbers of them.
○They lived in the a 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 ud for receiving underwater sound in modern whales. 车工工艺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.
