Section II:        It should be obvious that cetaceans-whales, porpoises, and dolphins-are mammals. They breathe through lungs, notthrough gils, and give birth to live young. Their streamlined bodies, the absence of hind legs, and the presence of a fluket andolowhole' cannot disguise their affinities with land dwelling mammals.■ How  ever, 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 whaleslooked like. Extinct but already fully marine cetaceans are known from the fossil record. How was the gap between a walkingmammal and a swimming whale bridged?■ Missing until recently were fossils clearly intermediate, or transitional, between landmammals and cetaceans. ■Very exciting discoveries have finally allowed scientists to reconstruct the most likely origins of cetaceans. WIn 1979, ateam looking for fossils in northern Pakistan found what proved to be the oldest fossil whale. The fossil was officially namedpakicetus in honor of the countr where the discovery was made. Pakicetus was found embedded in rocks formed from riverdeposits that were 52 million years old. The river that formed these deposits was actually not far from an ancient ocean known asthe Tethys Sea. The fossil consists of a complete skull of an archaeocyte, an extinct group of ancestors of modern cetaceans. Although limitedto a skull, the Pokicetus fossil provides precious details on the origins of cetaceans. The skull is cetacean-like but its jawbones lackthe enlarged space that is flled with fat or oil and used for receiving underwater sound in modern whales. Pakicetus probablydetected sound through the ear opening as in land mammals. The skull also lacks a blowhole, another cetacea an adaptation fordiving, Other features, however, show experts that Pakicetus is a transitional form between a group of extinct flesh-eatingmammals, the mesonychids, and cetaceans. It has been suggested that Pakicetus fed on fish in shallow water and was not yetadapted 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 insediments left by the Tethys Sea and now exposed in the Sahara desert. This whale lived around 40 milion years ago, 12 millionyears after Pakicetus. Many Incomplete skeletons were found but they included, for the first time in an archaeocyte, a completehind leg that features a foot with three tiny toes. Such legs would have been far too small to have supported the 50-foot-long .Basilosourus on land. Basilosourus 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 ("thewalking whale that swam") lived in the Tethys Sea 49 million years ago. It lived around 3 million years after Pakicetus but 9 millionbefore Basilosaurus. The fossil luckily includes a good portion of the hind legs. The legs were strong and ended in long feet verymuch like those of a modern pinniped. The legs were certainly functional both on land and at sea. The whale retained a tail andlacked a fluke, the major means of locomotion in modern cetaceans. The structure of the backbone shows, however, thatAmbulocetus swam like modern whales by moving the rear portion of its body up and down, even thoush a fluke was missing. TheJarge hind legs were used for propulsion in water. On Iand, where it probably bred and gave birth, Ambulocetus may have movedarond very much like a modern sea lion. It was undoubtedly a whale that linked life on land with life at sea.Fluke: the two parts that constitute the large triangular tail of a whaleaBlowhole: a hole in the top of the head used for breathing
【題組】4. In paragraph 1, what does the author say about the presence of a blowhole in cetaceans?
(A) It clearly indicates that cetaceans are mammals.
(B) It cannot conceal the fact that cetaceans are mammals.
(C) It is the main difference between cetaceans and land-dwelling mammals.