难句分析：
1. Their streamlined bodies, the absence of hind legs, and the
presence of a fluke and blowhole cannot disguise their affinities with
land dwelling mammals.
短语：cannot disguise their affinities with：不能掩盖它们与…的姻亲关系
land dwelling mammals：陆栖哺乳动物
释义：鲸类动物的流线型身体，后肢的缺少，三角状尾鳍的存在以及头顶的出气孔（以上都是典型的海洋动物特点）并不能掩盖它们与陆栖哺乳动物的姻亲关系。
2.
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.
短语：unlike the case of:与…的情况不同
it is not easy to envision:很难想象…的样子
释义：与海獭和其他鳍足动物不同，人们很难想象最初鲸类的模样。逻辑转换：人们很难想象最初鲸类动物的模样，但与之相反，设想海獭等动物的原始样貌应较为容易。
3. How was the gap between a walking mammal and a swimming whale bridged?
释义：本段主旨句，提出文章所讨论的中心问题：从陆栖哺乳动物到水生鲸类之间到底是如何进化的？
4.
Pakicetus was found embedded in rocks formed from river deposits that
were 52 million years old. 分句：“formed from river deposits that were 52
million years old”：形成于距今52
million年前的河流沉积物中，修饰rocks释义：化石P被发现镶嵌在岩石中，该岩石从距今52 million年前的河流沉积物中演化而来。
5. The fossil consists of a complete skull of an archaeocyte, an extinct group of ancestors of modern cetaceans.
短语：a complete skull：完整的头骨
分句：“an extinct group of ancestors of modern cetaceans”修饰archaeocyte,即archaeocyte是鲸类动物的一种已灭绝的祖先
释义：此句为事实1，化石P具有一个完整的鲸类始祖archaeocyte的头骨。
6.
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. 分句：“that is filled with fat or oil and used for
receiving underwater sound in modern whales”部分修饰enlarged
space释义：化石P的颚骨（jawbones）缺少现代鲸类具有的扩大性空间，这部分空间通常由脂肪和油脂填充并用来收听水下声音。此句为事实2。
7. Pakicetus probably detected sound through the ear opening as in land mammals.
释义：注意probably,此句为猜测1，化石P可能和陆栖动物一样通过耳孔来接受声音。
8. The skull also lacks a blowhole, another cetacean adaptation for diving.
释义：此句为事实3，P头骨缺少气孔，该气孔是鲸类适应潜水活动的结构。
9.
Other features, however, show experts that Pakicetus is a transitional
form between a group of extinct flesh-eating mammals, the mesonychids,
and cetaceans.
分句：“a group of extinct flesh-eating mammals”修饰“the mesonychid”，即M动物是一类已灭绝的食肉哺乳动物
主干：Pakicetus is a transitional form between the mesonychids and cetaceans
释义：此句指明化石P正是食肉哺乳动物M与鲸类的过渡阶段形态，为本段结论。
10.It has been suggested that Pakicetus fed on fish in shallow water and was not yet adapted for life in the open ocean.
短语：fed on fish：以鱼为食shallow water：浅水区
释义：注意suggested,此句为猜测2，即化石P可能生前靠浅水区鱼类为食并尚未适应海洋生活。
11. It probably bred and gave birth on land.
释义：“it”指代化石P，此句为猜测3，指出Pakicetus可能在陆地繁殖。
12.
Several skeletons of another early whale, Basilosaurus, were found in
sediments left by the Tethys Sea and now exposed in the Sahara desert.
词汇：exposed意为“暴露在”，此处指“处于，位于”。
分句：“left by the Tethys Sea”修饰sediments,即B骨架发现于T海遗留的沉积物中
释义：人们在Tethys海遗留的沉积物中发现了若干Basilosaurus的骨架，这些骨架现在位于撒哈拉沙漠中。
13. This whale lived around 40 million years ago, 12 million years after Pakicefus.
释义：此句为事实1，即Basilosaurus生活于40 million年前，比化石P晚12 million年。
14.
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.
分句：“a complete hind leg that features a foot with three tiny toes”即B具有完全的后肢，包括包括一只脚和三个脚趾
释义： 此句为事实2。注意archaeocyte是鲸类动物的一种已灭绝的祖先（已在上段中指出），由此可知原始鲸类Basilosaurus也是archaeocyte进化过程中的阶段性形态。
15.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.
短语：“nonfunctional, or vestigial, hind legs”：无实用作用的，退化了的后肢。
分句：“would have been too small to have supported the 50-foot-long Basilosaurus on land”:（这后肢）太小以至于不可能支撑50英尺长的Basilosaurus在陆地活动
释义：此句为本段结论，即Basilosaurus绝对已是（undoubtedly）完全的海洋鲸类。
16.
The now extinct whale Ambulocetus natans lived in the Tethys Sea 49
million years ago. It lived around 3 million years after Pakicetus but 9
million before Basilosaurus.
主干：Ambulocetus natans lived after Pakicetus but before Basilosaurus
释义：此句为事实1。指出Ambulocetus natans生活于49million年前的Tethys海中，它晚于Pakicetus 3million 年，但早于Basilosaurus 9 million年。
17.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.
短语：a good portion of:很完好的一部分
be ended in：末端是…
分
句：“ended in long feet very much like those of a modern
pinniped”：实际为“ended in long feet (which is)very much like those of a
modern pinniped”,修饰long feet
释义：此处为事实2。A化石具有完好的后肢，后肢很强壮且足部很长，如同现代鳍足动物（如海狮）一样，能够同时在陆地和海洋活动。
18.
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.
分句：“the major means of locomotion in modern cetaceans”修饰fluke,即现代鲸类在水中前进时所依靠的是其三角状尾鳍
主干：the structure of the backbone shows that Ambulocetus swam like modern whales
释义：此处为事实3。说明A化石缺少三角状尾鳍，但既便如此它仍能像现代鲸类一样游泳。
19. On land, where it probably bred and gave birth, Ambulocetus may have moved around very much like a modern sea lion.
释义：注意probably和may,此为猜测，即Ambulocetus可能在陆地繁殖后代，并且它在陆地运动时类似现代海狮。
20. It was undoubtedly a whale that linked life on land with life at sea.
释义：此句为本段结论，即通过上文描述A兼具海洋动物和陆地哺乳动物的特点得出它毫无疑问正是陆地哺乳动物到海洋鲸类过渡过程中的海陆两栖动物。
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