1.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。
2.Their streamlined bodies, the absence of hind legs, and the presence of a fluke and blowhole cannot disguise theiraffinities 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。
3.Pakicetus was found embedded in rocks formed from river deposits that were 52 million years old. 分句：“formed fromriver 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。
4.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？
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.he 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。
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 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.
9.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
10.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。