Pakicetid
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| Pakicetids Fossil range: Early Eocene–Middle Eocene |
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 Pakicetus.
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Pakicetids are the members of the family Pakicetidae, sometimes called the subfamily Pakicetinae, of extinct mammals that are the earliest known cetaceans. While modern-day cetaceans are all water-dwelling animals such as whales and dolphins, the pakicetids pre-date the transition from land. Because their fossils were found near bodies of water, they are presumed to have spent part of their life in water.
The known genera of pakicetids include the wolf-sized Pakicetus, Nalacetus, the fox-sized Ichthyolestes. Pakicetus was the first discovered in 1983 by Philip Gingerich, Neil Wells, Donald Russell, and S. M. Ibrahim Shah, and all three species are known from a few sites in Pakistan, hence the name of the first genera and the family as a whole. The region is believed to have been coastal to the Tethys Sea when the pakicetids lived, some 53 million years ago.
The pakicetids were carnivorous land animals, but are presumed to be ancestors of modern whales because of the three following features unique to whales: peculiarities in the positioning of the ear bones within the skull, the folding in a bone of the middle ear, and the arrangement of cusps on the molar teeth. The current theory is that modern whales evolved from archaic whales such as basilosaurids, which in turn evolved from something like the amphibious ambulocetids, which themselves evolved from something like the land-dwelling pakicetids.
