Pakicetus, extinct genus of early cetacean mammals known from fossils discovered in 48.5-million-year-old river delta deposits in present-day Pakistan. Pakicetus is one of the earliest whales and the first cetacean discovered with functional legs. In addition, it still retained many other features of terrestrial mammals, including an auditory system that was better for hearing in air than in water, a dentition not unlike that of its closest terrestrial relatives, such as the mesonychids, and functional feet capable of locomotion on land.

Pakicetus. Encyclopædia Britannica, Inc.

The body mass of Pakicetus was estimated at 45 kg (100 pounds), roughly the size of a wolf or large dog. The dentition of the animal indicates that it had a diet primarily of fish; however, its skeleton and skull suggest that it spent a considerable amount of time on land.

The climate of the early Eocene Epoch (56 million to 40 million years ago) was the warmest of the Cenozoic Era, nearly 10 °C (18 °F) warmer than the global average of the present day. Whales evolved during the Eocene in the warm, shallow tropical Tethys Sea, which lay sandwiched between the mainland of Asia and Europe to the north and Africa, Arabia, Madagascar, and the Indian subcontinent to the south. Most archaeocetes (first cetaceans) lived in the Tethys or along its margins. Pakicetus fossils, which include many broken teeth, skulls, and skeletons, were found in the Kuldana Formation in Pakistan, a site that was located near the northern edge of the Tethys Sea during the Eocene.

The later descendants of Pakicetus were fully aquatic. Modern whales are descended from the archaeocete basilosaurids, a group of toothed whales that had extremely long bodies and tails. The archaeocete basilosaurids appeared later in the Eocene and early Oligocene (34 million to 23 million years ago) and lived in the Tethys Sea and Atlantic Ocean.