 The pakicetids were digitigrade hoofed mammals that are thought to be the earliest known cetaceans , with Indohyus being the closest sister group . They lived in the early Eocene , around 50 million years ago . Their fossils were first discovered in North Pakistan in 1979 , located at a river not far from the shores of the former Tethys Sea . After the initial discovery , more fossils were found , mainly in the early Eocene fluvial deposits in northern Pakistan and northwestern India . Based on this discovery , pakicetids most likely lived in an arid environment with ephemeral streams and moderately developed floodplains millions of years ago . By using stable oxygen isotopes analysis , they were shown to drink fresh water , implying that they lived around freshwater bodies . Their diet probably included land animals that approached water for drinking or some freshwater aquatic organisms that lived in the river . The elongated cervical vertebrae and the four , fused sacral vertebrae are consistent with artiodactyls , making Pakicetus one of the earliest fossils to be recovered from the period following the Cetacea / Artiodactyla divergence event . 
