 Ambulocetus , which lived about 49 million years ago , was discovered in Pakistan in 1994 . It was probably amphibious , and resembled crocodiles in its physical appearance . In the Eocene , ambulocetids inhabited the bays and estuaries of the Tethys Sea in northern Pakistan . The fossils of ambulocetids are always found in near @-@ shore shallow marine deposits associated with abundant marine plant fossils and littoral mollusks . Although they are found only in marine deposits , their oxygen isotope values indicate that they consumed a range of water with different degree of salinity , with some specimens having no evidence of sea water consumption and others that did not ingest fresh water at the time when their teeth were fossilized . It is clear that ambulocetids tolerated a wide range of salt concentrations . Hence , ambulocetids represent a transition phase of cetacean ancestors between fresh water and marine habitat . 
