 Basilosaurids and dorudontines lived together in the late Eocene around 41 to 35 million years ago , and are the oldest known obligate aquatic cetaceans . They were fully recognizable whales which lived entirely in the ocean . This is supported by their fossils usually found in deposits indicative of fully marine environments , lacking any freshwater influx . They were probably distributed throughout the tropical and subtropical seas of the world . Basilosaurids are commonly found in association with dorudontines , and were closely related to one another . The fossilised stomach contents in one basilosaurid indicates that it ate fish . 
