Basilosaurids and dorudontines lived together in the late Eocene around 41 to 33.9 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.