Pakicetids were terrestrial animals that foraged in freshwater about 50 million years ago. These carnivorous and hoofed animals are the most primitive cetaceans known. Pakicetids include the genera Ichthyolestes, Pakicetus and Nalacetus, which fossils were discovered in river basins. Although being primarily terrestrial, these primitive animals showed some adaptations to the life in the water, including the ear structure.
Ambulocetids lived about 48 million years ago. Fossil records show body adaptations, such as shorter limbs and large feet shaped for paddling, which indicates an aquatic lifestyle. Ambulocetids ambushed prey in the water instead of chasing it, because their swimming was not very effective. Like pakicetids, ambulocetids lived in what is today India and Pakistan.
Protocetids were the first dolphin ancestors that were totally adapted to live in the water. They lived from 39 to 47 million years ago and were the first ancestors to leave the area that today is India. Several fossil of protocetids have been found in Africa and North America. These animals had long snouts, large eyes and short limbs, but unlike ambulocetids, they had webbed feet and hands.
Basilosaurids had long bodies, with fossil records indicating up to 52-foot body lengths. They lived in the late Eocene, about 35 to 37 million years ago. Basilosaurids had atrophied hind limbs, with a reduced number of toes. Basilosaurid fossils have been found in marine sediments in Egypt and North America. Dorudontids represent the link between basilosaurids and living dolphins. Dorudontids had body shapes and tails similar to modern day cetaceans, and also lived during the late Eocene period.