Archaeoceti is a suborder of cetaceans that represent the ancestors of modern-day whales. The bone structure of these ancient whales, dating back 50 million years (Early Eocene), is very similar to their decendants. Archaeoceti lived throughout the Eocene and were the first evidence of adaptation to a fully aquatic lifestyle. By the end of the Eocene, Archaeoceti had become extinct.

Some examples of ancient whales are: Pakicetus-latest Early Eocene, Ambulocetus-Early to Middle Eocene, Basilosaurus-Middle to Late Eocene.

Basilosaurus lived in the Eocene, around 35 to 40 million years ago.

Cenozoic Era Time Scale