The Archaeoceti, as their name implies, are the oldest group and consequently the most primitive in their structure. They flourished during the Eocene epoch, but most of them were extinct before the end of the Oligocene. A few species survived into the early Miocene.
Their teeth are completely different from those of modern whales for they are heterodont.
Basilosaurus is one of the most common of the primitive whales, called "archaeocetes" by paleontologists, that have found in exposures of Middle to Upper Eocene, 35 to 40 million year old, marine sediments within central Louisiana. The species of Basilosaurus found in Louisiana, Basilosaurus cetoides (Owen), had a stream-lined body that averaged 45 to 70 feet in length. Its body looked more like the body of a mythical sea-serpent rather then the body of a modern whale. Basilosauru had a wedged-shaped head up to 5 feet long with jaws containing two types of teeth. The teeth in the front, anterior, of the jaw had cone-shaped teeth which caught and held its prey while triangular-shaped teeth in the rear, posterior, of the jaw sliced them up. The type and amount of wear on the teeth of the Basilosaurus indicates that it likely ate fish and squid (Johnston 1991 Carpenter and White 1986).