Phylogeny of Whales

History
When Carl Linnaeus classified animals, he placed what would now be classified as cetaceans in the class Pisces, along with the fish.

It was once thought that cetaceans were descended from Mesonychids, based on dental similarities. Genetic evidence, however, shows that cetaceans are closest to the Hippopotamidae in the Artiodactyla. This has made several taxonomists abandon Artiodactyla and use Cetartiodactyla instead.