Analyses of mitochondrial genomes strongly support a hippopotamus-whale clade.

Although the sister-group relationship between Cetacea and Artiodactyla is widely accepted, the actual artiodactyl group which is closest to Cetacea has not been conclusively identified. In the present study, we have sequenced the complete mitochondrial genome of the hippopotamus, Hippopotamus amphibius, and included it in phylogenetic analyses together with 15 other placental mammals. These analyses separated the hippopotamus from the other suiform included, the pig, and identified the hippopotamus as the artiodactyl sister group of the cetaceans, thereby making both. Artiodactyla and the suborder. Suiformes paraphyletic. The divergence between the hippopotamid and cetacean lineages was calculated using this molecular data and was estimated at ca. 54 Ma BP.