the kogiids. Evolution The earliest sperm whale fossils are known from the late Oligocene – about 25 million years ago, with an ancestry tracing back from the latest Eocene before diverging from the remainder of the odontocetid line, leading to the dolphins, and porpoises. The fossil record suggests that sperm whales were more common in the Miocene, during which basal lineages (such as Zygophyseter and Brygmophyseter) existed; other fossil genera assigned to the Physeteridae include Ferecetotherium, Helvicetus, Idiorophus, Diaphorocetus, Aulophyseter, Orycterocetus, Scaldicetus, and Placoziphius, while kogiid fossil genera include Kogiopsis, Scaphokogia, and Praekogia. The earliest kogiids are known from the late Miocene, around 7 million years ago. The close relationship between extant Physeteridae and Kogiidae is confirmed in recent molecular studies using mitochondrial cytochrome b,; on the basis of these analyses, their nearest relatives appear to be the Ziphiidae on one hand, and the Mysticeti and Platanistidae on the other. The last cited paper also favours the grouping of Physeteridae and Kogiidae in a single superfamily, Physeteroidea, as has sometimes previously been suggested. Bianucci & Landini, 2006 suggest that Diaphorocetus, Zygophyseter, Naganocetus, and Aulophyseter antedate the inferred split of the Kogiidae and Physeteridae, thus would restrict the family Physeteridae to those genera that postdate this split (a cladistic view). Classification They are members of the Odontoceti, the suborder containing all the toothed whales and dolphins. Suggestions that the sperm whales might be a sister group to the baleen whales were refuted by molecular and morphological data, confirming the monophyly of Odontoceti including sperm whales. One extant species of the genus Physeter is placed in the family Physeteridae. Two species of the related extant genus Kogia, the pygmy sperm whale K. breviceps and the dwarf sperm whale K. simus, are sometimes also placed in this family, or else are placed in their own family, the Kogiidae. A summary of the classification of extant and extinct (†) taxa is: Physeteroidea, sperm whales Stem physeteroids Acrophyseter Albicetus Aulophyseter Brygmophyseter (= Naganocetus) Diaphorocetus Eudelphis Livyatan Miophyseter Orycterocetus Rhaphicetus Zygophyseter Physeteridae, sperm whales Cozzuoliphyseter Ferecetotherium Helvicetus Idiophyseter Idiorophus Orycterocetus Physeter, sperm whales Physeter macrocephalus, sperm whale Physeterula Placoziphius Preaulophyseter Scaldicetus Kogiidae Aprixokogia Kogia, small sperm whales Kogia breviceps, pygmy sperm whale Kogia sima, dwarf sperm whale Kogia pusilla Kogiopsis Praekogia Scaphokogia Thalassocetus Nomina dubia Eucetus Graphiodon Homocetus Hoplocetus Orcopsis Palaeodelphis Paleophoca Physetodon Physodon Physotherium Priscophyseter Prophyseter Scaptodon Ziphoides References Extant Chattian first appearances Taxa named