Aetiocetus is a genus of extinct basal mysticete, or baleen whale that lived , in the Oligocene in the North Pacific ocean, around Japan, Mexico, and Oregon, U.S. It was first described by Douglas Emlong in 1966 and currently contains known four species, A. cotylalveus, A. polydentatus, A. tomitai, and A. weltoni. These whales are remarkable for their retention of teeth and presence of nutrient foramina, indicating that they possessed baleen. Thus, Aetiocetus represents the transition from teeth to baleen in Oligocene mysticetes. Baleen is a highly derived character, or synapomorphy, of mysticetes, and is a keratinous structure that grows from the palate, or roof of the mouth, of the whale. The presence of baleen is inferred from the fossil record in the skull of Aetiocetus. Aetiocetus is known from both sides of the Pacific Ocean: it was first documented in Oregon, United States, but it is also known from Japan and Mexico. The genus is currently constrained to the Northern hemisphere and has little value in biostratigraphic studies of the Oligocene due to its limited occurrences across the Pacific. Etymology The genus name Aetiocetus comes from Ancient Greek via Latin 'cause, origin' and Latin 'whale', translating to "original whale". A. cotylalveus approximately means "bowl cavity", meaning 'cup' or 'bowl' in Ancient Greek, and alveus Latin for "hollow" or "cavity". A. tomitai is named in honor of then-mayor Akio Tomita of Ashoro, Hokkaido in Japan. A. weltoni is named in honor of Doctor Bruce J. Welton, who initially discovered the specimen and directed the excavation of the skeleton. A. polydentatus is named in reference to the diversity of tooth shapes (polydont dentition) present in the specimen. Phylogeny There has been some dispute about the relationships between Aetiocetus and stem Mysticetes. Barnes et al. (1995) expanded Emlong's original definition to encompass eight species in four genera. They suggested a monophyletic Aetiocetidae with three subfamilies: Chonecetinae, which includes Chonecetus spp., Morawanocetinae, which includes Morawanocetus yabukii, and Aetiocetinae, which contains Ashorocetus eguchii and Aetiocetus spp. In 2002, Sanders and Barnes hypothesized that there was a larger superfamily, Aetiocetoidea, which would include all known toothed mysticetes: Aetiocetidae, Llanocetidae, and Mammalodontidae. However, evidence suggests that this “Aetiocetoidea” is a grade taxon and does not actually form a natural group, as the retention of teeth is a symplesiomorphic condition for Cetacea and cannot be used as a synapomorphy for the group. Fitzgerald in 2006 proposed 6