years in age. A. weltoni is also known from the Yaquina Formation and occurs along the same cliff face as A. cotylalveus, but occurs higher in the stratigraphic section. This specimen was found in situ near the contact of the conformably overlying Nye Formation, which is Miocene in age. Thus, A. weltoni is very close to the Oligocene-Miocene boundary. A. tomitai was discovered in the Middle Hard Shale member of the Morawan Formation, Kawakami group in Japan. This is also late Oligocene in age and represents a basinal depositional environment. This specimen was not found in situ, but in a loose concretion, and could potentially be stratigraphically higher than the Middle Hard Shale, but Barnes et al. presume that the animal was not transported far from the location where it died. A. polydentatus was also discovered in the Morawan Formation of Japan, but from the Upper Tuffaceous Siltstone Member, which also represents a basinal depositional environment. The holotype was found in situ in the uppermost part of the member. It is currently the stratigraphically highest occurrence of an aetiocetid from the northwestern Pacific Ocean, meaning that it is the youngest known specimen of Aetiocetus. Description The type species for the genus is Aetiocetus cotylalveus. It is defined as being the monophyletic group encompassing the closest common ancestor of A. cotylalveus and A. polydentatus and all its descendants: the textbook definition for a monophyletic taxon. Skull Aetiocetus is a small, toothed whale with no more than three small denticles on the anterior and posterior margins of the posterior upper teeth. Their postcanine teeth are somewhat heterodont. The base of the rostrum, or snout, of the whale, is greater than 170% of the width of the occipital condyles where the skull meets the neck. These features are synapomorphies, or shared derived traits, of Aetiocetus. There is a distinct notch by the internal nostrils formed of the palatine, pterygoid, and vomer bones; this is a synapomorphy of Aetiocetus + Chonecetus. Synapomorphies of the aetiocetids present in Aetiocetus are: the coronoid process of the dentary, or lower jaw, is well developed; the zygomatic arch is expanded anteriorly and posteriorly but is narrow at the middle. Aetiocetus also shares several traits with all mysticetes. The mandibular symphysis, or the connection between both lower jaw bones, is not fused. The descending process of the maxilla becomes a toothless plate below the orbit. They possess a wide