This result is not entirely surprising, given its symplesiomorphic condition, meaning that Aetiocetus still retains many primitive features and few derived ones. Its phylogenetic placement among stem mysticetes is also in line with its late Oligocene stratigraphic occurrence, where crown Mysticeti had yet to appear in the fossil record. Discovery and history Aetiocetus cotylalveus was discovered in 1966 and described by Emlong, who initially ascribed Aetiocetus to the Archaeoceti based on its plesiomorphic, or primitive, dentition; he felt that the presence of teeth barred Aetiocetus from the Mysticeti. There are many distinct features that separates Aetiocetus from Odontoceti, or toothed whales, and Emlong did not see evidence of remodeling necessary for the modern odontocete skull. Notably, Emlong noted that Aetiocetus was some antecedent to the mysticete lineage, due to the degree of telescoping on the skull, indicating that the nostrils of the whale had migrated further back on the skull than seen in archaeocetes. Along with the relation between cranial features, Emlong allied Aetiocetus more closely with Mysticeti than Odontoceti. However, as A. cotylalveus retains teeth, Emlong considered it a highly derived archaeocete. Van Valen in his 1968 essay “Monophyly or diphyly in the origin of whales” placed Aetiocetus in its accepted position as a basal, or early, mysticete. In 1995, Lawrence G. Barnes and his co-authors Masaichi Kimura, Hitoshi Furusawa, and Hiroshi Sawamura described three new aetiocetids that allied with the genus Aetiocetus. These finds were unique in that they placed an aetiocetid within the same geologic formation as the type specimen, A. cotylalveus, and also placed new aetiocetids on the western coast of the Pacific, in Japan. This extended the geographic range of Aetiocetus dramatically. In 1998, L.G. Barnes listed a specimen of Aetiocetus within his list of fossil marine mammal assemblages in Mexico. However, this specimen remains aff. Aetiocetus sp., and cannot be ascribed to any particular species. This specimen was found in Baja California in the El Cien Formation, but as of yet no paper has been published describing this specimen. A. cotylalveus is known from the Yaquina Formation of Oregon. The Yaquina Formation is late Oligocene in age and at the cetacean's locality consists of a fine-grained grey sandstone with alternating layers of medium-grained light-grey sandstone and siltstone. The Yaquina Formation represents a coastal marine depositional environment, and is considered late late Oligocene in age (Chattian) based on foraminifera and mollusc stages; approximately 24-25 million