major toothed mysticete lineages, in which Aetiocetidae was paraphyletic, with a Chonecetus clade and an Aetiocetus clade. A. polydentatus was considered by Fitzgerald to not be a member of Aetiocetus at all because of its seemingly derived features in comparison to other members of the genus Aetiocetus, for instance, its polydont dentition and greatly enlarged nasal bones. The debate regarding relationships within the Aetiocetidae highlight the importance of this clade to the understanding of basal mysticete evolution and hypotheses surrounding the loss of adult teeth and the development of baleen. The larger scale phylogenetic placement of Aetiocetus has remained fairly constant throughout modern studies. Geisler and Sanders’ 2003 paper, “Morphological Evidence for the Phylogeny of Cetacea” used the genus in their morphological cladistics study and their results support a monophyletic Aetiocetidae (Aetiocetus + Chonecetus). Here, the Aetiocetidae is the sister taxon to Eomysticetus+ Micromysticetus + Diorocetus + Pelocetus + crown Mysticeti. In their sample, Aetiocetus is the second-most basal mysticete; two undescribed museum specimens, ChM, are considered the most basal mysticetes in this phylogeny. In Geisler et al.’s 2011 study entitled “A supermatrix analysis of genomic, morphological, and paleontological data from crown Cetacea”, there is higher resolution of Aetiocetus’ phylogenetic relationship with other mysticetes, as well as more taxa considered. In this study, Aetiocetus is still basal and is still the sister taxon to Eomysticetus + Micromysticetus + Diorocetus + Pelocetus + crown Mysticeti, all of which possess baleen and no teeth. There are two competing hypotheses supported by the supermatrix: 1) that Aetiocetus is not the sister group to Chonecetus, suggesting that the Aetiocetidae is a paraphyletic group, a group in which not all descendants are considered, or 2) that they indeed form a monophyletic group. Both results have been supported in previous studies. In an even more recent paper, entitled “A Phylogenetic Blueprint for a Modern Whale”, more than one species in the genus Aetiocetus is used: A. cotylalveus, A. weltoni, and A. polydentatus. These three taxa form a polytomy with Chonecetus, where the relationships of the four taxa cannot be more determined with the present resolution. However, this result suggests monophyly of the Aetiocetidae, or that all aetiocetids are derived from a single common ancestor. In this phylogeny, the Aetiocetidae is the sister taxon to Eomysticetus + Cetotheriidae + crown Mysticeti. Almost all phylogenies agree that Aetiocetus is a stem mysticete with no affiliation with crown Mysticeti.