Aetiocetus is a genus of extinct basal mysticete, or baleen whale that lived 33.9 to 23.03 million years ago, in the Oligocene in the North Pacific ocean, around Japan, Mexico, and Oregon, U.S.

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.

In 2002, Sanders and Barnes hypothesized that there was a larger superfamily, Aetiocetoidea, which would include all known toothed mysticetes: Aetiocetidae, Llanocetidae, and Mammalodontidae.

[9] 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.

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.

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.

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.

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.

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.

[13] 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.

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.

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.

This indicates that loss of functionality in relevant enamel genes, such as ameloblastin (AMBN), enamelin (ENAM), and amelogenin (AMEL), had not yet taken place in Aetiocetus.

This is interpreted to be the basic placental mammalian dental formula with three incisors, one canine, four premolars, and three molars on both upper and lower jaws.

Baleen is made of keratin (the same material that comprises claws, hooves, nails, and hair) that grows throughout the whale's life.

These nutrient foramina, present on the maxillae of the whale, are associated with grooves and sulci, or fissures, which in life are occupied by branches of the superior alveolar artery and nerve.

This superior alveolar artery supplies nutrients to the epithelial, or surface cells of the body, from which the baleen continuously develops.

This close association leads Deméré and Berta to hypothesize that Aetiocetus displays an ancient ontogeny, or growth sequence.

Compared to other edentulous, or toothless, mysticetes, the pattern of nutrient foramina is most similar to extant balaenopterids (blue whales and other rorquals) and fossil cetotheres.

[15] The teeth of Aetiocetus resemble those of archaeocetes and odontocetes, which employ a bite-and-swallow feeding strategy, but they also possessed expanded palates.

However, an argument exists that Aetiocetus was in fact a bulk feeder, who fed by gulping and straining prey from the water through their interlocking cusped cheek teeth.

Fitzgerald argued against the model of tooth-aided filter feeding, based on the lack of closely pressed teeth and the presence of simple postcanine crowns.

There is no reason to assume a priori that all bulk filter-feeders eat small prey, given the large diversity of food items consumed by modern mysticetes.

Deméré hypothesizes that Aetiocetus{{}}' bulk feeding behavior could have targeted large prey, such as schooling fish or squid.

However, a more likely explanation is that the fossil record for Aetiocetus is poor, or that a sampling bias is present and not enough work has been done in late Oligocene deposits in the south Pacific Ocean.