Aetiocetus

Aetiocetus is a genus of extinct basal mysticete, or baleen whale that lived 33.9 to 23.03 million years ago, in the late 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.[1] 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
Aetiocetus means “causal whale”, with aetio- being the Greek root for “cause” or “origin”, and –cetus meaning “whale”. This name was given to the genus by Douglas Emlong in his 1966 monograph of Aetiocetus cotylalveus, though he failed to present an etymology.

A. cotylalveus approximately means “bowl cavity”, cotyl- meaning “cup” or “bowl” in Greek, while alveus is Latin for “hollow” or “cavity”. However, the author’s original reasoning for the genus and species name for A. cotylalveus remains unknown.

A. tomitai is named in honor of then-mayor Akio Tomita of Ashoro-cho, Hokkaido, Japan,[2] who was a huge support for their work on Japanese cetaceans.

A. weltoni is named in honor of Doctor Bruce J. Welton, who initially discovered the holotype and directed the excavation of the skeleton.[2]

A. polydentatus is named in reference to the polydont dentition present in the specimen; poly- being the Greek root for “many” and –dentatus Latin for “toothed”.

Geological and Environmental Context
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.[3] 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 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.[2]

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.[2] 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.