Aetiocetidae is an extinct family of toothed baleen whales known from the Oligocene. The whales are from the North Pacific Ocean and ranged in size from 3 to 8 metres (10 to 26 ft) long. Many of the described specimens were discovered from the Upper Oligocene of the Japanese Morawan Formation, the largest known one from the Morawan's Upper tuffaceous siltstone. Other formally described extinct toothed mysticetis from this time are smaller, from 3 to 4 metres (10 to 13 ft) in length. Mysticeti with true baleen are seen in fossils from the Upper Oligocene. The monophyly of the family is still uncertain, as are the evolutionary relationship between the early toothed baleen whales (Aetiocetidae, Mammalodontidae, and Llanocetidae) and the early and extant edentulous baleen whales. However, the