The family Cetotheriidae and the genus Cetotherium (sensu lato) have been used as wastebaskets for all kinds of baleen whales, most notably by Brandt 1873, Spassky (1954) and Mčedlidze 1970. Based on more recent phylogenetic studies and revisions of many 19th century genera, much smaller monophyletic Cetotheriidae and Cetotherium sensu stricto is limited to a single or only a few species. For example, Gol'din, Startsev & Krakhmalnaya 2013 included only C. rathkii and C. riabinini in the genus and only ten genera in the family.
Cetotheriidae were thought to have gone extinct during the Pliocene until 2012, when it was hypothesized that the Pygmy right whale was the sole surviving species of this family.
Formerly assigned to CetotheriumEdit
The following species were originally described as nominal species of Cetotherium but have been either re-assigned to other genera or removed from Cetotherium:
- Cetotherium furlongi Kellogg, 1925, is known from a partial skull from the Burdigalian of the Vaqueros Formation in California, but the holotype is lost.
- Cetotherium gastaldii Strobel, 1875, known from the early Pliocene-age Sabbie d'Asti Formation of the Piedmont region in Italy, is now the type species of the eschrichtiid genus Eschrichtioides.
- Cetotherium klinderi Brandt, 1871, is known from an isolated earbone from Miocene sediments in Chişinău, Moldova. Although fragmentary, it does not appear to be congeneric with either of the two valid species of Cetotherium.
- Cetotherium maicopicum Spasski, 1951, based on a specimen from the late Miocene of the Russian Caucasus, has been re-assigned to the genus Kurdalagonus from the same region.
- Cetotherium mayeri Brandt, 1871, known from a partial skeleton, is apparently not congeneric with Cetotherium.
Cetotheres came into existence during the Oligocene epoch. The cetotheres have been divided into two sub-groups. One group includes Cetotherium. From evolutionary perspective, these whales share some characteristics of the Balaenopteridae and Eschrichtiidae.