| Joumocetus Temporal range: | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Family: | Cetotheriidae |
| Genus: | † Joumocetus Kimura & Hasegawa, 2010 |
| Species: | †J. shimizui |
| Binomial name | |
| †Joumocetus shimizui | |
Joumocetus is a genus of extinct baleen whale in the family Cetotheriidae containing the single species Joumocetus shimizui. The species is known only from a partial skeleton found in Miocene age sediments of Japan. [1]
Joumocetus shimizui was described from a mostly complete cranium and associated portions of both mandibles, cervical and thoracic vertebrae. The skeleton was recovered from an outcrop of the earliest late Miocene, Serravallian age Haraichi Formation exposed along the Kabura River near Yoshii, Gunma Prefecture, Japan ( 36°18′N139°00′E / 36.3°N 139.0°E , paleocoordinates 37°48′N137°12′E / 37.8°N 137.2°E ). [2] The Haraichi Formation includes several tuff layers which have been used for dating. The type skeleton was found between the older Baba tuff and the younger Kamikoizawa tuff. Biotite specimens taken from the Baba tuff have yielded an 40argon–39argon date of 11.26 million years ago, while sanidine specimens gave a 40argon–39argon date of 11.29 million years ago. The overlying Kamikoizawa tuff has not been dated, sedimentation rates have been used to estimate an age of the earliest late Miocene. [1]
Two other Cetotheriidae genera are known from sediments older than Joumocetus, however the validity of the placement for those fossils has been questioned. The Early Miocene species Heterocetus brevifrons from the Netherlands is known from a poor specimen and lacks many diagnostic features. The other described species in Heterocetus are from the early middle Miocene to Pliocene leaving a distinct age gap. The second oldest species Cetotherium furlongi , from the middle Miocene of California, has been questioned as to its placement in the genus and family. Similar to the lack of diagnostic features in H. brevifrons, the skull from the C. furlongi type specimen is lost, making taxonomic placement questionable. If the two species are excluded, it would make Joumocetus shimizui the oldest occurrence of a cetotheriid in the fossil record. [1]
The genus and species is known from only the holotype, number GMNH-PV-240 which is currently preserved in the paleontology collections housed at the Gunma Museum of Natural History in Gunma, Gunma Prefecture, Japan. The specimens were studied by paleontologists Toshiyuki Kimura and Yoshikazu Hasegawa, both of the Gunma Museum of Natural History. Kimura and Hasegawa published their 2010 type description for J. shimizui in the Journal of Vertebrate Paleontology . [1] They coined the generic name Joumocetus as a combination of an older district name, "Joumo" and the Latin cetus meaning "whale". The etymology of the chosen specific name shimizui is in recognition of Masaru Shimizu who originally found the specimen and donated it to the Gunma Museum of Natural History. [1]
The skull of Joumocetus shimizui is over 755 millimetres (29.7 in) long with over 60% being composed of the rostrum. The deep V-shaped patterning of the rostrum is considered a primitive trait of the cetotheriids. The V-shape forms a shallower wedge than is seen in other members of the family. The widely exposed parietals and interparietal in Joumocetus form a sagittal crest which is distinct and longer than that seen in other cetotheriids. [1]
Neobalaenidae is a family of baleen whales including the extant pygmy right whale. Although traditionally considered related to balaenids, a recent phylogenetic study by Fordyce and Marx (2013) recovered the living pygmy right whale as a member of Cetotheriidae, making it the only extant cetotheriid, but not all authors agree with this argument.
Cetotherium is an extinct genus of baleen whales from the family Cetotheriidae.
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. It was first described by Douglas Emlong in 1966 and currently contains known four species, A. cotylalveus, A. polydentatus, A. tomitai, and A. weltoni. 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.
Brygmophyseter, known as the biting sperm whale, is an extinct genus of toothed whale in the sperm whale family with one species, B. shigensis. When it was first described in 1994, the species was placed in the genus Scaldicetus based on tooth morphology, but this was later revised in 1995. In 2006, it was classified into the genus Naganocetus, which is considered to be a junior synonym. The only known specimen, a nearly complete skeleton, was dated to be around 14–15 million years old. Brygmophyseter is thought to have been 7 meters (23 ft) long, and it probably had 11 or 12 teeth in the upper and lower jaws. Brygmophyseter is part of a group of macroraptorial sperm whales which tended to be apex predators using their large teeth to catch struggling prey such as whales. It had a spermaceti organ which was probably used for biosonar like in the modern sperm whale. The whale has made an appearance on The History Channel's TV series Jurassic Fight Club.
Australodelphis mirus is an extinct Pliocene dolphin. A. mirus is known from fossils found in the Sørsdal Formation, Mule Peninsula, Vestfold Hills, East Antarctica. The genus has been described as an example of convergent evolution with beaked whales.
Plesiobalaenoptera is a genus of extinct rorqual which existed in Italy during the late Miocene epoch. The type species is P. quarantellii. It is the oldest known rorqual from the Mediterranean basin. Fossils have been found from sediments of the Stirone River in northern Italy that were deposited during the Tortonian age, around 11 to 7 million years ago.
Cetotheriidae is a family of baleen whales. The family is known to have existed from the Late Oligocene to the Early Pleistocene before going extinct. Although some phylogenetic studies conducted by Fordyce & Marx 2013 recovered the living pygmy right whale as a member of Cetotheriidae, making the pygmy right whale the only living cetotheriid, other authors either dispute this placement or recover Neobalaenidae as a sister group to Cetotheriidae.
Titanocetus is a genus of extinct cetaceans closely related to the family Cetotheriidae.
Herpetocetus is a genus of cetotheriid mysticete in the subfamily Herpetocetinae.
Peripolocetus is a genus of balaenid baleen whale from the middle Miocene of Kern County, California.
Metopocetus is an extinct genus of baleen whale belonging to the family Cetotheriidae. The type species is Metopocetus durinasus.
Huaridelphis is an extinct genus of river dolphins from the Early Miocene. The type species is H. raimondii, found in the Chilcatay Formation of the Pisco Basin.
Tranatocetus is an extinct genus of mysticete from the late Miocene (Tortonian) of Jutland, Denmark. The type and only species is Tranatocetus argillarius.
Brandtocetus is a genus of cetotheriid mysticete in the subfamily Cetotheriinae. The type and only species is Brandtocetus chongulek from the late Miocene (Tortonian) of the Kerch Peninsula in Crimea.
Kurdalagonus is a genus of cetotheriid mysticete in the subfamily Cetotheriinae from the Miocene of the Russian Caucasus.
Eucetotherium is a genus of cetotheriid mysticete from Miocene (Tortonian) marine deposits in the Russian Caucasus.
Tiucetus is an extinct genus of cetotheriid baleen whale known from the Late Miocene Pisco Formation of Peru.
Otradnocetus is an extinct genus of baleen whale from the middle Miocene of the Russian Caucasus.
Ciuciulea is a genus of cetotheriid mysticete found in middle Miocene marine deposits in Moldova.