''Joumocetus'' is a genus of extinct

baleen whale Baleen whales (systematic name Mysticeti), also known as whalebone whales, are a parvorder of carnivorous marine mammals of the infraorder Cetacea (whales, dolphins and porpoises) which use keratinaceous baleen plates (or "whalebone") in their ...

in the family

Cetotheriidae Cetotheriidae is a family of baleen whales (parvorder Mysticeti). The family is known to have existed from the Late Oligocene to the Early Pleistocene before going extinct. Although some phylogenetic studies conducted by recovered the living py ...

containing the

single Single may refer to: Arts, entertainment, and media * Single (music), a song release Songs * "Single" (Natasha Bedingfield song), 2004 * "Single" (New Kids on the Block and Ne-Yo song), 2008 * "Single" (William Wei song), 2016 * "Single", by ...

species ''Joumocetus shimizui''. The species is known only from a partial skeleton found in

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

age sediments of

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

History and classification

''Joumocetus shimizui'' was described from a mostly complete

cranium The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

and associated portions of both

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

cervical In anatomy, cervical is an adjective that has two meanings: # of or pertaining to any neck. # of or pertaining to the female cervix: i.e., the ''neck'' of the uterus. *Commonly used medical phrases involving the neck are **cervical collar **cerv ...

and

thoracic vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae and they are intermediate in size b ...

. The skeleton was recovered from an outcrop of the earliest

late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...

Serravallian The Serravallian is, in the geologic timescale, an List of time periods, age or a stage (stratigraphy), stage in the middle Miocene epoch (geology), Epoch/series (stratigraphy), Series, which spans the time between 13.82 annum, Ma and 11.63 Ma (m ...

age Haraichi Formation exposed along the Kabura River near Yoshii, Gunma Prefecture, Japan (, paleocoordinates ).. Retrieved October 2013. The Haraichi Formation includes several

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

layers which have been used for dating. The type skeleton was found between the older Baba tuff and the younger Kamikoizawa tuff.

Biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

specimens taken from the Baba tuff have yielded an ⁴⁰argon–³⁹argon date of , while

sanidine Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system ...

specimens gave a ⁴⁰argon–³⁹argon date of . The overlying Kamikoizawa tuff has not been dated, sedimentation rates have been used to estimate an age of the earliest late Miocene. 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. The genus and species is known from

only Only may refer to: Music Albums * ''Only'' (album), by Tommy Emmanuel, 2000 * ''The Only'', an EP by Dua Lipa, 2017 Songs * "Only" (Anthrax song), 1993 * "Only" (Nine Inch Nails song), 2005 * "Only" (Nicki Minaj song), 2014 * "The Only", by ...

the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

, number GMNH-PV-240 which is currently preserved in the

paleontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

collections housed at the

Gunma Museum of Natural History is a museum of the natural sciences in Tomioka, Gunma Prefecture, Japan. History The museum first opened in 1978 under its old Japanese name, after repairs to the former . In 1996, the old museum closed, and its collection was transferred, befor ...

Gunma is a prefecture of Japan located in the Kantō region of Honshu. Gunma Prefecture has a population of 1,937,626 (1 October 2019) and has a geographic area of 6,362 km2 (2,456 sq mi). Gunma Prefecture borders Niigata Prefecture and Fukushima P ...

Gunma Prefecture is a prefecture of Japan located in the Kantō region of Honshu. Gunma Prefecture has a population of 1,937,626 (1 October 2019) and has a geographic area of 6,362 km2 (2,456 sq mi). Gunma Prefecture borders Niigata Prefecture and Fukushima ...

, Japan. The specimens were studied by

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

s Toshiyuki Kimura and Yoshikazu Hasegawa, both of the Gunma Museum of Natural History. Kimura and Hasegawa published their 2010

type description A species description is a formal description of a newly discovered species, usually in the form of a scientific paper. Its purpose is to give a clear description of a new species of organism and explain how it differs from species that have be ...

for ''J. shimizui'' in the ''

Journal of Vertebrate Paleontology The ''Journal of Vertebrate Paleontology'' is a bimonthly peer-reviewed scientific journal that was established in 1980 by Jiri Zidek (University of Oklahoma). It covers all aspects of vertebrate paleontology, including vertebrate origins, evolu ...

''. They coined the generic name ''Joumocetus'' as a combination of an older district name, "''Joumo''" and the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

' meaning "whale". The

etymology Etymology ()The New Oxford Dictionary of English (1998) – p. 633 "Etymology /ˌɛtɪˈmɒlədʒi/ the study of the class in words and the way their meanings have changed throughout time". is the study of the history of the Phonological chan ...

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.

Description

The skull of ''Joumocetus shimizui'' is over long with over 60% being composed of the

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ships * Ros ...

. 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 A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are exceptiona ...

which is distinct and longer than that seen in other cetotheriids.

References

{{Taxonbar, from=Q6294606 Baleen whales Fossil taxa described in 2010 Miocene cetaceans Prehistoric monotypic mammal genera Prehistoric cetacean genera Miocene mammals of Asia