Joumocetus
Encyclopedia
Joumocetus is an extinct baleen whale
genus in the family Cetotheriidae
and containing the single
species Joumocetus shimizui. The species is known only from a partial skeleton found in Miocene
age sediments of Japan
.
s, 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. 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 , while sanidine
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 then 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 gape. 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 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
. 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.
. The deep v-shaped patterning of the rostrum is considered a primitive trait of the cetotheriids. The v-shape forms a shallower wedge then 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 then that seen in other cetotheriids.
Baleen whale
The Baleen whales, also called whalebone whales or great whales, form the Mysticeti, one of two suborders of the Cetacea . Baleen whales are characterized by having baleen plates for filtering food from water, rather than having teeth. This distinguishes them from the other suborder of cetaceans,...
genus in the family Cetotheriidae
Cetotheriidae
Cetotheriidae is an extinct family of baleen whales in the suborder Mysticeti. The family existed from the Late Oligocene to the Late Pliocene before going extinct.-Taxonomy:...
and containing the single
Monotypic
In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...
species Joumocetus shimizui. The species is known only from a partial skeleton found in Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
age sediments of Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
.
History and classification
Joumocetus shimizui was described from a mostly complete cranium and associated portions of both mandibleMandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
s, cervical
Cervical vertebrae
In vertebrates, cervical vertebrae are those vertebrae immediately inferior to the skull.Thoracic vertebrae in all mammalian species are defined as those vertebrae that also carry a pair of ribs, and lie caudal to the cervical vertebrae. Further caudally follow the lumbar vertebrae, which also...
and thoracic vertebrae
Thoracic vertebrae
In human anatomy, twelve thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. They are intermediate in size between those of the cervical and lumbar regions; they increase in size as one proceeds down the spine, the upper...
. The skeleton was recovered from an outcrop of the earliest late Miocene
Late Miocene
The Late Miocene is a sub-epoch of the Miocene Epoch made up of two stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch....
, Serravallian
Serravallian
The Serravallian is in the geologic timescale an age or a stage in the middle Miocene epoch/series, that spans the time between 13.65 ± 0.05 Ma and 11.608 ± 0.005 Ma...
age Haraichi Formation exposed along the Kabura River near Yoshii, Gunma Prefecture
Yoshii, Gunma
was a town located in Tano District, Gunma, Japan.As of September 1, 2007, the town had an estimated population of 24,758 and a density of 424.30 persons per km². The total area was 58.35 km².-Geography:...
, Japan. The Haraichi Formation includes several tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
layers which have been used for dating. The type skeleton was found between the older Baba tuff and the younger Kamikoizawa tuff. Biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
specimens taken from the Baba tuff have yielded an 40Argon-39Argon date of , while sanidine
Sanidine
Sanidine is the high temperature form of potassium feldspar 4O8. Sanidine most typically occurs in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures...
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 then 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 gape. The second oldest species Cetotherium furlongi
Cetotherium
Cetotherium is a genus of the extinct cetaceans from the family Cetotheriidae .-Known species:...
, 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 the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
, number GMNH-PV-240 which is currently preserved in the paleontology
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
collections housed at the Gunma Museum of Natural History in Gunma, Gunma Prefecture
Gunma Prefecture
is a prefecture of Japan located in the northwest corner of the Kantō region on Honshu island. Its capital is Maebashi.- History :The remains of a Paleolithic man were found at Iwajuku, Gunma Prefecture, in the early 20th century and there is a public museum there.Japan was without horses until...
, 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
Journal of Vertebrate Paleontology
The Journal of Vertebrate Paleontology was founded in 1980 at the University of Oklahoma by Dr. Jiri Zidek. It is a scientific journal that publishes original contributions on all aspects of the vertebrate paleontology, including vertebrate origins, evolution, functional morphology, taxonomy,...
. They coined the generic name Joumocetus as a combination of an older district name, "Joumo" and the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
cetus meaning "whale". The etymology
Etymology
Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time.For languages with a long written history, etymologists make use of texts in these languages and texts about the languages to gather knowledge about how words were used during...
of the chosen specific name
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
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 755 millimetres (29.7 in) long with over sixty percent being comprised of the rostrumRostrum
Rostrum may refer to:* A piece of furniture suitable for holding papers used by a person who is addressing a group; a lectern* Australian Rostrum, an association of Australian public speaking clubs...
. The deep v-shaped patterning of the rostrum is considered a primitive trait of the cetotheriids. The v-shape forms a shallower wedge then is seen in other members of the family. The widely exposed parietals
Parietal bone
The parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....
and interparietal
Parietal bone
The parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....
in Joumocetus form a sagittal crest
Sagittal crest
A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull of many mammalian and reptilian skulls, among others....
which is distinct and longer then that seen in other cetotheriids.