''Yamatocetus canaliculatus'' is an extinct species of eomysticetid

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 ...

from the Late Oligocene of Japan.

Taxonomy

The

holotype specimen 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 ...

, a partial skeleton, was discovered by curator Toshiyuki Kamei in October 1981 in the Ashiya Group of the Jinnobaru Formation on the island of

Kyushu is the third-largest island of Japan's five main islands and the most southerly of the four largest islands ( i.e. excluding Okinawa). In the past, it has been known as , and . The historical regional name referred to Kyushu and its surroun ...

, and stored in the Kitakyushu Museum of Natural History and Human History. The genus name honors the old name of Japan, Yamato, and

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 ...

''cetus'', "whale." The species name translates to "with groove" in reference to the grooves in its mouth which indicate functional

baleen Baleen is a filter-feeding system inside the mouths of baleen whales. To use baleen, the whale first opens its mouth underwater to take in water. The whale then pushes the water out, and animals such as krill are filtered by the baleen and ...

. It was placed into the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Eomysticetidae Eomysticetidae is a family of extinct mysticetes belonging to Chaeomysticeti (toothless mysticetes). It is one of two families in the basal chaeomysticete clade Eomysticetoidea (the other being Cetotheriopsidae). Description Eomysticetids a ...

, a primitive group of

Description

As in other

s, the skull is wide and flat. The skull, along the sides, has several narrow, straight grooves–eight in total–and there are several foramina in the skull used as passage for blood vesselsthough not as many as modern baleen whales–which indicate it had baleen in its mouth. The skull contains several tooth sockets, but no teeth were discovered; it is possible these sockets were

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

and held no teeth, or the teeth naturally fell out over the course of the animal's life. The blowhole was located midway along the snout, near the position of eyes. Unlike the closely related ''

Eomysticetus ''Eomysticetus'' is an extinct genus of baleen whale from the late Oligocene (Chattian) Chandler Bridge Formation of South Carolina. (148Mb) Taxonomy ''Eomysticetus'' is a member of the family Eomysticetidae, which also includes '' Micromystic ...

'', the head is relatively flat, and the snout is blunt. The auditory bulla in the ear is more similar to primitive baleen whales and archaeocetes. ''Yamatocetus'' had seven

neck vertebra In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In ...

e, and the complete series was preserved. The neural spine projecting up from the vertebra increases along the series. Thoracic vertebrae one through seven and nine were preserved. The ribs had a double-headed joint with the vertebrae. The

shoulder blade The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either ...

was concave on the side facing the ribs, a primitive feature for baleen whales. The left forearm and fragments of the right were discovered. As in modern whales, the elbow joint probably could not rotate. The

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

is proportionally longer than that of modern baleen whales, a characteristic of eomysticetid whales, suggesting eomysticetids had less swimming capabilities.

Paleoecology

''Yamatocetus'' was an early

filter feeder Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

, and its teeth, if it had any, were likely vestigial and served no purpose. Since the whale had several double-headed vertebral joints, and modern baleen whales have a reduced number of these to increase the flexibility of the ribcage to aid in deep-sea diving, ''Yamatocetus'' likely stayed near the surface. Several marine vertebrates were found in the Ashiya Group: the baleen whales "'' Metasqualodon''," ''

Chonecetus ''Chonecetus'' is an extinct genus of primitive baleen whale of the family Aetiocetidae that lived in the Oligocene period. Its fossils have been found in Canada, in the northeast Pacific. It was first named by L.S. Russell in 1968, and conta ...

'', and an unidentified aetiocetid; and an unidentified squalodontid dolphin; a '' Trionyx'' softshell turtle; the tortoise '' Indotestudo takasago''; an unidentified

sea turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhead, ...

; an unidentified crocodile; the seabirds ''

Copepteryx ''Copepteryx'' is an extinct genus of flightless bird of the family Plotopteridae, endemic to Japan during the Oligocene living from 28.4 to 23 mya, meaning it existed for approximately . Taxonomy ''Copepteryx'' was named by Olson and Hasegawa ...

'' and an unknown

pelagornithid The Pelagornithidae, commonly called pelagornithids, pseudodontorns, bony-toothed birds, false-toothed birds or pseudotooth birds, are a prehistoric family of large seabirds. Their fossil remains have been found all over the world in rocks dating ...

; and unknown dugong. An unknown amynodontid, a terrestrial hippo-like animal, was also found.

References

{{Taxonbar, from=Q25385774 Baleen whales Oligocene cetaceans Prehistoric cetacean genera Mammals described in 2012 Oligocene mammals of Asia