''Pseudorca yokoyamai'' is an extinct species of

oceanic dolphin Oceanic dolphins or Delphinidae are a widely distributed family of dolphins that live in the sea. Close to forty extant species are recognised. They include several big species whose common names contain "whale" rather than "dolphin", such as the ...

from the Calabrian stage of the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in ...

of Japan, an extinct relative of the modern day

false killer whale The false killer whale (''Pseudorca crassidens'') is a species of oceanic dolphin that is the only extant representative of the genus '' Pseudorca''. It is found in oceans worldwide but mainly in tropical regions. It was first described in 184 ...

(''P. crassidens'').

Discovery and taxonomy

''Pseudorca yokoyamai'' was described in 1926 by Matsumoto Hikoshichiro of the

Tohoku Imperial University , or is a Japanese national university located in Sendai, Miyagi in the Tōhoku Region, Japan. It is informally referred to as . Established in 1907, it was the third Imperial University in Japan and among the first three Designated National ...

. 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 ...

is represented by two teeth–probably the very last two in the back of the mouth–and a

paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype nor a syntype). O ...

specimen consists of the right

ramus of the 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 teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

with seven teeth, though four teeth are incomplete. The holotype was found in Hommoku,

Yokohama is the second-largest city in Japan by population and the most populous municipality of Japan. It is the capital city and the most populous city in Kanagawa Prefecture, with a 2020 population of 3.8 million. It lies on Tokyo Bay, south of T ...

–the

species name In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

honoring the place of its discovery–and the paratype in the

Kazusa Province was a province of Japan in the area of modern Chiba Prefecture. The province was located in the middle of the Bōsō Peninsula, whose name takes its first ''kanji'' from the name of Awa Province and its second from Kazusa and Shimōsa province ...

on the opposite side of

Tokyo Bay is a bay located in the southern Kantō region of Japan, and spans the coasts of Tokyo, Kanagawa Prefecture, and Chiba Prefecture. Tokyo Bay is connected to the Pacific Ocean by the Uraga Channel. The Tokyo Bay region is both the most populous ...

. It may represent an intermediary phase between the modern false killer whale and

pilot whale Pilot whales are cetaceans belonging to the genus ''Globicephala''. The two extant species are the long-finned pilot whale (''G. melas'') and the short-finned pilot whale (''G. macrorhynchus''). The two are not readily distinguishable at sea, ...

s (''Globicephala'' spp.).

Description

The teeth of ''P. yokoyamai'' are largely conical, though bend backward, with a smooth enamel coating. On the holotype, the second-to-last and last teeth measured respectively in height. ''P. yokowamai'' had smaller, though a greater number of, teeth than the modern false killer whale. The paratype represents the last seven teeth in the jaw, and there were perhaps three or four more teeth in front of it. The diameter of the

tooth crown In dentistry, crown refers to the anatomical area of teeth, usually covered by enamel. The crown is usually visible in the mouth after developing below the gingiva The gums or gingiva (plural: ''gingivae'') consist of the mucosal tiss ...

at the

gumline The gums or gingiva (plural: ''gingivae'') consist of the mucosal tissue that lies over the mandible and maxilla inside the mouth. Gum health and disease can have an effect on general health. Structure The gums are part of the soft tissue linin ...

is , and the series of seven teeth is long.

References

{{Taxonbar, from=Q56276531 Oceanic dolphins Pleistocene mammals of Asia Mammals described in 1926