The Takikawa sea cow (''Hydrodamalis spissa'') is an extinct herbivorous
marine mammal
Marine mammals are mammals that rely on marine ecosystems for their existence. They include animals such as cetaceans, pinnipeds, sirenians, sea otters and polar bears. They are an informal group, unified only by their reliance on marine enviro ...
of the Late
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58recently extinct
Steller's sea cow
Steller's sea cow (''Hydrodamalis gigas'') is an extinction, extinct sirenian described by Georg Wilhelm Steller in 1741. At that time, it was found only around the Commander Islands in the Bering Sea between Alaska and Russia; its range exte ...
(''H. gigas''). In 1988, fossils of sea cows were discovered in
Hokkaido
is the list of islands of Japan by area, second-largest island of Japan and comprises the largest and northernmost prefectures of Japan, prefecture, making up its own list of regions of Japan, region. The Tsugaru Strait separates HokkaidÅ fr ...
and were originally assigned to the Takikawa sea cow, a newly described species, even though this
taxon
In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...
is thought of by some scientists as a
synonym
A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...
of the
Cuesta sea cow
The Cuesta sea cow (''Hydrodamalis cuestae'') is an extinct Herbivore, herbivorous marine mammal and is the direct ancestor of the Steller's sea cow (''Hydrodamalis gigas''). They reached up to in length, making them among the biggest sirenians ...
(''H. cuestae''). It is uncertain whether or not the Takikawa sea cow was simply a local variant of the Cuesta sea cow or a completely separate lineage. However, the Steller's sea cow and Takikawa sea cow share more morphological similarities than the Takikawa sea cow and Cuesta sea cow.