''Enhydra'' is a genus of mustelid that contains the
sea otter
The sea otter (''Enhydra lutris'') is a marine mammal native to the coasts of the northern and eastern Pacific Ocean, North Pacific Ocean. Adult sea otters typically weigh between , making them the heaviest members of ...
and two extinct relatives. It is the only extant genus of the bunodont otters group, referring to otters with non-blade
carnassial
Carnassials are paired upper and lower teeth modified in such a way as to allow enlarged and often self-sharpening edges to pass by each other in a shearing manner. This adaptation is found in carnivorans, where the carnassials are the modified f ...
s with rounded cusps.
Sea otters probably diverged from other otters during the
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Enhydritherium'', a bunodont otter endemic to North America during the late
Miocene
The Miocene ( ) is the first epoch (geology), 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 mea ...
and early Pliocene epochs.
'' Enhydra reevei'', the oldest known species, has its origins in the Atlantic, suggesting this may have been where sea otters originated. Fossil evidence indicates the ''Enhydra'' lineage became isolated in the
North Pacific
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is bounded by the contine ...
approximately 2 million years ago, giving rise to the now-extinct '' Enhydra macrodonta'' and the modern sea otter.