The smew (Mergellus albellus) is a species of duck, and is the only living member of the genus Mergellus. This genus is closely related to Mergus and is sometimes included in it, though it might be closer to the goldeneyes (Bucephala). The smew has interbred with the common goldeneye (B. clangula).
A seaduck fossil from the Middle Miocene shows that birds similar to smew existed up to 13 million years ago. The extant species dates back to the Pleistocene. source