Smew

The Smew (Mergellus albellus) is a small duck which is somewhat intermediate between the typical mergansers (Mergus) and the goldeneyes (Bucephala). It is the only member of the genus Mergellus; sometimes included in Mergus, this genus is well distinct (though closely related) and might actually be a bit closer to the goldeneyes. The Smew has interbred with the Common Goldeneye (B. clangula).
An unnamed fossil seaduck, known from a humerus found in the Middle Miocene Sajóvölgyi Formation (Late Badenian, 13-12 million years ago) of Mátraszõlõs, Hungary, was assigned to Mergus. However, the authors included the Smew therein, and consequently, the bone is more properly assigned to Mergellus – especially as it was more similar to a Smew's than to Bucephala remains also found at the site. It is sometimes argued that the Mátraszõlõs fossil is too old to represent any of the modern seaduck genera, but apparently these were all well-distinct even back then.
The living species is known to exist since about 2 to 1.5 million years, as attested by fossils from the earliest Pleistocene found in England.