Guillemot is the common name for several species of seabird in the auk family, comprising two genera: Uria and Cepphus. This word of French origin apparently derives from a form of the name William, cf. the Gwillim or the Guillaume.
The Uria are known as murres in North America and, together with the Razorbill, Dovekie and the extinct Great Auk, make up the tribe Alcini. They have distinctly white bellies, thicker, longer bills than Cepphus and form very dense colonies on cliffs during the reproductive season.
The three species of Cepphus - for which the term "guillemot" is generally reserved in North America - form a tribe of their own: Cepphini. They are smaller than the Uria species, have black bellies, rounder heads and bright red feet.
Some prehistoric species are also known:
U. brodkorbi is interesting insofar as it is the only known occurrence of the Alcini tribe in the temperate to subtropical Pacific, except for the very fringe of the range of U. aalge. It suggests that the Uria species, which are the sister taxon to all the other Alcini, and like them are usually believed to have evolved in the Atlantic, may have evolved in the Caribbean or possibly close to the Isthmus of Panama. The modern Pacific distribution would then be part of a later arctic expansion, whereas most other auk lineages form clades with a continuous range in the Pacific, from Arctic to subtropical waters.
As in other genera of auks, fossils of prehistoric forms of Cepphus have been found:
The latter two resemble the extant species, but because of the considerable distance in time or space from their current occurrence, they may represent distinct species.