As a species, the American mink represents a more specialized form than the European mink in the direction of carnivory, as indicated by the more developed structure of the skull.[5] Fossil records of the American mink go back as far as the Irvingtonian, though the species is uncommon among Pleistocene animals. Its fossil range corresponds with the species' current natural range. The American minks of the Pleistocene did not differ much in size or morphology from modern populations, though a slight trend toward increased size is apparent from the Irvingtonian through to the Illinoian and Wisconsinan periods.[6]