The muskrat (Ondatra zibethicus Linnaeus, 1766), a rodent of North American origin, was introduced into Europe in 1905; it spread quickly and is now abundant along coasts, lakes, rivers, and tributaries throughout western Europe.
The stratigraphic implications of Ondatra were first published by Semken (1966), who prepared a bivariate graph of M1 length versus width. The resulting chronocline, which indicated a gradual size increase from Ondatra idahoensis through Ondatra zibethicus, was correlated with relatively rapid increases in dentine track height (including hypsodonty) and increasing amounts of cement in the reentrant angles. The next year, Hibbard and Zakrzewski (1967) suggested that Pliopotamys was probably an ancestor of Ondatra. Zakrzewski (1969) graphically added Pliopotamys minor and Pliopotamys meadensis to the M1 length/ width bivariate and convincingly demonstrated that Pliopotamys was ancestral to Ondatra. This evolutionary lineage was reinforced by Schultz, Tanner, and Martin (1972) and Martin (1979, 1984) with additional specimens from critical local faunas of the central Great Plains. Nelson and Semken (1970) added a paleoecological indicator, via modern topocline, to the analysis of North American muskrats. The lineage Pliopotamys minor–meadensis–Ondatra idahoensis–annectens–nebrascensis– zibethicus evolved featuring increases in size, hypsodonty, dentine track height (perhaps a function of hypsodonty), and Mx crown complexity, as well as addition of cement in the reentrant angles. This evolution was parallel to that of the Mimomys occitanus–Arvicola terrestris lineage in Eurasia (Chaline, 1987).