Evolution and systematics

The mountain beaver, or sewellel (Aplodontia rufa), is not related to the beaver but is in its own distinct family. Classification of suborders of rodents is based, in part, on the anatomy of the jaw muscles. In early classification schemes, three suborders are recognized. Alternative schemes recognize two suborders, with no assignment given to several families whose lineage is uncertain. Regardless of which scheme is used, Aplodontidae are substantively different from other families of rodents, and considered the most ancient of living rodents. Their origins trace back 50 million years to the Eocene. They and related groups flourished through to the Miocene and once occupied parts of Asia, Europe, and western North America. Today, the family is represented by a single species, mountain beaver, confined to a small portion of western North America.

The taxonomy for this species is Aplodontia rufa (Rafinesque, 1817), Oregon, United States.

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