The Family Aplodontiidae consists of a single living species, the mountain beaver (Aplodontia rufa), found in the forests and riparian areas of western North America. Despite its name, it is not a true beaver; it is considered a primitive rodent and the sole surviving member of its family. Mountain beavers are nocturnal, burrowing herbivores, feeding on ferns, shoots, and other vegetation. They have robust bodies, short tails, and strong limbs adapted for digging extensive burrow systems. Ecologically, they influence soil aeration and vegetation patterns, and they are of interest to evolutionary biologists as a living fossil, retaining many ancestral rodent traits.