The mountain beaver is considered to be a living fossil, due to the presence of a host of primitive characteristics, particularly the protrogomorphous zygomasseteric system.

[citation needed] In the protrogomorphous condition, the medial masseter muscle does not pass through the infraorbital foramen as it does in guinea pigs and mice.

Likewise, the lateral masseter muscle attaches to the base of the zygomatic arch and does not extend to the region in front of the eye as is seen in squirrels and mice.

[5] The mountain beaver is the only living rodent with this primitive cranial and muscular feature (except perhaps the blesmols, which clearly evolved protrogomorphy from a hystricomorphous ancestor).

[citation needed] According to the fossil record, the Aplodontiidae split from the squirrels in the Middle or Late Eocene as indicated by the extinct genera Spurimus and Prosciurus.

Their superficial similarity with true beavers reflects only their relatively large size (for rodents), strong odor, preference for living in extremely watery / moist habitats, and propensity to gnaw bark and cut branches.

[6] They are known to climb trees a few meters to acquire food in the form of branches and leaves, but otherwise their diet consists mostly of ferns, especially species that are toxic to other animals.

[4] Mountain beavers have an unusual projection on each molar and premolar tooth, which is unique among mammals and allows for easy identification of teeth.

Two upper and one lower premolars are present, along with all the molars, giving a dental formula of 1.0.2.31.0.1.3[10] Mountain beavers cannot produce concentrated urine.

They are thought to be physiologically restricted to the temperate rain forest regions of the North American Pacific coast and moist microenvironments inland due to their inability to obtain sufficient water in more arid environments.

The entrances to these burrows often contain clumps of wilted vegetation which the animal likely uses as a kind of food cache as well as a source of nesting material.

[7] They appear to build hay mounds at some burrow entrances, but whether this behavior is related to water regulation, curing food, or gathering nest materials is debated.

[19] Mountain beavers are considered pests in areas of the Pacific Northwest because of the extensive damage they cause to forest trees due to basal-grinding (removal of bark), branch cutting, or clipping small sapling and seedlings.

[11] This damage often poses a problem for forest management and reforestation when smaller seedlings are buried or uprooted during mountain beaver feeding and can result in tree deformities, growth suppression, and mortality.

[15] While mountain beavers are generally solitary creatures, they may form groups due to suitable habitat and food resources being in a small area; population densities vary widely because of this.

[24] Originally listed in 1991, the Point Arena mountain beaver is distinguished by its black colouration and by characteristic body proportions, including a smaller overall size.

[25] The principal threat to this subspecies is habitat loss and fragmentation; its remaining range comprises a disjunct region of 24 square miles (62 km2) in western Mendocino County.