"Some scientists think the Mountain Beaver is the world's most primitive living rodent, similar in appearance and behavior to animals that lived 60 million years ago. They have small eyes and ears and luxurious whiskers, and are, like many other burrowing rodents, armed with good senses of smell and touch. They rely much less on sight and hearing. They eat plants, including bark, and are able to feed on species such as rhododendron and stinging nettle, which are toxic or noxious to many other mammals. Mountain Beavers are found in coniferous forest at all elevations. Although their geographic distribution is limited, they are common within their range. They are not closely related to water-dwelling beavers (genus Castor), although both are rodents."

Adaptation: The many similarities in the form of the skull of an early rodent from the Paleocene, Paramys, and the living Mountain Beaver, Aplodontia, suggest roughly similar functional patterns of foraging and feeding in the two. 

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account