By Phil Myers
This tiny family includes a single genus and species, restricted in distribution to the Pacific northwest of North America, from central California to southern British Colombia. Mountain beavers are large rodents, weighing up to about 1.5 kg. Their bodies are heavyset and covered with reddish or grayish brown fur. They have short tails and limbs, and both the forefeet and hindfeet have five digits. The digits on the forefeet are relatively long and capable of grasping. Eyes and ears are small.
The most notable aspect of the 
skulls of mountain beavers is the zygomatic region. The zygomatic plate is narrow and oriented horizontally, and the zygomatic arches spread widely. The infraorbital foramen is moderately large, and it transmits a small part of the masseter (the entire masseter arises from the zygomatic ). The arrangement of zygomatic plate and infraorbital foramen in mountain beavers is termed protrogomorphous and may represent the primitive condition for all rodents. The lower jaw is sciurognathous.
The skulls of mountain beavers are flattened in side view. In dorsal view, they are much widened posteriorly. Postorbital processes are lacking. Ventrally, the auditory bullae are flask-shaped. Anterior to the bullae, the palate ends posterior to the cheekteeth and seems broad. The coronoid process of the lower jaw is large and curved posteriorly. The dental formula is 1/1, 0/0, 2/1, 3/3 = 22; and the cheekteeth have an unmistakeable occlusal pattern. The cheekteeth are evergrowing.
Mountain beavers live in small colonies, occupying areas with plentiful green vegetation and cover. These colonies may in fact be concentrations due to limited appropriate habitat. Mountain beavers feed on a number of species of forbs, and on the bark of several species of trees. They dig complex burrows with many openings.
Their fossil record extends to the Miocene. An early offshoot of the Aplodontidae, the now-extinct Myagaulidae, were animals the size of woodchucks that had prominent horns on their rostra.
References and literature cited:
Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.
McLaughlin, C. A. 1984. Protrogomorph, sciuromorph, castorimorph, myomorph (geomyoid, anomaluroid, pedetoid, and ctenodactyloid) rodents. Pp. 267-288 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.
Paradiso, J. L. 1975. Walker's Mammals of the World, Third Edition. Johns Hopkins University Press, Baltimore.
Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.
Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.
Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.
Contributors
Phil Myers (author), Museum of Zoology, University of Michigan.
