The mountain beaver (Aplodontia rufa) is a North American rodent. It is the only living member of its genus, Aplodontia, and family, Aplodontiidae. It should not be confused with true North American and Eurasian beavers, to which it is not closely related, being more closely related to squirrels. There are 7 subspecies of mountain beavers, 6 of which are found in California and 3 of which are endemic to the state. Spelling and etymology Most references use the spelling Aplodontidae for the family name. This has been deemed incorrect due to the technical rules of converting a genus name into a family name. The proper conversion of Aplodontia to a family name is to drop the -a only and add -. Thus, Aplodontiidae is technically correct. This spelling is gaining acceptance in modern texts and is the standard spelling currently recognized by the Integrated Taxonomic Information System. Alternate spellings of the genus name have also been reported, with as many as 30 variants historically. These include Haplodontia, Haplodon, Aploodontia, Apluodontia, and Aplodontie, among others. The name Aplodontia ("simple tooth") is in reference to the single large basin comprising the bulk of each cheek tooth. The specific epithet, rufa means red or reddish. Taxonomy and phylogeny The mountain beaver is considered a living fossil, due to the presence of a host of primitive characteristics, particularly the protrogomorphous zygomasseteric system. This condition is similar to what is found in most mammal groups, such as rabbits, where no extreme specialization of the masseter muscle has evolved. In the protrogomorphous condition, the masseter muscle does not pass through the infraorbital foramen as it does in guinea pigs and mice. Likewise, the medial 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. 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). The mountain beaver was once thought to be related to the earliest protrogomorphous rodents, such as the ischyromyids (Paramys). Both molecular and morphological phylogeneticists have recently suggested a more distant relationship to these animals. Molecular results have consistently produced a sister relationship between the mountain beaver and the squirrels (family Sciuridae). This clade is referred to as Sciuroidea, Sciuromorpha (not to be confused with the sciuromorphous