zygomasseteric system), or Sciurida, depending on the author. According to the fossil record, the Aplodontioidea split from the squirrels in the Middle or Late Eocene as indicated by the extinct genera Spurimus and Prosciurus. The fossil record for the genus Aplodontia extends only to the Late Pleistocene of North America. Closest relatives 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 zygomasseteric system), or Sciurida, depending on the author. According to the fossil record, the Aplodontioidea split from the squirrels in the Middle or Late Eocene as indicated by the extinct genera Spurimus and Prosciurus. The fossil record for the genus Aplodontia extends only to the Late Pleistocene of North America. Subspecies At present, seven subspecies of Aplodontia rufa are recognized: A. r. californica (Peters, 1864): distributed throughout the Sierra Nevada in Northern California and extreme western Nevada A. r. humboldtiana Taylor, 1916: restricted to the far northwestern coast of California A. r. nigra Taylor, 1914: restricted to a small region in southern Mendocino County, California A. r. pacifica Merriam, 1899: distributed across coastal Oregon A. r. phaea Merriam, 1899: found mostly in Point Reyes, northwest of San Francisco, California A. r. rainieri Merriam, 1899: found across the Cascade Range from southern British Columbia to northern California A. r. rufa (Rafinesque, 1817): found along