Whereas aplodontiid rodents are represented today by a single living species, their fossil record is diverse in number of species (>100), in morphology, and in ecology. Recent phylogenetic efforts are beginning to make sense of the relationships among diverse members of the clade, but there is still much to learn about the relationships within the morphologically coherent subfamilies of this large group. The subfamily Aplodontiinae, including the extant Aplodontia, ranges from the mid-Miocene through recent, including only eight described species. This is the least diverse clade of aplodontiids, even though it is the only extant group. This study describes three new species, increasing the diversity of described fossil aplodontiids by more than a third. The description herein of Liodontia bathypotamos, n. sp., from the late Hemingfordian of Montana, Liodontia dailyi, n. sp., from the late Hemingfordian of Nevada, and Aplodontia minor, n. sp., from the late Hemphillian of Oregon extends the geographic and temporal range of the subfamily. A phylogenetic analysis of all known aplodontiines finds support for slow diversification and possible anagenetic change within lineages while also suggesting substantial missing time in the aplodontiine fossil record. However, the morphological changes, including rapid increases in hypsodonty, along the aplodontiine lineage yield an extant species, Aplodontia rufa, that, despite its early divergence from the rest of crown group Rodentia, is not at all representative of the ‘primitive’ rodent it is commonly assumed to be.