A new genus and three new species of early aplodontoid rodent are described from the Orellan (early Oligocene) of Montana, Brachygaulus nicholsi (type species), Brachygaulus leistneri, and Brachygaulus xerobothrus. A fourth indeterminate species is also recognized but not named. The occlusal dental morphology of Brachygaulus has a number of similarities to that of the earliest mylagaulids, the Promylagaulinae (early Arikareean: late Oligocene). However, the brachydont cheek teeth of Brachygaulus are more similar to primitive contemporaneous basal aplodontoids (=Prosciurinae). Brachygaulus represents an ancestral morphotype for the Mylagaulidae, demonstrating that the family can be derived from a prosciurine-like ancestor independent of the Meniscomyinae.