The best-known and most diverse mammalian faunas from the Fort Union Group of North Dakota are the Brisbane and Judson local faunas, both historically considered as mid Tiffanian (Ti3, mid-late Paleocene). Our review of the stratigraphic context of the localities provides no lithologic reasons for continued use of the terms Slope and Bullion Creek Formations, and we propose abandoning these terms in favor of the Tongue River Formation. Revised faunal lists were completed for this paper using taxonomic revisions published after the original 1978 study. The chronostratigraphic ranges of Brisbane and Judson taxa were then used to determine the biochronology of the faunas. The Brisbane local fauna is corroborated as being Ti3 based upon a combination of first appearance (Leptacodon tener, Thryptacodon australis, Dorraletes diminutivus) and last appearance (Pararyctes pattersoni, Ectocion collinus) data, together with taxa reported only from Ti3 faunas (Bisonalveus holtzmani, Plesiadapis rex, Aletodon quadravus). The Judson local fauna is reinterpreted as being late Tiffanian (Ti4) on the basis of the presence of Plesiadapis sp. cf. P. churchilli, or more importantly, the absence of P. rex. This reinterpretation of the age of the fauna means that Prochetodon foxi and Neoplagiaulax nanophus are restricted to the Ti4 biochron and Litocherus zygeus and Protictis paralus occur in the Ti3 and Ti4 biochrons.