Cretaceous mammals have been recovered from the lowermost Kirtland Shale (Quarry 1 of the Fossil Forest study area). Of the 13 therian mammal taxa described, 6 are new (Alphadon parapraesagus, n. sp., Ectocentrocristus foxi n. gen. and sp., Pediomys fassetti n. sp., Aquiladelphis paraminor, n. sp., Gypsonictops clemensi n. sp., and Cimolestes lucasi n. sp.). Faunal comparison with other known Cretaceous sites show that Fossil Forest mammals are most closely related to faunas described by Sahni (1972) from the Judith River Formation and by Fox (1977, 1979a, 1979b, 1979c, and 1981) from the Oldman Formation (Judith River equivalent). The age of the lowermost part of the Kirtland Shale must be near the Campanian-Maastrichtian boundary based on mammalian evidence. The long normal magnetochrons (30 and 31 of Lindsay and others [1981]) must, therefore, be renumbered. Two alternatives for renumbering are discussed; the fossil mammal evidence favors renumbering the magnetochrons as anomaly 33. This interpretation requires that one or more unconformities be present between the lower part of the Kirtland Shale and overlying formations on the south flank of the San Juan Basin. This results in the loss of approximately 6.4 Ma of rock record that probably includes the Cretaceous-Tertiary boundary event.