With the discovery and description of the Bug Creek faunas in 1965, it became necessary to reexamine the sequence of mammal ages (Lancian-Puercan) spanning the Cretaceous-Tertiary boundary. Bug Creek faunal assemblages have been viewed as being in part coeval with the Lancian assemblages or slightly younger. Because of the lack of Lancian sites above the Bug Creek–type sites and the discontinuous nature of the sediments preserving the latter type faunas, it appears that the Bug Creek faunas postdate Lancian faunas. Although the Bug Creek faunas cannot be well constrained biostratigraphically, the appearance of a number of new taxa and the...