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 continuation of these or closely related taxa into the Puercan is strong evidence for a biochronologic sequence of Lancian–Bug Creek–Puercan. When the faunal contents of Lancian, Bug Creek, and earliest Puercan (Pu 1 ) sites are scrutinized, it is clear that important faunal introductions occur with the commencement of Bug Creek assemblages. In contrast, between Bug Creek and earliest Puercan (Pu 1 ) assemblages there are very few major faunal introductions and thus more faunal continuity. Therefore, the definition and concept of the Puercan Land Mammal Age is modified, and the Bug Creek faunas are formally defined and characterized as the Protungulatum/Peradectes Interval-Zone (Pu 0 ) of the Puercan Land Mammal Age. The Protungulatum/Peradectes Interval-Zone (Pu 0 ) postdates the (latest Cretaceous) Lancian Land Mammal Age and commences the (latest Cretaceous?–early Paleocene) Puercan Land Mammal Age. This interval-zone is defined as including faunas that occur during the time between the first appearance of the arctocyonid ungulate Protungulatum and the first appearance of the didelphid marsupial Peradectes. Certain biochronological criteria (first appearances and “cladochronology”) within this interval-zone strengthen the view that the original sequence (from oldest to youngest) of Bug Creek Anthills, Bug Creek West, and Harbicht Hill is correct. Therefore, three informal biochrons are recognized, the Protungulatum/Mimatuta (bk 1 ), Mimatuta/Oxyprimus (bk 2 ), and Oxyprimus/Peradectes (bk 3 ) biochrons. Dinosaurs and Lancian mammals are found at all Pu 0 localities. Although the possibility of reworking cannot be completely dismissed, the abundance of Lancian mammals and the nature of dinosaur material at certain Pu 0 sites suggest some temporal overlap. Similarly, the stratigraphic placement of palynological change and an iridium anomaly relative to Pu 0 faunas remains equivocal.