The record of dinosaurs over the last 10 m.y. of the Cretaceous, as well as surrounding the Cretaceous-Paleogene boundary, helps to define extinction scenarios. Although Late Cretaceous dinosaur fossils occur on all present-day continents, only in North America do we find a terrestrial vertebrate fossil record spanning the Cretaceous-Paleogene boundary, although promising work may yield comparable records in South America, India, China, and Europe. For the present then, the North American record represents the proxy for our knowledge of dinosaur extinction. Over the last 10 m.y. of the Cretaceous (late Campanian to late Maastrichtian) in the northern part of the western interior of North America, the number of nonavian dinosaur species dropped from 49 to 25, almost a 50% reduction, even though a 16% greater extent of fossil-bearing exposures record the last dinosaurs in the latest Cretaceous in the western interior. Important, but less-well-exposed, nonavian-dinosaur–bearing units suggest this drop occurred around, or at least commenced by, the Campanian-Maastrichtian boundary. These losses began during climatic fluctuations, occurring during and possibly in part caused by the last major regressive cycle of the Cretaceous, which also reduced the expanse of the low coastal plains inhabited by nonavian dinosaurs. The pulse of Deccan Trap emplacement that began some time later in the latest Cretaceous was also likely a major driver of climatic change. As for the dinosaur record near the Cretaceous-Paleogene boundary, even the best-known records from North America remain enigmatic and open to interpretation. Newer studies suggest some decline in at least relative abundance approaching the Cretaceous-Paleogene boundary, but the cause (or causes) for the final extinction (if it was the case) of non-avian dinosaurs remains unresolved, although the Chicxulub impact undoubtedly played a major role.

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This chapter documents survivorship across the Cretaceous/Tertiary (K/T) (and Lancian/Puercan) boundary for all 111 species of nonmarine vertebrates from Garfield and McCone counties, Montana. Species-level survivorship appears to be between about 53 and 64 percent after the artifacts caused by paleobiogeography, rarity of some species, and differing evolutionary rates are taken into account. Without correcting for these artifacts, survival is an artificially low 32 percent. If specimens from the Bug Creek interval are early Paleocene (Puercan) in age, survivorship is an almost certainly incorrectly high value of 82 percent. This is because an undetermined number of latest Cretaceous (Lancian) species in the Bug Creek interval probably were reworked from Cretaceous sediments and were extinct by Bug Creek time. Comparison of the Lancian/Puercan transition with the older Judithian/Lancian and younger Puercan/Torrejonian transitions suggests percentage survival is similar among the three (55, 48, and 58 percent, respectively) after the Lazarus effect is considered. These results are not easily explained by a catastrophic mass-extinction scenario for the K/T transition, at least for nonmarine vertebrates. Rather, a geologically rapid but noncatastrophic change, such as the loss of range and habitat diversity during the Late Cretaceous marine regression, is commensurate with the analysis.

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.