SLOW BURN, BLAZING FINALE, OR SOMETHING ELSE? THREE-PHASE MODEL FOR DINOSAURIAN EXTINCTION IN THE NORTH AMERICAN WESTERN INTERIOR

Extinction scenarios concerning non-avian dinosaurs across the Cretaceous-Tertiary boundary have been polarized into two groups: 1) catastrophic extinction in which healthy ecosystems are almost instantly destroyed; and 2) gradualist extinction in which long-term (10 m.y.) "declines" in taxonomic diversity are noted before the final K-T event. Clearly, part of the problem lies in the definition of "catastrophic" and "gradual." If dinosaurian extinction was drawn out over 10s to 100s of k.y., it cannot realistically be considered "catastrophic." Similarly, pulsed extinction events that span a 10 m.y. interval cannot be considered in the framework of a gradual decline. The inability to resolve time in Upper Cretaceous sedimentary rocks at intervals lower than 100s of k.y., is an obstacle to discerning such an extinction from a catastrophic (apparent or actual) event. Combining paleontological, paleoclimatologic, stratigraphic, and sedimentological data, a pattern of linked, climate-induced extinction and pulsed-turnover events becomes apparent in non-avian dinosaur faunas of the North American Western Interior during the Campanian-Maastrichtian interval. The gradual global cooling that characterized the later Campanian (75-71.6 Ma) was punctuated by an abrupt drop in global temperature at about 73 Ma. This abrupt pulse of cooling may have been a result of the Manson Impact in eastern North America (74 Ma). Dinosaurian faunas of the Western Interior responded with a dramatic loss in taxonomic diversity of large to giant herbivores. Hadrosaurids decreased from 9-4 genera (56% loss) and ceratopsians from 10-5 genera (50% loss). Smaller, foraging taxa (ornithomimids, hypsilophodonts, and pachycephalosaurs) and carnivores (theropods) were not as clearly affected. Global warming at 70 Ma resulted in a second extinction amongst large-bodied herbivores, with only two genera of hadrosaurids and two or genera of ceratopsids surviving. Rapid global warming at 65.4 Ma followed by abrupt cooling at 65.2 Ma accompanied the crash in marine and terrestrial ecosystems, including the extinction of all non-avian dinosaurs. Similar patterns of climate-induced continental faunal turnover during the Campanian-Maastrichtian transition can be recognized in Asia, but nowhere else is the sequence of events so well-represented as in western North America.