A quantitative assessment of evolutionary and ecological change in mammalian faunas leading up to and across the Cretaceous-Tertiary boundary in northeastern Montana

The Cretaceous-Tertiary (K-T) mass extinction and subsequent biotic recovery represent a key interval in earth history. Paleontological and geological records from terrigenous deposits of the Hell Creek and Tullock Formations of Garfield County, Montana offer an opportunity to address evolutionary and stratigraphic questions regarding this interval.; Fieldwork was undertaken in the lower part of the Hell Creek Formation in the study area that led to the recovery of over 450 mammalian fossils from 26 new localities. These collections and existing collections housed in the University of California Museum of Paleontology were assembled into an extensive fossil database that broadly samples the Hell Creek Formation. The stratigraphic section was correlated to the global record through revisions and additions to the existing chronostratigraphic framework that extend it two million years back into the latest Cretaceous.; The pattern and mechanism of the K-T mass extinction in the study area were examined using this fossil database and chronostratigraphic framework. Results show changes in relative abundances, mean individual body size, and taxonomic composition of mammalian faunas that are interpreted as "normal" responses to latest Cretaceous climate change. The extinction of 22--27 mammalian species at or near the K-T boundary cannot be explained by climate change alone. The fossil data are consistent with either a non-linear response pattern for the extinction, resulting from the accumulated stress of multiple long- and short-term environmental perturbations, or a single, short-term cause (an extraterrestrial impact).; Most workers suggest that following the K-T boundary, eutherian mammals rapidly invaded a wide array of body sizes and ecological specializations, but few studies have directly quantified the pattern of morphological change or tested causal hypotheses. Results from geometric morphometric analyses on dental specimens from latest Cretaceous and earliest Paleocene mammalian faunas suggest that the ecological expansion of eutherians in the Paleocene had a slower start than generally perceived and that multiple factors, including ecological release from non-avian dinosaurs, related changes to the habitat structure, and immigration from other areas, likely triggered this key episode in eutherian evolution. Additional independent studies will determine whether these findings apply more broadly to areas beyond northeastern Montana.