Vegetation and Environmental Changes Across the Cretaceous-Paleogene (K-Pg) Boundary in Northeastern Montana

Thesis (Ph.D.)--University of Washington, 2022The Cretaceous-Paleogene (K/Pg) mass extinction was a pivotal event in Earth history; not only did this include the global extinction of non-avian dinosaurs but also the local disappearance of as much as 75% of vertebrate and invertebrate species. Our understanding of plants across the K/Pg is comparatively sparse, despite the central role of plants in shaping terrestrial ecosystems and as a food source. This gap in our understanding of plants across this significant time period motivates my study of plants from a ca. 2.3 Myr time interval around the K/Pg in northeastern Montana. Eleven megafloral localities are discussed here in terms of taxonomic affinities and diversity, morphologic diversity, climate implications, and ecological interpretation. This floral assemblage is also compared with contemporaneous fossil assemblages from around the globe. The earliest fossil assemblage in this study, Seafood Salad, is a particularly diverse, angiosperm-dominated flora, interpreted as a “pre-disaster” community from the Late Cretaceous. The taxa at Seafood Salad are commonly unique (15 of 34 taxa are unique to this study), but the flora broadly shows an affinity with floral assemblages from similar stratigraphic intervals in the region, indicating shared regional taxonomy. The K-Pg event in northeastern Montana culminated in the disappearance of 63% of latest Cretaceous taxa and a ~28% drop in richness. Despite the dramatic turnover at the K-Pg boundary, however, the recovery was relatively rapid; early Paleocene floras regained pre-mass extinction levels of richness within 80 to 900 kyr after the boundary. Some aspects of plant ecological diversity (the diversity of ecological strategies) remained restricted, however, and fast-return taxa continued to dominate the floras. Overall, the plant communities in northeastern Montana experienced a significant restructuring (diminishment of dominant functional groups) even though there was no significant loss of ecomorphological richness and taxonomic diversity rebounded relatively quickly. These results point to a pattern of plant community response during biotic crises: ecological changes and diversity loss, short-term taxonomic decline, but few to no major (family-level) taxonomic extinctions. Moreover, this work indicates global and regional heterogeneity in the extinction magnitude and timing of both extinction and recovery across the K-Pg boundary