VEGETATION AND ENVIRONMENT CHANGE ACROSS THE K-PG BOUNDARY IN THE HELL CREEK OF MONTANA

The Cretaceous-Paleogene (K/Pg) boundary marks a major mass extinction resulting in global faunal turnover, notably the extinction of non-avian dinosaurs. The pattern of floral extinction at the K/Pg has been studied in areas such as North Dakota and New Zealand with results indicating an extinction rate as high as 80%. The Hell Creek and Fort Union Formations in northeastern Montana contain some of the most well studied vertebrate localities for this mass extinction, however very little is known of the floral record for eastern Montana. This has prevented meaningful tests of the role of plant communities in the observed faunal changes. The goal of this study is to use paleobotanical and vegetational data to investigate ecosystem evolution and climate change in the Hell Creek Area of Montana.

We outline initial results as well as proposed future work using sedimentological and paleobiological methods to decipher and interpret the signal of climate, vegetational restructuring, and floral turnover across the K-Pg boundary. Fieldwork to date has led to the collection of over 1000 leaf macrofossils across the latest Cretaceous and earliest Paleogene. These will be used to reconstruct temperature, environment, and vegetation through leaf physiognomy. Additionally, these data will be used to estimate plant investment (estimated based on leaf mass per area) and floral diversity through this period. We have also collected sediment samples from sections across the Hell Creek and lowest Fort Union Formations to analyze for phytolith and pollen content. Taken together, these lines of evidence will allow us to assess changes in environment and potential correlations between paleofaunal and paleofloral trends across this mass extinction.

Preliminary results indicate a high turnover of plant taxa at the boundary, with declining diversity leading to the K/Pg boundary and approaching pre-extinction levels of diversity by the upper Tullock. Flora remained dominated by angiosperms with a minor component of conifer taxa. Future analyses will aim to quantify diversity, ecology, and structure of these assemblages, and link these sites with the faunal record in the Hell Creek Area. These results will add to our knowledge of how mass extinctions affect ecosystems and the interplay between biotic turnover and environment.