FLORAL RECOVERY AFTER THE END-CRETACEOUS MASS EXTINCTION IN THE DENVER BASIN, COLORADO

The fossil flora of the Denver Basin, Colorado, records a history of extinction and recovery across the Cretaceous-Paleogene boundary (KPB; 66 Ma). More than any other KPB-spanning study area, the Denver Basin appears to preserve a heterogeneous floral response to the end-Cretaceous mass extinction event: compared to localities from the low-diversity swampy basin center, localities from the mountainous western margin of the basin are more diverse and sometimes contain leaves with physiognomies suggestive of a rainforest biome not found anywhere else in the early Paleocene of North America. This geographic heterogeneity of floral assemblage within the Denver Basin provides an excellent opportunity to identify and understand some of the nuances of how plants respond to mass extinction.

More than 30 previously collected Denver Basin localities, ranging in age from just before the mass extinction (~66.1 Ma) to the early Paleocene (~63.8 Ma), have been newly morphotyped into over 500 locality-level morphospecies following the protocol described in The Manual of Leaf Architecture (Ellis et al. 2009). We used these new morphospecies to make spatially and temporally constrained mean annual temperature and precipitation reconstructions across the Denver Basin. Additionally, locality-level morphospecies with high degrees of architectural similarity were linked across sites, allowing for basin-wide comparisons of species presence and absence during different slices of time following the mass extinction event. The architectural characters assigned to each morphospecies were also used to construct a high-resolution record of multivariate morphospace across the geographic and temporal range of the Denver Basin, which can illustrate differences in floral recovery that species-level analysis would miss. Combined with recent advances in the dating of Denver Basin sediments, the new Denver Basin leaf morphotype series enables a detailed and contextualized analysis of floral recovery after the KPB.