We report a time-calibrated stratigraphic section in Colorado that contains unusually complete fossils of mammals, reptiles, and plants and elucidates the drivers and tempo of biotic recovery during the poorly known first million years after the Cretaceous–Paleogene mass extinction (KPgE). Within ~100 thousand years (ka) post-KPgE, mammalian taxonomic richness doubled, and maximum mammalian body mass increased to near pre-KPgE levels. A threefold increase in maximum mammalian body mass and dietary niche specialization occurred at ~300 ka post-KPgE, concomitant with increased megafloral standing species richness. The appearance of additional large mammals occurred by ~700 ka post-KPgE, coincident with the first appearance of Leguminosae (the bean family). These concurrent plant and mammal originations and body-mass shifts coincide with warming intervals, suggesting that climate influenced post-KPgE biotic recovery.

The Chicxulub bolide impact has been linked to a mass extinction of plants at the Cretaceous—Paleogene boundary (KPB; ∼66 Ma), but how this extinction affected plant ecological strategies remains understudied. Previous work in the Williston Basin, North Dakota, indicates that plants pursuing strategies with a slow return-on-investment of nutrients abruptly vanished after the KPB, consistent with a hypothesis of selection against evergreen species during the globally cold and dark impact winter that followed the bolide impact. To test whether this was a widespread pattern we studied 1,303 fossil leaves from KPB-spanning sediments in the Denver Basin, Colorado. We used the relationship between petiole width and leaf mass to estimate leaf dry mass per area (LMA), a leaf functional trait negatively correlated with rate of return-on-investment. We found no evidence for a shift in this leaf-economic trait across the KPB: LMA remained consistent in both its median and overall distribution from approximately 67 to 65 Ma. However, we did find spatio-temporal patterns in LMA, where fossil localities with low LMA occurred more frequently near the western margin of the basin. These western margin localities are proximal to the Colorado Front Range of the Rocky Mountains, where an orographically driven high precipitation regime is thought to have developed during the early Paleocene. Among these western Denver Basin localities, LMA and estimated mean annual precipitation were inversely correlated, a pattern consistent with observations of both fossil and extant plants. In the Denver Basin, local environmental conditions over time appeared to play a larger role in determining viable leaf-economic strategies than any potential global signal associated with the Chicxulub bolide impact.

PREMISE The Eocene–Oligocene transition (EOT; 34–33 Ma) was marked by global cooling and increased seasonality and aridity, leading to a shift in North American floras from subtropical forests to deciduous hardwood forests similar to today. This shift is well documented taxonomically and biogeographically, but its ecological nature is less known. METHODS Using the relationship between petiole cross‐sectional area and leaf mass, we estimated leaf dry mass per area (LMA), a functional trait tied to plant resource investment and expenditure, at 22 western North American sites spanning the EOT to determine how the broad restructuring of vegetation during this time was reflected in leaf economics. RESULTS There was no overall shift in LMA between pre‐EOT and post‐EOT floras; instead, changes in LMA across sites were mostly driven by a negative correlation with dry‐season precipitation and a positive correlation with paleoelevation. These patterns held for both whole sites and subsets of sites containing taxa with similar biogeographical histories (taxa that persisted in the highlands across the EOT or that migrated to the lowlands) and are consistent with most observations in extant floras. CONCLUSIONS Our data provide a geological context for understanding environmentally paced changes in leaf‐economic strategies, particularly linking leaf economic strategies to dry‐season precipitation and paleoelevation.

thank you. I would also like to thank Sabrina Koetter, Miles Brooks, and Shuo Wang for their work processing photographs. Thanks to Reid Perkins and Molly Wagner for their moral support and for embodying the live más lifestyle. Finally, thanks to Dr.