NO GLOBAL SHIFT IN LEAF MASS PER AREA ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY

For plants, the end-Cretaceous mass extinction (~66 Ma), linked to the devastating effects of the Chicxulub bolide impact, was a moment of immense species loss. How this loss in species richness was tied to selectivity, and whether it ultimately led to a shift in plant ecological strategies, is understudied. Previous work found that in the Williston Basin, North Dakota, plants no longer pursued strategies with a slow return-on-investment of nutrients immediately after the Cretaceous-Paleogene boundary (KPB). To test whether this was a widespread response, we studied fossil leaves from the KPB spanning Denver Basin, Colorado, where we used the relationship between petiole width and leaf mass to estimate leaf dry mass per area (LMA), a leaf functional trait which negatively correlates with the rate of return-on-investment.

We did not observe a loss of high LMA species after the KPB: the expectation if plants stopped following slow return-on-investment strategies. Indeed, we found no evidence for a shift in leaf economic strategies across the KPB: neither the median species LMA nor the distribution of species in LMA-space appreciably changed for over two million years straddling the KPB. Around 64 Ma, LMA declined at sites interpreted as rainforests near the western margin of the basin; similarly low values are commonly observed in present-day rainforests. The western margin localities where we observed this downward shift are found close to the Front Range of the Rocky Mountains, where an orographic precipitation regime is thought to have developed during the deposition of the Denver Basin. We found that among these Front Range adjacent localities, LMA and mean annual precipitation were inversely correlated, a pattern consistent with observations of extant plants.

Overall, in the Denver Basin we found that local climate and biome type played a larger role in determining leaf ecological strategies than a global signal associated with the Chicxulub bolide impact.