Macrofloral extinction and recovery dynamics across the Cretaceous-Paleogene transition in Argentine Patagonia support a globally heterogeneous floral response

STILES, ELENA; WILF PETER; ARI IGLESIAS; GANDOLFO, MARÍA A.; CÚNEO JOHNSON WILF SCASSO GANDOLFO IGLESIAS

Tukson

Congreso; Botanical Society of America Anual Meeting; 2019

Botanical Society of America

The Cretaceous-Paleogene (K-Pg) mass extinction appears to have been globally heterogeneous for someorganismal groups. For example, Southern Hemisphere data from calcareous nannoplankton, palynomorphs, andinsect feeding-damage suggest that extinction rates were lower and recovery faster than in the NorthernHemisphere. Explanatory hypotheses involve the bolide?s impact angle, greater distance of fossil sites from theMexican impact crater, and oceanic buffering of the impact winter in the Southern Hemisphere. In the SouthernHemisphere, the severity of the floral extinction remains poorly understood because few plant-bearing sites arerecognized, and no K-Pg macrofloral turnover studies exist yet. Previous studies on stratigraphically constrainedlatest Cretaceous (Maastrichtian) and early Paleocene (Danian) localities in Chubut, Argentine Patagonia, showlower palynological extinction rates and faster insect-feeding damage diversity recovery than in the well-studiedNorth American (NAM) sections. Here, we address the macrofloral extinction dynamics for these Patagonian sites.We studied macrofloras from Maastrichtian strata of the Lefipán Formation (67-66 Ma) and the Danian Salamancaand Peñas Coloradas formations (ranging 65.7-62.2 Ma), spanning about 400 km along the paleocoastline. A K-Pgmacrofloral species extinction of >90% was estimated based on 63 Maastrichtian and 49 Danian dicot leafmorphotypes, contrasting sharply with the previously estimated <12% Lefipán angiosperm pollen extinction.Consistent with extinction, rarefaction analysis showed a decrease in diversity across the K-Pg, and rich Maastrichtian tidal flat deposit floras were replaced by lower-diversity and compositionally homogeneous Danian assemblages that persisted from marginal to fully terrestrial facies. However, morphospace analysis of Maastrichtian and Danian leaf assemblages showed significant overlap and higher post-boundary morphological diversity, indicating continuity and expansion of leaf types and their underlying ecological and phylogenetic diversity. Furthermore, we found that all recognized Maastrichtian families persisted in the Danian. Leaf margin analysis indicates cooling of ca. 5°C between the Maastrichtian and Danian, consistent with global proxy data and indicating a separate cause of turnover biasing against finding more K-Pg survivors. Published Danian Lefipán and Salamanca palynomorph data showed geographic heterogeneity in the species composition along the coastline, indicating a spatial bias against finding higher macrofloral survivorship. In summary, despite temporal and spatial biases favoring an overestimated macrofloral extinction, the significant drop in rarefied species diversity and remarkably homogeneous Danian assemblages are indicators of extinction broadly similar to NAM. However, the evidence for phylogenetic and ecological floral continuity, coupled with an increase in morphological disparity, are notable characteristics of Patagonian samples that appear to differ from NAM.