MORPHOSPACE ANALYSIS OF CRETACEOUS-PALEOGENE LEAVES IN ARGENTINE PATAGONIA REVEALS RAPID SHIFT IN MORPHOSPACE OCCUPATION, COOLING, AND STABLE BUT DIVERSE DANIAN ASSEMBLAGES

The response of Southern Hemisphere floras to the Cretaceous-Paleogene (K-Pg) bolide impact and mass extinction is poorly understood compared with the Northern Hemisphere. The vast majority of plant-bearing continental localities is reported in the Western Interior of North America (WINA), which is relatively close to the impact site on the Yucatán Peninsula in Mexico. New leaf collections from Chubut in Argentine Patagonia include 4,000+ specimens from well-dated sites in the latest Cretaceous (Maastrichtian) Lefipán Fm. (67-66 Ma) and earliest Paleogene (Danian) Salamanca and Peñas Coloradas fms (65.7-62.2 Ma), offering the opportunity to quantify macrofloral K-Pg turnover for the first time in the Southern Hemisphere. These collections reveal diverse floras, with 63 and 57 dicot leaf morphotypes for Maastrichtian and Danian floras, respectively. Palynological records of these sites indicate lower extinction and faster recovery of diversity following the K-Pg relative to WINA. However, the extensive leaf collections have not been compared previously. Here, we quantify their morphological diversity to track changes in morphospace occupation as a first step towards evaluating species turnover across the K-Pg. Each morphotype is described in a matrix of 55 discrete architectural characters and plotted using Principal Coordinates Analysis (PCoA). Results of PCoA show a significant change in morphospace occupation across the K-Pg, with Danian leaves occupying a greater area of the total space due to a higher morphological diversity. Similar to WINA floras, Argentine leaf assemblages show an increased proportion of toothed morphotypes across the boundary; from 38% to 62%. However, opposing the WINA trend, there is an increase in lobed morphotypes, from 12% to 32%. The significant shift in morphospace occurs within two m.y. at most, yet the Danian morphospace varies little for the three m.y. following the boundary. This suggests a period of rapid turnover from one rich flora to another across the K-Pg, with Danian floras remaining stable and homogeneous but much more diverse than those of the same age observed in WINA. The trend in leaf margins points to a net cooling between the Maastrichtian and Danian of Patagonia as in WINA; supporting a global cooling scenario.