A PRELIMINARY REPORT ON THE DIVERSITY OF LATEST CRETACEOUS FLORAS FROM NORTHERN PATAGONIA, ARGENTINA

Latest Cretaceous floras from the Southern Hemisphere in general, and South America in particular, are poorly understood, especially with regard to their response to K-T events. For the first time, an integrated and systematic approach to these floras and their paleoenvironmental/paleoclimatic context has been initiated in Patagonia. The present study is focused on the Lefipan Fm. in NW Chubut province. This unit is considered Maastrichtian in age based primarily on marine invertebrates, and it also contains a rich macro and microflora. The Lefipán Fm. represents a series of tidal dominated deltaic deposits of approximately 300 m thickness. We report new, unbiased collections of more than 500 compression-impression plant specimens that are concentrated in the lower and upper parts of the section. In the lower part, several localities yielded a diverse assemblage mainly composed of dicot leaves (including large aquatic Nelumbo leaves and fruits) and conifers. A younger assemblage occurs in the upper part of the section in two localities, associated with heterolithic facies in a mixed littoral deposit. This assemblage includes a highly diversified assemblage of dicot leaves with probably more than 70 species, as well as some monocots, podocarp conifers, and ferns. The dicots show a high ratio between entire/toothed foliar margins, suggesting warm climatic conditions. This preliminary report shows that latest Cretaceous floras were probably more diverse than, and had little taxonomic overlap with, those known from Patagonia during the Paleocene. This suggests that the K-T event was probably also catastrophic in the terrestrial ecosystems of southern latitudes and that recovery of floral diversity probably took most of the Paleocene until we see similar levels of plant richness by the early Eocene. Even though we are far from the quantity of data known from North America, a preliminary comparison suggests that approximately similar patterns of plant extinction and recovery took place in both hemispheres, from rich, highly diversified latest Cretaceous floras, through a relatively impoverished Paleocene, and back to high plant diversity by the early Eocene as a response to the global P/E thermal maximum.