Patagonia’s diverse but homogeneous early paleocene forests: Angiosperm leaves from the Danian Salamanca and Peñas Coloradas formations, San Jorge Basin, Chubut, Argentina

Abstract

Early Paleocene macrofloras from the Southern Hemisphere are little known, despite their significance for understanding plant evolution, biogeography, and global variation in recovery after the end-Cretaceous extinction. As a foundation for systematic and paleoecological work, we describe 51 angiosperm leaf morphotypes from three distinct, precisely dated early to late Danian time intervals, using collections from the Salamanca and Peñas Coloradas formations in the San Jorge Basin, Chubut Province, Patagonia, Argentina. These rich floras were previously analyzed but with minimal descriptions. The assemblages comprise the first stratigraphically controlled and quantitatively collected floras for the early Paleocene of the Southern Hemisphere. Botanical affinities of the angiosperm morphotypes are not formally assigned here, but we informally associate some of them with families including Arecaceae, Fabaceae, Cunoniaceae, Lauraceae, Nothofagaceae, Rhamnaceae, and Rosaceae; in addition, leaves of Menispermaceae and other Rhamnaceae were formally described in previous work. Other families potentially present in these assemblages include Akaniaceae, Anacardiaceae, Apiaceae, Araceae, Bixaceae, Juglandaceae, Malvaceae, Sapindaceae, and Urticaceae. Remarkably, there is little floral turnover or change in dominance through the Danian floral sequence spanned by the studied localities, even among estuarine vs. continental depositional environments. This finding indicates a homogeneous, generalist, long-lived floral association following the K-Pg extinction, similar in these respects to many North American Danian floras. However, the richness of the Danian Patagonian floras, from paleolatitudes >50 degrees South, along with other lines of evidence from the region, suggests differences in the response of terrestrial ecosystems in southern South America to the terminal Cretaceous event from those of the Northern Hemisphere. The flora appears to be largely paleo-endemic in nature and shows several compositional links to the Eocene floras of Patagonia, emphasizing the importance of diversification within Patagonia after the end-Cretaceous event as a factor leading to the hyperdiverse Eocene regional floras.