PATAGONIA’S DIVERSE BUT HOMOGENEOUS EARLY PALEOCENE FORESTS

Early Paleocene macrofloras from the Southern Hemisphere are virtually unknown, despite their enormous potential significance for understanding plant evolution, biogeography, and global variation in recovery from the end-Cretaceous extinction. We here report 51 angiosperm leaf morphotypes from 3,522 specimens recovered from three distinct, early to late Danian time intervals (geomagnetic polarity chrons C29n, C28n, and C27n, respectively) of the Salamanca and Peñas Coloradas formations, San Jorge Basin, central Patagonia, Argentina. These assemblages were noted previously but with minimal descriptions and inadequate age constraints; they now comprise the first stratigraphically controlled and quantitatively collected sequence of early Paleocene macrofloras from the Southern Hemisphere. Preliminary botanical affinities of the morphotypes include Nothofagus, Fabaceae, and Menispermaceae, which are potentially the oldest macrofloral records of these lineages. Other families likely to be present include Arecaceae, Araceae, Cunoniaceae, Lauraceae, ?Malvaceae, Rosaceae, Sapindaceae, and Urticaceae (angiosperm reproductive organs from these sites, as well as ferns, araucarian and podocarpaceous conifers, insect body fossils and leaf-feeding traces, and feathers are under separate investigation). A striking result of this study is the general lack of change in floral composition or dominance through the Danian, even though the depositional environments shift from estuarine to continental. Like nearly all North American floras of this time, those from the San Jorge Basin of Patagonia appear to represent a homogenous, generalist, persistent vegetation type. However, the considerably higher richness of Danian floras from Patagonia, along with much other evidence from Southern Hemisphere continental environments, supports the idea of geographic variation in biotic response related to distance from the Chicxulub impact site and other potential factors.