ADVANCES IN THE PALEOGENE PALEOBOTANY OF PATAGONIA, ARGENTINA

JOHNSON WILF CÚNEO GANDOLFO IGLESIAS GONZÁLEZ LABANDEIRA

Gainesville, Florida

Conferencia; . Advances in Paleobotany Conference recognizing David Dilcher and Jack Wolfe; 2006

The utility of Cretaceous and Paleogene megafloras for assessing floral diversity and paleoclimate has been greatly enhanced by employing large sample sizes from quantitatively collected bench quarries in a geochronologically-constrained stratigraphic framework. Beginning in 1997, we have been recollecting known floras and discovering new ones from Paleogene strata of Chubut and Río Negro Provinces with the intent of building an understanding of southern mid-latitude terrestrial paleoclimate and biodiversity. In order of decreasing age, the floras collected to date include the early Paleocene (Danian, ~62 Ma) Palacio de los Loros flora and associated sites from the Salamanca Formation near Sarmiento; the early Eocene (~52 Ma) Laguna del Hunco flora from a volcanic lake sequence in the Río Chubut valley of Central Chubut; and early Eocene Pampa de Jones and middle Eocene (~47.5 Ma) Río Pichileufú floras from the Chubut-Neuquén borderlands near San Carlos de Bariloche. Our approach has been to prospect for and extensively quarry sites that preserve abundant megafossils. Each of the quarries is collected in a manner that accounts for all fossils and thus represents an unbiased sample of the buried flora. The Salamanca floras occur in clay plug deposits of a abandoned channels and in channel sands while all of the younger sites occur in tuffaceous lacustrine sediments. We have collected more than 1000 identifiable specimens from each locality except Pampa de Jones. All floras are dominated by angiosperms but all also contain a distinctive conifer element. Both the angiosperms and the conifers show a strong Gondwanic affinity, particularly to extant humid tropical and warm temperate floras of Australasia. Examples include Araucaria section Eutacta, Acmopyle, Dacrycarpus, and cf. Papuacedrus in the conifer group, and Akania, Gymnostoma, diverse Cunoniaceae, and possible Eucalyptus in the angiosperms. Rarefaction analyses of the floras shows that the Eocene floras are significantly more diverse than the Paleocene ones, a pattern that mimics what is known from identically collected, comparable, well-sampled floras in North America, and that both Paleocene and Eocene Patagonian floras are much more diverse than their North American counterparts. We recognize almost 200 plant organ morphotypes from the Laguna del Hunco flora alone. Moreover, the Patagonian floras bear extraordinarily diverse insect damage, and at Laguna del Hunco the diversity of insect damage is the highest we have observed for the Cenozoic. These results show an ancient history of high diversity in thermophilic South American floras and their associations with insects, and they raise the interesting possibility that the end-Cretaceous event was less severe in Patagonia, far from Chicxulub, than in the Western Interior U.S. Collectively these floras begin to define the floral composition and taxonomic and ecological diversity of the Patagonian Paleogene and set the stage for planned exploration into the poorly sampled Late Cretaceous rocks of Patagonia.