Latest Cretaceous, Paleocene and Eocene floras from Patagonia: a critical piece of Gondwana’s floral history comes into focus

IGLESIAS ARI; WILF PETER; CÚNEO RUBEN; GANDOLFO M. ALEJANDRA; M.C. ZAMALOA; KIRK JOHNSON; RUDY SLINGERLAND

La Plata

Congreso; X Congreso Argentino de Paleontología y Bioestratigrafía y VII Congreso Latinoamericano de Paleontología; 2010

Universidad de La Plata

Patagonia contains a rich and fairly continuous paleobotanical record from the late Maastrichtian to the middleEocene, an interval known globally for mass extinction and significant climate changes. From this time interval, wehave quantitatively collected and stratigraphically studied over 600 species and 20000 specimens from eight wellpreservedcompression macrofloras from north-west and central Patagonia. Cretaceous and early Paleocene florasare from coastal environments, whereas the Eocene floras are from volcaniclastic lake sediments. Several sites andstratigraphic levels in the early Paleocene Salamanca Formation show higher richness than comparable NorthernHemisphere floras, but they are relatively impoverished when compared to those of the upper Maastrichtian LefipanFormation. Probable middle Paleocene floras from the Peñas Coloradas Formation share most species with theSalamanca Formation, but are preserved in fluvial facies. Bedded tuffs at the Eocene sites have yielded precise 40Ar-39Ar dates of 54.24 ±0.45 Ma for the Pampa de Jones flora, 51.91 ±0.22 Ma for the extremely diverse Laguna del Huncoflora, and 47.46 ±0.05 Ma for the similarly rich Río Pichileufú flora. The history of Patagonian plant diversity in theearly Paleogene could be linked to a relatively lower effect of the terminal Cretaceous event and high diversificationand immigration during Eocene warming. Although the biogeographic signature of the floras is clearly Gondwanic,during the Paleocene there appears to be greater compositional distinctiveness from coeval Australasian floras thanin the Eocene, suggesting some degree of endemism after the K-T. The Eocene floras include beautifully preserved examples of extant Australasian tree genera that are now extinct in South America, including Agathis, Papuacedrus,Dacrycarpus, Eucalyptus, Cassuarinaeceae, and Akaniaceae. Paleoclimate analyses based on the macrofloras indicatesubtropical and moist growing conditions; during the Eocene there is evidence for true rainforest conditions. Theseresults contribute a considerably improved framework to understanding the plant evolution in Gondwana.