Plants and their dates tell the animals' west side story (Early Paleogene of Patagonia, Argentina, West Gondwana)

WILF PETER; VIVIANA BARREDA; CLYDE WILL; CÚNEO RUBEN; GANDOLFO M. ALEJANDRA; ELIZABETH HERMSEN; ARI IGLESIAS

Gainsville

Congreso; 10° North American Paleontological Convention; 2014

North American Paleontological Society

We synthesize recent advances from integrated paleobotany and geochronology that have high relevance for understanding Patagonia's important early Paleogene vertebrate faunas. Palynofloral data from the Lefipán Formation (Chubut) show a rapid recovery of coastal ecosystems across the Cretaceous-Paleogene boundary, supporting faunal data that show a large number of surviving vertebrate groups. The correspondingly rich coastal-forest macrofloras from the upper Salamanca Formation (Chubut) can now be assigned to C28n (64.67-63.49 Ma), and the laterally exposed Peligran vertebrates, which include a monotreme, are presumably nearly coeval. The overlying Peñas Coloradas Formation has a tuff with a U-Pb age of 61.984 +/- 0.041 Ma, which dates the formation's macrofloras and provides a working age for its "<i>Carodnia<i>" zone vertebrates. From the Eocene, the Pampa de Jones site (Neuquén) and its plants and ontogenetic series of pipid frogs have an Ar-Ar date of 54.56 +/- 0.45 Ma. The early Eocene Laguna del Hunco (LH, Chubut) and early middle Eocene Río Pichileufú (RP, Río Negro) sites feature outstandingly diverse and well-preserved plants, frogs, and fish in volcanic fossil-lake beds, bracketing in time the nearby Paso del Sapo mammalian faunas (∼49?47 Ma), which have several Antarctic links. The fossiliferous section at LH is constrained by two paleomagnetic reversals and three Ar-Ar dated tuffs. The most precise age, on sanidine, is 52.22 +/- 0.22 Ma, the time of the Early Eocene Climatic Optimum and the maximum presumed potential for trans-Antarctic linkage to Australia. At RP, the fossil horizons are closely associated with three Ar-Ar dated tuffs with a combined age of 47.74 ± 0.05 Ma, placing the biota at the earliest phases of Antarctic separation. The LH and RP floras are both outstanding for having large numbers of plant genera that are extant and often in association at great distance in Australasia and/or southeast Asia, but that are extinct in South America today. Several of the taxa indicate dispersal or pollination syndromes with vertebrates. Strikingly similar modern assemblages can be found in lower montane rainforests of the Australian subtropics and wet tropics, New Guinea, and Borneo. Many of the plant genera also have fossil records from the Paleogene of southern Australia, showing that they were part of a vast, now fragmented and relocated trans-Antarctic rainforest biome. The RP flora is already lacking several of the trans-Antarctic elements found in the older LH floras. South American Paleogene mammals were classically considered "isolated," and Australian faunas lack sufficient resolution to make firm comparisons. However, the trans-Antarctic history of Eocene rainforest plants strongly suggests a non-isolated model for the mammals, when also considering Antarctic links of the Paso del Sapo mammals and the many non-mammalian fossil vertebrates from Patagonia that have Australasian affinities.