Holocene paleoagrostological impressions from the Eastern Chaco Region (Argentina)

The Chaco region is a sedimentary basin limited by the Andes and the Brazilian craton. This South American subtropical plain is defined as large alluvial fan sediments of the main rivers that were repeatedly deposited during the late Quaternary under diverse climatic conditions. The Eastern Chaco is characterized by semideciduous forests intermingled with extensive palm-savannas, with gallery forests being the common vegetation along the banks of the main rivers and their western tributaries. These vegetation patterns of the Chaco have been proposed to be the result of the Pleistocene-Holocene alluvial dynamics of the plain as well as of the intensive migration of fluvial belts of the main rivers. Although no data are available on the paleoflora of the Chaco plain, plant fossil impressions are found in the banks of the low Bermejo River, which flows along the Eastern Chaco phytogeographic region. Here, we analyze paleoagrostological impressions recovered from two fossiliferous localities in order to provide insights into the environmental history of the Chaco region. Sixty complete Poaceae spikelet impressions were studied morphologically and compared with extant Chaco grass species. For each fossil spikelet, 11 morphological characters were described and statistically analyzed. All spikelets were found to belong to the Panicoideae subfamily, and some have consistent morphological features with this subfamily; hence, we assign them to the extant genera Cenchrus, Echinochloa, Hymenachne, and Paspalum. These impressions are the first floral remains found in the Eastern Chaco and collectively suggest that environmental and climatic conditions from these sediments corresponded to a subtropical humid climate and that vegetation was similar to the extant Eastern Chaco flora. (C) 2015 Elsevier B.V. All rights reserved.