THE QUEBRADA HONDA BASIN, SOUTHERN BOLIVIA: A WINDOW INTO THE LATE MIDDLE MIOCENE OF TROPICAL SOUTH AMERICA

The late Middle Miocene saw declining CO₂ levels and global temperatures following the Miocene Climatic Optimum (~17-14 Ma). Biotic change during this time has been documented primarily from mid-high latitude localities, whereas low-latitude sites of this age that preserve faunal or floral data remain extremely rare. An exception is the late Middle Miocene Quebrada Honda Basin (QHB) in southern Bolivia, which ranks among the best-characterized terrestrial vertebrate sites in South America, with >40 mainly mammal species documented. Work to date and describe the stratigraphy and paleosols of QHB sediments has suggested that the most fossiliferous intervals span ~13-12 Ma, making them in part coeval with the Monkey Beds at La Venta, Colombia (>13.03 Ma). Further, paleosols and ichnofossils from the lower part of these strata have pointed to deposition in a seasonal, sub-humid to semi-arid savanna with Mean Annual Precipitation of ~100 cm, consistent with results from ecological diversity analyses of QHB mammals. However, direct evidence for the local flora has so far been missing. Here, we present vegetation data based on 25 phytolith assemblages (11 well preserved, 14 of fair-poor preservation) extracted from several sections in the QHB. Principal Component Analysis of phytolith relative abundance data points to two main assemblage types: one with moderately abundant palms and other forest indicators and warm-adapted, presumably open-habitat PACMAD grasses, and the other typically dominated by grasses interpreted as closed-habitat grasses (e.g., bamboos). Assemblages rich in palm and open-habitat grass morphotypes are associated with laminated sandy layers, suggestive of more open, riparian vegetation. In contrast, assemblages of mainly closed-habitat grass phytoliths show less clear facies association, but potentially represent more terra firme plant communities. The variation seen through the section does not indicate major changes through time but, rather, small-scale spatial heterogeneity. Similarly, a chi-squared comparison of vertebrates of the different local areas within the QHB reveals no major taxonomic differences, but turtle and rodent abundances suggest microhabitat variation among areas.