The flora, fauna, and paleoenvironment of the late Middle Miocene Quebrada Honda Basin, Bolivia (Eastern Cordillera, Central Andes)

Abstract

Miocene ecosystem change in the Central Andes is not well understood because of a dearth of well-dated fossil sites from the region. The late Middle Miocene (∼13–12 Ma) Quebrada Honda Basin (QHB) in southern Bolivia (22° S) helps fill this gap and provide vital insights into Neotropical paleoenvironments. The site is among the best-characterized Middle Miocene terrestrial vertebrate sites of South America and has a robust temporal, spatial, and lithostratigraphic framework for analyzing its sedimentary facies, fossils, and paleoenvironment. Here, we present new plant silica (phytolith) assemblage data from the QHB as well as new analyses of QHB faunal data. Phytolith assemblage data indicate two broad vegetation types: one suggestive of more open habitats (≥ 60% presumably open-habitat grasses) and the other of more closed habitats (typically dominated by potential bamboos and other forest indicators). Compositional overlap suggests that these vegetation types represent distinct plant communities within a broader biome that lacks an exact modern analog among studied Neotropical vegetation; however, it was likely akin to modern Neotropical semi-deciduous/dry forest to wooded savanna. No clear temporal or spatial trends in phytolith composition are evident in the QHB, and the same is broadly true for QHB vertebrates based on analyses of 872 identified specimens. Abundances of some mammals (certain rodents, armadillos, turtles, and the notoungulate Hemihegetotherium) vary slightly among well-sampled local areas and stratigraphic intervals, paralleling phytolith assemblage data suggesting local heterogeneity. The new floral and faunal data, combined with previous studies of paleosols, ichnofossils, ectothermic vertebrates, and mammal ecological diversity of the QHB, point to a mosaic landscape in lowland subtropical to tropical conditions that did not change substantially during the preserved interval. These results add critically to our understanding of Neotropical landscape evolution, suggesting that the QHB had not undergone substantial uplift, counter to recent reconstructions of Andean orogeny in the Eastern Cordillera.