RECONSTRUCTING THE MIDDLE MIOCENE PALEOENVIRONMENT OF QUEBRADA HONDA, BOLIVIA, USING ICHNOLOGY AND PALEOPEDOLOGY

The Neotropics are home to exceptional levels of mammalian diversity, but few fossil-producing localities document the history of this unique fauna. La Venta, Colombia, and Quebrada Honda, Bolivia are well-sampled, contemporaneous (13-12 Ma) sites that preserve the remains of many extinct nonvolant Neotropical mammals (59 and 30 genera, respectively). Almost no mammalian genera are shared between the two sites, and this could reflect climate and/or habitat differences. In this study, we use paleopedology and ichnology as independent lines of evidence to elucidate the habitat of Quebrada Honda and test this hypothesis. The paleosols of Quebrada Honda are weakly to moderately developed and composed of brown-to-red mudstones, silts and silty sands. The paleosols are interpreted as Entisols and Inceptisols that formed in distal floodplains. The suite of ichnofossils present in the paleosols include cm-scale mottled lined and unlined horizontal-to-subvertical burrows and non-mottled sinuous branching burrows. Horizons of Celliforma and Coprinisphaera are present in the paleosols, as are dcm-scale burrows with chambers. These ichnofossils are interpreted as dwelling and breeding structures of solitary social insects, and dwelling structures of small mammals, respectively. Rhizoliths include mm-scale rhizotubules, cm-scale rhizohaloes and dcm-scale rhizocretions. The rhizotubules and rhizohaloes are interpreted as roots of grasses and other small herbaceous plants while the rhizocretions are interpreted as taproots of medium-to-large plants such as shrubs and trees. Taken together, the paleosol and ichnofossil data from Quebrada Honda suggest a mixture of grasslands and savannahs located proximal to alluvial systems. This interpretation contrasts with published paleoenvironmental interpretations for La Venta (a mixture of river-associated tropical forests and successional stages thereof) and indicates that dissimilar habitats could account for many of the differences between the mammal faunas of these two important fossil sites.