USING ICHNOFOSSILS AND PALEOSOLS TO RECONSTRUCT THE MIDDLE MIOCENE PALEOENVIRONMENT OF QUEBRADA HONDA, BOLIVIA

The Neotropics are home to exceptional levels of mammalian diversity, but few fossil-producing sites document the history of this unique fauna. La Venta, Colombia (LV), and Quebrada Honda, Bolivia (QH) are well-sampled, roughly 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 QH and test this hypothesis. The paleosols of QH are weakly to moderately developed and composed of brown-to-red silts, silty sands, and mudstones. In thin section, the framework grains are primarily biotite, feldspar and quartz; the micromorphology is characterized by granular to agglomeroplasmic grain fabrics and silasepic to insepic microfabrics. The paleosols occur in compound and composite profiles and are interpreted as Entisols and Inceptisols that formed in floodplains. The suite of ichnofossils present within the paleosols include mottled, passively filled burrows, meniscate and pelleted back-filled burrows. Horizons of Celliforma and Coprinisphaera are present in the paleosols, as are calcified burrows with cylindrical chambers. These ichnofossils are interpreted as dwelling, feeding, and breeding structures of solitary social insects, and dwelling structures of small mammals, respectively. Rhizoliths include rhizotubules, rhizohaloes, and rhizocretions. The rhizotubules and rhizohaloes are interpreted as roots of grasses and small herbaceous plants while the rhizocretions are interpreted as taproots of medium to large plants such as shrubs and trees. Together, paleosol and ichnofossil data suggest that QH fossils were preserved in savannahs proximal to alluvial systems in a seasonal, humid to sub-humid climate with mean annual precipitation (MAP) of ca. 1000 mm. This inferred paleoenvironment differs from that of LV, which has been reconstructed as a mixture of river-associated tropical forests and successional stages thereof with MAP of 1,500–2,000 mm, and indicates that dissimilar habitats could account for many of the differences between the mammal faunas of these two important fossil sites.