RECONSTRUCTING MIOCENE NEOTROPICAL PALEOENVIRONMENTS: A CASE STUDY FROM THE PALEOSOLS AND ICHNOFOSSILS OF CERDAS, BOLIVIA

Bolivia is one of the few countries in South America that preserves a rich fossil record of middle Miocene Neotropical mammals. Nevertheless, little is known about the paleoenvironments that fostered the development of this ancient mammalian diversity. Cerdas, an early middle Miocene (16 – 15 Ma) locality in southern Bolivia has produced remains of 15 mammal species in seven orders and nine families. In this study, we use paleopedology and ichnology to investigate its paleoenvironment. The paleosols of Cerdas are thin (5 – 55 cm), weakly to moderately developed, and primarily composed of red-to-brown silty sandstone with an angular blocky texture. In thin section, the framework grains are primarily feldspar and quartz; the micromorphology is characterized by intertextic to agglomeroplasmic grain fabrics and insepic to mosepic microfabrics. The molecular weathering ratios are characterized by low oxidation and lessivage values and moderate calcification and leaching values. The paleosols occur in compound profiles and are interpreted as eutric Inceptisols that formed proximal to braided streams. Burrows and rhizoliths are common in these paleosols. Burrows are linear and J-shaped, lined and unlined, and are horizontally to subvertically oriented; they are interpreted as feeding and dwelling structures of various soil invertebrates. Rhizoliths include cm-scale grey rhizohaloes in the base and middle of the section and mm-scale rhizotubules in the top of the section. The rhizohaloes are interpreted as roots of small- to medium-sized plants that grew in imperfectly drained soils, whereas the rhizotubules are interpreted as roots of grasses and small plants that grew in well-drained soils. Cerdas ichnofossils represent soil organisms that are not preserved as body fossils. Based on the characteristics of the paleosols and ichnofossils, the paleoenvironment of Cerdas is interpreted as a humid to subhumid shrubland located proximal to an alluvial system. These interpretations also fit with previous paleotemperature and paleoelevation estimates based on clumped isotope analyses. The use of paleopedology and ichnology is essential for accurate paleoenvironmental interpretations, especially in localities where insufficient sampling precludes body-fossil based reconstructions.