BIAS IN THE TERRESTRIAL FOSSIL RECORD EVALUATED WITH DISTRIBUTIONS OF MODERN SPECIES IN THE CHACO PLAIN AND PANTANAL WETLANDS OF SOUTH AMERICA

Since only fluvial sediments deposited in a sedimentary basin will ultimately be preserved in the rock record, only animals that live in these basins have a chance of becoming fossilized. Species that occur in sedimentary basins have a higher probability of becoming preserved, due to tectonic subsidence, than those living outside of these basins, e.g., in adjacent highlands, where erosion dominates. Possible bias in the number of species represented in the fossil record may exist if two distinct groups of species inhabit these different areas. We evaluate whether there is a distinct separation of the composition of species between the Chaco Plain of the Andean Foreland, the Pantanal basin, and outlying highlands. The Chaco Plain and Pantanal Basin were delineated using Landsat imagery and elevations from the Shuttle Radar Topography Mission. Areas of current depositions on the megafans and axial rivers were also delineated to identify actively aggrading locations within the basin. Occurrence data for modern mammals and reptiles were obtained from the Global Biodiversity Information Facility database for Argentina, Bolivia, Brazil, Chile, Paraguay, and Uruguay. We related these occurrence data to environmental layers that summarize climate, vegetation, and elevation using MaxEnt, an algorithm for producing models of species distributions. MaxEnt identifies areas that are likely to be favorable for a species to survive. Though some species have modeled distributions in both settings, preliminary findings indicate that species within sedimentary basins are distinctly different than those outside the sedimentary basins. These results suggest species preserved in the fossil record may only represent the relatively limited number of species held within the sedimentary basin, while largely missing the species that lived outside these basins.