AN ICHNOLOGICAL PROXY FOR THE RESPONSE AND RECOVERY OF SOIL FAUNA ACROSS THE K-PG EXTINCTION INVERVAL: BIG BEND NATIONAL PARK

We present the results of an ichnological proxy for ecosystem stress applied to a continental Cretaceous-Paleogene (K-Pg) mass-extinction interval at Big Bend National Park, which reveals an ecological perturbation that is otherwise not apparent in the lithology or traditional paleontology. The trace-fossil record of soil-dwelling organisms within alluvial deposits and associated paleosols from the Dawson Creek section contains Planolites, Celliforma, Skolithos and Naktodemasis (adhesive meniscate burrows; AMB), however our quantitative efforts focused on AMB (n=801) due to the pervasiveness of this trace throughout the study interval. Previous studies demonstrate AMB were likely generated by burrower bugs or cicada nymphs. As a proxy for the body size of the tracemaker, our dataset indicates that a smaller burrow diameter is statistically (α < 0.05) correlated with poor drainage (mean = 6.4±0.2 mm) in comparison to moderately (7.1±0.2 mm) or well-drained conditions (7.1±0.1 mm) within paleosols (n=15). Soil order also has a correlation to body size within the soil-dwelling community - Entisols contain smaller-diameter burrows (6.5±0.1 mm) than Inceptisols (7.2±0.1 mm) or Vertisols (7.1±0.2 mm). Whereas the precise stratigraphic placement of the K-Pg boundary is unknown at Big Bend due to the lack of diagnostic geochemical indicators and sparsity of age-specific body fossils, a previous approximation of the position based on a combination of absolute ages and rates of pedogenesis has tentatively placed the boundary at 5 m above a channel sandstone containing the highest dinosaur remains and 26 m below a Pg assemblage. Our dataset reveals an abrupt decrease in AMB diameter (5.6±0.3 mm) within a paleosol (referred to as P22) immediately superjacent to the highest dinosaur remains. P22 is a well-drained Vertisol, therefore the anomalous shift cannot be attributed to drainage or paleosol order. Furthermore, AMB diameters remain smaller on average above P22 (6.6±0.1 mm) in comparison to the burrows constructed during the interval known to be Cretaceous in age (7.3±0.1 mm). We interpret the trace fossils in P22 to signify the post-extinction recovery community and propose that the stratigraphic position of the K-Pg boundary in Big Bend is represented by the dinosaur-bearing channel sandstone subjacent to P22.