TRACK-MAKING BEHAVIOR OF MODERN ELEPHANTS AS AN ANALOG FOR FOOTPRINT FORMATION BY SAUROPOD DINOSAURS

The purpose of this study is to use experiments with modern elephants to quantify the relationship between sedimentary and physicochemical track-forming variables. Our goal is to determine an equation that can be applied to fossil megafaunal footprints to quantify original sediment moisture.

Classic models of deep-footprint formation view vertebrate feet as approximately cylindrical indenters that cause characteristic sediment deformation. We observed such sediment deformation attributed to sauropod dinosaurs in the Upper Jurassic Morrison Formation, Bighorn Basin, Wyoming, in a previous ichnological investigation. We used sedimentary structures, paleopedological features, and ichnofossils associated with the tracks to qualitatively interpret original sediment consistency. A quantitative model for footprint formation, however, does not exist.

We are conducting track-making experiments with modern elephants at the Topeka Zoo, Topeka, Kansas. The elephant footprints and associated sedimentary structures are very similar to the sauropod footprints observed in the Morrison Formation. Elephants are also good modern analogs for sauropod dinosaurs because of similarities in foot structure. In each experiment, footprint volume is measured as the dependent variable to account for the effects of foot surface area on track depth. The independent variables are sediment grain size, sediment density, sediment moisture, animal weight, and animal speed. Multiple regression is used to determine an equation relating all variables; this equation is solved for moisture content. Fossil footprint volume is measured in situ at tracksites. Ratios of clay-, silt-, and sand-sized grains are measured from thin sections of rock adjacent to footprints. Values of animal weights and typical sediment densities are taken from the literature. Animal speed can be calculated from trackway measurements, but preliminary analyses show that speed during a constant gait does not significantly influence footprint depth.