Modern Elephant Footprints in Alluvium as Analogs for Sauropod Dinosaur Tracks in Ancient Floodplain Sediments

We studied deep sauropod dinosaur tracks in the Upper Jurassic Morrison Formation at locations in Bighorn County, Wyoming, and Garfield County, Utah. Tracks are preserved in convex hyporelief on the bases of sheet sandstones. Sheet sandstones are underlain by mudstones that contain downwardly deformed bedding below tracks. Tracks appear as elliptical cylinders up to 25 cm deep with rare vertical surficial striations and skin impressions.

Sheet sandstones are interpreted as crevasse-splay deposits. Preservation indicates the tracks were formed in mudstone and filled with sand during crevassing events. Mudstones are interpreted as overbank deposits in proximal floodplain environments.

We hypothesize that the volumes of these tracks can be used to interpret original sediment-moisture conditions. We use volume because it incorporates track depth and surface area. To test this hypothesis we are conducting experiments with modern elephants. Elephants are often cited as the closest modern analogs to sauropod dinosaurs because of their large size and similar foot structure.

We are conducting experiments at the Topeka Zoo, Topeka, Kansas, with one Asian elephant (Elephas maximus) and one African elephant, (Loxodonta africana). The elephants are weighed and walked through pits filled with sediment of known grain size, moisture, and density. Footprint dimensions are noted and used in multiple regression analyses to quantify the relationship between variables.

Previous elephant footprint experiments in pure sand showed that sediment density and moisture significantly influence track volume. A graph of density versus moisture plots as a convex-down parabola, a relationship predicted by soils engineering. We use mixtures of sand, silt, and clay in current experiments because they are better analogs for ancient floodplain sediments. Preliminary results show a similar moisture-density relationship.