Effectiveness of Harvester Ant Mounds as Sample Sources Based on Geographic Comparison of Oxfordian (Jurassic) Marine Fauna, Wyoming, USA

Schimmel/Conrades Science Center Atrium

Western harvester ants (Pogonomyrmex occidentalis) are a species of large red ant native to the American West. During the formation of their nests, harvester ants generate distinct, well-sorted mounds of excavated material and waste. The size of material usually falls between 2-64 mm and is constrained by the maximum carrying capacity of the ants. Paleontologists studying small fossils often collect samples from harvester ant mounds. However, this method of collection may be prone to sample bias and the ability of anthill samples to accurately represent paleoenvironment and faunal diversity is unclear. A pilot study was conducted on Oxfordian (Jurassic) marine fauna collected from harvester ant mounds in the Redwater Shale Member of the Sundance Formation. Twenty-one samples from four different locations across Wyoming were analyzed to obtain the relative abundances of different taxa. A general shallowing-eastward model was used to estimate relative water depths, from which predictions of the community content and variation were derived. All the taxa present in the samples were predicted in the expected model; no anomalous or unexpected fossil specimens were found. These results indicate that the harvester ants stayed within the predicted range and the mound did not become mixed with material from higher in the stratigraphic column, so samples from anthill mounds are effectively constrained. Analysis of the relative abundances of taxa between the locations revealed no clear trends, so these results do not support the expected model. However, the expected model is very general and the samples may be reflecting slight variations in water depth or other paleoenvironmental factors. Anthill-collected material may still accurately reflect the paleobiogeography of the area, and is still an effective and useful tool. More conclusive results may be obtained with a larger sample size and area.