ASSESSING VERTEBRATE MICROFOSSIL CONTENT OF ANTHILL SEDIMENTS IN PARK COUNTY, WY

Harvester ants (Pogonomyrmex occidentalis) collect sediment grains under approximately 3 mm in diameter to construct their hills. Previous studies have noted that, in areas where local sediments are derived from modern erosion of fossiliferous bedrock, ant activity can concentrate vertebrate microfossils. We examined bulk samples of mixed sediment and fossil fragments collected by Bighorn Basin Paleontological Institute field teams over the summer of 2016. Bulk samples were surface-collected from 5 anthills within an approximate 0.6 km radius in Park County, Wyoming, where local sediments are derived from outcrops of the Latest Cretaceous Lance Formation. Our objective was to determine concentrations of identifiable and unidentifiable fossil material within each anthill sample to evaluate (1) usefulness of anthill surveys for searching for productive in situ fossil sites in the vicinity, and (2) preserved richness of the fauna in this area during the Late Cretaceous. Sediment grains were separated from fossils using binocular dissecting microscopes. Fossils were further separated into identifiable specimens and unidentifiable fragments. Weight percentage of fossil material recovered per site ranged from 1.33% to 80.95%, while identifiable fossils ranged from 0.73% to 2.02%. Preliminary findings indicate a high abundance of teeth and tooth fragments among identifiable fossils (64.42% - 83.65% by weight at the two most fossiliferous sites), many of which were identifiable as ray teeth (8.17% - 24.53% percent weight of identifiable material). Other common fossils identified include champsosaur teeth and gar scales. Overall, high variability in total fossil content among sites indicates that anthill surveys in this region may be useful indicators of local in situ fossil concentrations. However, it is worth examining a greater number of sites for fossil content in the future, as the low observed percentage of identifiable fossil material from our least fossiliferous sites suggests that the anthills are not productive sources of paleofaunal data.