EASTERN WYOMING HARVESTER ANT MOUNDS REVEAL RICH VERTEBRATE MICROFOSSIL ASSEMBLAGE

Recently a microfossil assemblage in an eastern Wyoming deposit of the uppermost Cretaceous (Maastrichtian) Lance Formation was discovered via a North American harvester ant (Pogonomyrmex occidentalis) mound that yielded a dense concentration of vertebrate microfossils. We set out to discover just how fossiliferous this deposit was, so we collected and screened a 3,050 gram sample of sediment and picked out all fossil material present. Of this sediment, 225.4 grams of fossil material were collected, and 7.4 grams of this fossil material consisted solely of teeth. Ant mounds from this area have yielded fossils from three genera of crocodiles, a species of hybodont shark, a species of skate, at least one species of fish, over three different species of mammals, two ornithischian dinosaur genera, and at least three genera of theropod dinosaurs. Of particular interest is the large abundance of teeth from the small theropod dinosaur Troodon formosus. Sediment collected from one ant hill yielded 327 individual teeth. This is an unusual and high concentration of teeth from Troodon. The density of vertebrate microfossils found in this ant mound gave us valuable insight to the presence of a fossiliferous microfossil bonebed nearby. It should be noted that other ant hills in the vicinity had decreasing concentrations of fossil material the farther away they were from this central ant mound. This allowed us to pinpoint the approximate location of the bonebed simply by sampling ant mounds in the area. As we theorized, surrounding the hill this central mound was nested on was an outcrop of a fossiliferous microfossil bonebed. We have concluded that ant mounds from the North American harvester ant can provide valuable data about nearby deposits. Additionally, ant mounds have proven to be a valuable and effective tool for paleontologists to pinpoint fossil exposures and gather data about an area of interest.