Caves are important sites of fossil preservation, especially for Quaternary vertebrates. Taphonomic processes operating in caves are not well understood and have never been experimentally examined. This study focuses on the potential role of bat guano, which impacts environmental biogeochemistry and serves as the base of the food chain in cave ecosystems. The presence or absence of guano is expected to be a major control of preservation potential in caves. Bats first appear in the fossil record in the early Eocene so bat guano likely influenced cave preservation only during the Cenozoic. This is a probable megabias of the cave fossil record. Microcosm experiments were used to determine the impact of guano presence and composition, moisture, temperature, and time on preservation potential of small mammal bones, leaves, and crickets. Guano came from insectivorous and frugivorous bats. The guano of insectivorous bats has an acidic pH, while the guano of frugivorous bats is close to neutral. Lab studies were supplemented with field experiments at Crumps Cave, Kentucky. Leaves and crickets were better preserved in the guano of insectivorous bats, while bones showed recrystallization after burial. Leaves and crickets buried in the guano of frugivorous bats were quickly colonized by fungi and mostly destroyed, while only a few bones showed signs of fungus and degradation. Microcosms with a higher moisture content showed greater degradation, while time and temperature had less of an effect. Bones buried in both types of guano decayed much more rapidly than in sand. Bones buried in situ in cave sediments showed little degradation over three months. These results provide insight into the variable impact of environmental conditions on the taphonomy of Quaternary vertebrates and plants.