Describing a Newly Excavated Packrat Midden Near Natural Trap Cave, Wyoming to Help Assess Biases in Fossil Microfaunal Accumulation

Abstract

Microfauna communities resulting from packrat accumulation provide crucial information about how natural communities respond to environmental changes. However, fossil accumulation mechanisms can be complex, making it difficult to interpret these community responses. Natural Trap Cave (NTC) is an open-mouth cave with an 85-foot drop that often lies unseen by passing animals, making it a major location for fossil accumulation. NTC, located in the Bighorn Mountains of Wyoming, has accumulated a paleontological record of the local community’s biota dating back to the Late Pleistocene, 25,000 years ago. The rim of the cave’s mouth is home to packrat middens located directly above the major deposits of microvertebrate (<5 kg) fossils. It is hypothesized that middens along the rim have accumulated seeds, bones, teeth, and bird pellets from the local communities over time and have periodically washed into the cave below. We seek to assess the bias in microvertebrate community composition introduced by packrat accumulation. We identified bones and teeth collected from a 9-section grid of a modern midden located adjacent to the cave, which allow us to see which species dominate the NTC environment. We calculate species evenness, richness, and relative abundances using MNI. To assess local bias resulting from packrat accumulation, we compare the modern midden with live-and camera-trap data of small mammal communities. To assess which NTC fossils result from packrat accumulation, we compare the modern midden with fossil material from within the cave, ranging in age from 2 to 20 ka. Once we understand these biases present in the fossil accumulation, we can associate species’ relative abundances with changes in climate, such as drought, increasing temperatures, and flooding. By knowing how climate has affected NTC species’ abundances through time, we will be prepared to aid in conservation efforts by predicting how species numbers will be affected by future climate change.