The utility of alpine cave fossil assemblages for zoological census: an example from northern Utah, United States

Assemblages of mammal skeletal remains provide a powerful tool for censusing wildlife populations to establish zoological baselines required for evaluating biogeographic trends over varying timescales. Caves provide an ideal depositional setting to preserve these skeletal remains despite potential time averaging and taphonomic filtering. We describe a Holocene paleontological assemblage from Boomerang Cave in the Bear River Range of Cache County, northern Utah, United States, at an elevation of 2,231 m, and at the boundary between the Great Basin and Rocky Mountain biogeographic provinces. We analyzed 1,228 surface-collected specimens from six areas within the cave, and identified a minimum of 22 nonoverlapping mammalian taxa, comprising all size classes present in the region. Compared to museum records for mammals from the Bear River Range and individuals trapped or observed in the vicinity of the cave, specimen-based rarefaction demonstrates that our assemblage captures most of the mammalian diversity expected in the area. This is particularly apparent for carnivorans and soricids, which are particularly well-represented in the Boomerang Cave assemblage, with the former clade represented by at least nine taxa. This high level of diversity can be attributed to the relatively random nature of natural trap cave deposition, reducing accumulation biases due to size or diet. We also record the first occurrence of Merriam’s Shrew (Sorex merriami) from the Bear River Range. Our analysis does not indicate any mammalian changes between late Holocene and present-day communities, but these data establish a new zoological baseline for an alpine community at the interface between two key biogeographic provinces in western North America. Our work highlights the value of collecting skeletal remains from cave assemblages as a convenient and fast method for censusing terrestrial mammalian communities.