Over the past thirty years, late Quaternary environments in the arid interior of western North America have been revealed by a unique source of fossils: well-preserved fragments of plants and animals accumulated locally by packrats and quite often encased, amberlike, in large masses of crystallized urine. These packrat middens are ubiquitous in caves and rock crevices throughout the arid West, where they can lie preserved for tens of thousands of years. More than a thousand of these deposits have been dated and analyzed, and middens have supplanted pollen records as a touchstone for studying...