Packrat Midden

Fossil packrat (or woodrat) middens provide information on past environments because they are a rich source of debris collected by packrats in the past. Midden is an archeological term meaning roughly "garbage pile". In order to conserve water in an arid environment, the packrat produces very viscous urine. And the packrat often urinates on its garbage pile, marking its territory and building the midden. When this urine crystalizes, it acts as a glue holding the entire garbage pile together. Fossil debris held within the midden becomes mummified, preserving it indefinitely. As long as the midden is protected from water, such as under a rock ledge, it will persist. Packrat middens are aged using radiocarbon dating. Fossil middens have been found that were older than 50,000 years, the practical limit of radiocarbon dating.

The packrat's garbage pile is usually located somewhere close to its nest, often in a rock crevice. Most of the mass of the midden consists of packrat fecal pellets, twigs, and rocks. But other things collected by the packrat are abundant, such as leaves, seeds, fruits, and bones. All of these items occurred close enough to the fossil site in the past that a packrat was able to collect them, probably within 30 to 100 meters. Thus, this technique provides a very powerful tool for reproducing past biotic communities at a specific site. Additional items present in the midden such as pollen, lizard scales, and arthropods, further add to the reconstruction of past environments. Other mammals in arid regions leave similar fossil deposits such as the Ringtail Cat (Bassariscus), a North American carnivore that leaves middens with small bones, and the rock hyrax (Procavia capensis) of Arabia and Africa.

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