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.
Middens are analyzed in specialized laboratories such as the USGS-NAU Macrobotanical Laboratory. The midden is first dissected, the fossil parts are extracted from the matrix of crystalized urine by washing, and then the fossils are identified through the use of comparative collections of plants, pollen, bones, and other materials.
Other web publications containing Packrat Midden debris
- Colorado Plateau Packrat Midden Database and Plant Macrofossil Digital Library USGS Colorado Plateau Research Station Global Change Program web site with links to a midden database and digital images of identified modern and fossil plant parts from North American Deserts.
- North American Packrat Midden Database. An online query-able database of fossil records.
- Past Climate and
Vegetation Changes in the Southwestern United States An overview of the
results of numerous studies on the paleoecology of the western United States including packrat
midden data.(R. S. Thompson and K. H. Anderson)
- NOAA Packrat Midden Slide Set. A set of 26 photographic slides illustrating the method that can be viewed online or ordered for $25.
- Migrational History of Utah Juniper in Wyoming. Packrat Middens used to illustrate juniper migration over the last 10,000 years.
- Packrat Middens: The Last 40,000 Years of Biotic Change. The most complete book on packrat middens.
- Quantitative Paleoclimatic Reconstructions from Late Pleistocene Plant Macrofossils of the Yucca Mountain Region (Thompson, Anderson, and Bartlein). Packrat macrofossils used to project ice-age climates at Yucca Mountain.
- Land Use History of North America - (LUHNA): The Paleobotanical Record (Allen, Betancourt, and Swetnam). Summary of the historical background of southwestern landscapes.
- Ecology and Paleoecology of the Upper Gunnison Basin, Colorado (Emslie and Stiger).
- Impacts of nineteenth century grazing at Capitol Reef National Park as determined through packrat midden analysis Fossil packrat middens were used to reconstruct changes of the last 500 years and compare them with late Holocene changes. (K. L. Cole; this site).
- Past, Recent, and Twenty-first Century Vegetation Change in the Arid Southwest A USGS global change program research project using packrat middens (K.L. Cole).
- The Holocene and
Pleistocene vegetation history of the Grand Canyon Fossil plant assemblages
from packrat middens have been used to reconstruct the last 30,000 years of vegetation history
from the Grand Canyon.(K. L. Cole; this site).
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