Paleo Slide Set: Packrat Middens: Vegetation & Climate Variability in the Southwestern United States
Pleistocene midden from Burro Canyon, Kofa Mountains, Yuma County, Arizona

Plant remains are both abundant and well preserved in packrat middens, such as needles (leaves) 
of a singleleaf pinon (Pinus monophylla,) clearly visible in this 12,000 year-old sample from the 
Kofa Mountains, 200 km west of Phoenix, Arizona. Open piņon-juniper-oak woodlands occurred at this 
Sonoran Desert site during the last ice age. The preservation of plant remains in such middens is 
remarkable, even at the molecular level, allowing a variety of morphological, geochemical and genetic 
studies. Researchers are working to use polymerase chain reaction (PCR) techniques to extract, amplify 
and sequence ancient DNA from plant remains found in middens. Such DNA "fingerprinting" can be used to 
confirm species identifications based on leaf morphology, to identify hybrid zones, and to determine if 
changes in genotype frequencies accompanied large-scale migrations during the last deglaciation. 

Photo Credits:
Thomas R. Van Devender 
Arizona Sonora Desert Museum