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Appendix 5. A Brief Evolutionary History of Tassel-Eared Squirrels
- University of New Mexico Press
- Chapter
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184 A Brief Evolutionary History of TasselEared Squirrels [W]e appear to have in the Tassel-eared Squirrels a fine example of evolution in its first stages, not yet well defined in its trends and not yet in operation for a very long period, but having definite opportunities to develop well isolated forms along independent lines. —E. D. McKee, “Distribution of the Tassel-Eared Squirrels,” 1941 Jefferson’s squirrel, Douglassciurus jeffersoni, is recognized as the earliest fossil squirrel. This fossil is from the late Eocene, approximately thirty-six million years ago, and was found in Wyoming in 1975 (ref. 1). Evidence suggests that tree squirrels originated in the Northern Hemisphere (ref. 2) and expanded west into Asia and Europe (ref. 3). Members of the genus Sciurus have been found in North America and Europe dating from the early Miocene, approximately twenty-two million years ago, but the origin has not been determined (ref. 3). The genus Sciurus was described as a “living fossil” based on the distinct similarities in its osteology with the discovered fossil remains (ref. 4). The fossil record is “woefully inadequate” for following the dispersal of the numerous arboreal squirrels through the Tertiary period (ref. 5). As the Wisconsin glacial stage ended about 35,000 years ago, ponderosa pine forests began advancing into higher elevations as the Southwest began experiencing higher aridity. It is not known from the fossil records if the range of ponderosa pine forests and the distribution of tassel-eared squirrels were continuous from Colorado to Mexico “but presumably this was the case” (ref. 6). An excellent summation of the climatic conditions and the forest flora of North America during the middle of the late Tertiary, approximately fifteen million years ago, is given in Arizona’s Tree Squirrels. Moisture and temperature regimes changed, causing much colder and drier climates. Northern forests became more restrictive within particular habitats based on their temperature and moisture requirements. Conifers, which tolerate snow and colder, drier climates, endured in the higher elevations. The present configuration of coniferous forests within the southwestern Appendix 5 A B r i e f E v o l u t i o n a r y H i s t o r y 185 United States was established about 11,000 years ago. An accurate account of the actual arrival of tree squirrels in the Southwest is inexact because of the paucity of the fossil record. However, based on climatic data, Brown argues that the range of tree squirrels was established within the Holocene, at least 4,000 years ago (ref. 7). The distribution of tassel-eared squirrels across the southwestern United States and Mexico was studied in 1977 by examining mtDNA of the six subspecies. The data gathered in this research supported a post-Pleistocene (10,000 years ago to present) distribution hypothesis proposing that tassel-eared squirrels dispersed northward as the range of ponderosa pine expanded (ref. 8). References for Appendix Five 1. Thorington, R. W., Jr., and K. Ferrell. 2006. Squirrels: The Animal Answer Guide. Baltimore, MD: Johns Hopkins University Press. 2. Mercer, J. M., and V. L. Roth. 2003. The effects of Cenozoic global change on squirrel phylogeny. Science 299:1568–71. 3. Gurnell, J. 1987. The Natural History of Squirrels. New York: Facts on File. 4. Emry, R. J., and R. W. Thorington Jr. 1984. The tree squirrel Sciurus (Sciuridae , Rodentia) as a living fossil. In Living Fossils, ed. N. Eldredge and S. M. Stanley. New York: Springer Verlag. 5. Black, C. C. 1972. Holarctic evolution and dispersal of squirrels (Rodentia: Sciuridae). Evolutionary Biology 6:305–22. 6. McKee, E. D. 1941. Distribution of the tassel-eared squirrels. Plateau 14:12–20. 7. Brown, D. E. 1986. Arizona’s Tree Squirrels. Phoenix: Arizona Game and Fish Department. 8. Lamb, T., T. R. Jones, and P. J. Wettstein. 1997. Evolutionary genetics and phylogeography of tassel-eared squirrels (Sciurus aberti). Journal of Mammalogy 78 (1): 117–33. ...