Evolution and systematics

The ground-dwelling squirrels are grouped into three tribes: 1. Xerini—the African ground squirrels (Xerus and At-lantoxerus) and a central-Asian ground squirrel (Spermophilop-sis); 2. Marmotini—the marmots (Marmota), antelope squirrels (Ammospermophilus), prairie dogs (Cynomys), ground squirrels (Spermophilus), and Chinese rock squirrels (Sciuro-tamias); and 3. Tamiini—the chipmunks (Tamias). The classification of the 25 species of chipmunks has a long history of debate over whether the group should be one, two, or three genera. Jameson, in 1999, recommended that the single genera Tamias is incorrect and that the group should be split into three genera (Tamias, Eutamias, and Neotamias) based on the evolutionary relationships of chipmunk ectoparasites. The molecular work of Piaggio and Spicer in 2001 support Jameson's recommendation.

Climate change, plate tectonics and fluctuations in sea level produced pulses of rapid evolution in the Sciuridae beginning with the initial divergence of the Sciuridae in the late Eocene to early Oligocene. Global cooling and the expansion of grasslands during the late Oligocene gave rise to the evolution of the early ground squirrels. According to Mercer and Roth in 2003, African ground squirrels arose from a single colonization by an Eurasian ancestor after the joining of Africa with Asia approximately 18 to 20 million years ago (mya). Asia and North America were joined across the Bering land bridge allowing the exchange and radiation of tree squirrels, however forests followed by the formation of a marine barrier (7.4 to 4.8 mya) prevented the movement of North American Marmota sp. and Spermophilus sp. into Asia until after an unforested land bridge was established as glaciers expanded and sea level declined. Tamias crossed the land bridge at this time as well but a North American or Eurasian origin of this group is still debated.

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