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Ground squirrels are of the squirrel family (Sciuridae) that generally live on the ground or in , rather than in trees like the . The term is most often used for the medium-sized ground squirrels, as the larger ones are more commonly known as marmots (genus Marmota) or prairie dogs, while the smaller and less bushy-tailed ground squirrels tend to be known as (genus Tamias).
Together, they make up the "marmot tribe" of squirrels, Marmotini, a clade within the large and mainly ground squirrel subfamily Xerinae, and containing six living genera. Well-known members of this largely Holarctic group are the ( Marmota), including the American groundhog, the , the ( Spermophilus), and the ( Cynomys). They are highly variable in size and habitus, but most are remarkably able to rise up on their hind legs and upright posture comfortably for prolonged periods. They also tend to be far more gregarious than other squirrels, and many live in colonies with complex social structures. Most Marmotini are rather short-tailed and large squirrels. At up to or more, certain marmots are the heaviest squirrels.
The of the genus Tamias frequently spend time in trees. Also closer to typical squirrels in other aspects, they are occasionally considered a tribe of their own (Tamiini).Steppan et al. (2004)
Where the Marmotini originated is unclear. The subtribes probably diverged in the early to mid-Oligocene, as primitive marmots and chipmunks are known from the Late Oligocene of North America. The fossil record of the "true" ground squirrels is less well known, beginning only in the mid-Miocene, when modern susliks and prairie dogs are known to have inhabited their present-day range already.
Whether the Marmotini dispersed between North America and Eurasia via "island-hopping" across the Bering Straits or the Greenland region—both of which were temperate habitat at that time—and from which continent they dispersed to which, or if both continents brought forth distinct subtribes which then spread to the other, is not known and would probably require more fossil material to be resolved. In any case, the fairly comprehensive fossil record of Europe—at the relevant time separated from Asia by the Turgai Sea—lacks ancient Marmotini except the indeterminate Palaeosciurus, which might be taken to indicate an East Asian or western North American origin with trans-Beringia dispersal being the slightly more satisfying hypothesis. This is also supported by the enigmatic Chinese genus Sciurotamias, which may be the most ancient living lineage of this group, or—if the chipmunks are not included here—close to the common ancestor of the Tamiini and the Marmotini sensu stricto.
In any case, expansion of the Marmotini to Africa was probably prevented by competitive exclusion by their close relatives the Protoxerini and Xerini—the native terrestrial and palm squirrels of that continent, which must have evolved at the same time as the Marmotini did.
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