Evolution and systematics

Tree squirrels are fascinating rodents whose arboreal roots are thought to go back to the Eocene (54-38 million years ago [mya]). The first members of the squirrel family which were "tree squirrel like" in their dental morphology appear in the fossil record during the Oligocene (37-25mya). These include specimens from France, Sciurus dubius, dated to the early to middle Oligocene and members of the genus Proto-sciurus, which occurred in North America from the early Oligocene to the early Miocene (24-5 mya). The Miocene fossil record also contains a large number of genera of flying and ground squirrels from Europe, Asia, and North America as well as genera of tree squirrels that are still extant today.

The Sciuridae squirrels are divided into ground and tree squirrels in the subfamily Sciurinae and flying squirrels in the subfamily Pteromyinae. The number of recognized genera and species varies among authors. Including arboreal giant and pygmy squirrels, there are 21 described genera with approximately 117 species: Tree squirrels (Callosciurus, Funam-bulus, Funisciurus, Glyphotes, Heliosciurus, Microsciurus, Paraxerus, Prosciurillus, Sciurus, Sundasciurus, Syntheosciurus, and Tamias-ciurus), giant squirrels (Epixerus, Protoxerus, Ratufa, Reithrosci-urus, and Rubrisciurus), pygmy squirrels (Exilisciurus, Myosciurus, Nannosciurus, and Sciurillus).

Some genera such as Sciurus or Callosciurus also contain a significant number of described subspecies. However, the precise status of some of these is in doubt and as new morpho logical and genetic data become available the number of recognized species may change.

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