of white-tailed antelope squirrels, found at least 10% of his 609 specimens' stomachs contained some type of vertebrate, mostly lizards and rodents. Morgart observed a white-tailed antelope squirrel capturing and eating a silky pocket mouse. Taxonomy The living squirrels are divided into five subfamilies, with about 58 genera and some 285 species. The oldest squirrel fossil, Hesperopetes, dates back to the Chadronian (late Eocene, about 40–35 million years ago) and is similar to modern flying squirrels. A variety of fossil squirrels, from the latest Eocene to the Miocene, have not been assigned with certainty to any living lineage. At least some of these probably were variants of the oldest basal "protosquirrels" (in the sense that they lacked the full range of living squirrels' autapomorphies). The distribution and diversity of such ancient and ancestral forms suggest the squirrels as a group may have originated in North America. Apart from these sometimes little-known fossil forms, the phylogeny of the living squirrels is fairly straightforward. The three main lineages are the Ratufinae (Oriental giant squirrels), Sciurillinae and all other subfamilies. The Ratufinae contain a mere handful of living species in tropical Asia. The neotropical pygmy squirrel of tropical South America is the sole living member of the Sciurillinae. The third lineage, by far the largest, has a near-cosmopolitan distribution. This further supports the hypothesis that the common ancestor of all squirrels, living and fossil, lived in North America, as these three most ancient lineages seem to have radiated from there; if squirrels had originated in Eurasia, for example, one would expect quite ancient lineages in Africa, but African squirrels seem to be of more recent origin. The main group of squirrels can be split into five subfamilies. The Callosciurinae, 60 species mostly found in South East Asia; the Ratufinae 4 cat-sized species found in south and southeast Asia; the Sciurinae contains the flying squirrels (Pteromyini) and the tree squirrels, 83 species found world wide; Sciurillinae a single South American species; and Xerinae includes three tribes of mostly terrestrial squirrels, including the Marmotini (marmots, chipmunks, prairie dogs, and other Holarctic ground squirrels), Xerini (African and some Eurasian ground squirrels), and Protoxerini (African tree squirrels). Taxonomy list Basal and incertae sedis Sciuridae (all fossil) Hesperopetes Kherem Lagrivea Oligosciurus Plesiosciurus Prospermophilus Sciurion Similisciurus Sinotamias Vulcanisciurus Subfamily Cedromurinae (fossil) Subfamily Ratufinae – Oriental giant squirrels (1 genus, 4 species) Subfamily Sciurillinae – neotropical pygmy squirrel (monotypic) Subfamily