The momonga or Japanese flying squirrel, Pteromys momonga: less than a foot in length and possibly the most revoltingly adorable animal in existence. Photo from here.
The Sciuridae are a family of about 280 species of rodent native to all continents except Australia (where they are currently represented only by a population of the Indian palm squirrel Funambulus pennanti living in and around Perth Zoo that was first established in 1898). The family is primarily arboreal though some lineages in the subfamily Xerinae have become terrestrial. According to the classification adopted by Thorington & Hoffmann (2005), living squirels are divided between five subfamilies. There are also two currently standing fossil subfamilies that have been recognised in the Sciuridae.
The Cedromurinae (a name that can be translated as "cedar-mice") are a small group from the Oligocene of the North American Great Plains (Korth & Emry 1991). Cedar-mice lack certain features of the dentition of living squirrels, placing them outside the sciurid crown group. Most notable of these is the lack of sciuromorphy, a derived mode of attachment of the jaw muscles. The muscle attachment sites for cedar-mice do not extend as far forward on the skull as in crown squirrels. Sciuromorphous squirrels are known from even earlier in the fossil record than cedar-mice and the two groups would have been contemporary.
Also quite distinct in its dentition was the Chinese Pleistocene Aepyosciurus orientalis, placed in its own subfamily when originally described (Wang & Qiu 2003) though this probably needs further investigation*. Aepyosciurus had the highest tooth-crowns of any squirrel, probably an adaptation to living on a much tougher diet on the arid Tibetan plateau.
*Chinese vertebrate palaeontology seems to be awash with monotypic suprageneric taxa established for 'distinctive' species without any attempt to actually place the species phylogenetically. Many of these are later sunk into previously established, more inclusive groups (such as happened with Yuesthonyx). Explanatory factors for this situation probably include lack of communication between Chinese and foreign researchers and/or lack of opportunities to study non-Chinese specimens and publications, and a perceived link between personal prestige and the numbers and significance of taxa described.
|Indian giant squirrel, Ratufa indica. Including the tail, giant squirrels can reach close to a metre in length. Photo by Rajiv Lather.|
The known distributions of the cedar-mice and many of the earliest squirrels indicates a North American origin for the Sciuridae. However, squirrels invaded Eurasia early in their history with numerous subsequent migrations back to North America; probably the great majority of living American squirrels are descended from Eurasian ancestors (Mercer & Roth 2003). Perhaps the only living squirrel that might have a claim to a lineage untainted by Eurasian influence is the South American Sciurillus pusillus which may be the sister group to all other living squirrels. Also very basal are the Ratufa giant squirrels of southern Asia. The remaining squirrels form a clade divided between the Callosciurinae (the Asian palm squirrels), Sciurinae (including the 'typical' and flying squirrels) and Xerinae (African squirrels, chipmunks and ground squirrels).
Korth, W. W., & R. J. Emry. 1991. The skull of Cedromus and a review of the Cedromurinae (Rodentia, Sciuridae). Journal of Paleontology 65 (6): 984-994.
Mercer, J. M., & V. L. Roth. 2003. The effects of Cenozoic global change on squirrel phylogeny. Science 299: 1568-1572.
Thorington, R. W., Jr & R. S. Hoffmann. 2005. Family Sciuridae. In: Wilson, D. E., & D. M. Reeder (eds). Mammal Species of the World: A Taxonomic and Geographic Reference 3rd ed., vol. 2 pp. 754-818. John Hopkins University Press: Baltimore.
Wang B. & Qiu Z. 2003. Aepyosciurinae——a new subfamily of Sciuridae (Rodentia, Mammalia) from basal loess deposits at the northeastern border of Tibetan Plateau. Chinese Science Bulletin 48 (7): 691-695. (A Chinese version of this paper appears to have been published two months earlier.)