Sciuridae

Belongs within: Rodentia.
Contains: Xerinae, Callosciurini, Pteromyini, Sciurini.

The Sciuridae are the squirrels, a family of rodents found on all continents except Australia and Antarctica. Ancestrally, squirrels are probably arboreal, but a number of lineages have become more terrestrial. Living squirrels have a sciuromorphous skull, with the attachment sites for the jaw muscles extending forward onto the rostrum, and teeth with well-developed lophs instead of individual cusps, but the Palaeogene Cedromurinae have a protrogomorphous skull with less extensive muscle attachments and bunodont teeth with distinct individual cusps (Wang & Qiu 2003).

Squirrels and cedar-mice

Published 28 June 2010

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.

*Update: Since this post was written, the Perth population may have been eradicated.

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. 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.

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).

Systematics of Sciuridae

Characters (from Wang & Qiu 2003): Sciuromorphous skull (small infraorbital foramen, medial masseter starting from inner side of anterior end of zygoma without passing through infraorbital foramen, broad zygomatic plate, lateral masseter extending onto rostrum) (absent in Douglassciurus [Mercer & Roth 2003] and Cedromurinae); palate usually broad, terminating at level of or behind third molar; dental formula: 1.0.1–2.3/1.0.1.3; rooted cheek teeth, P⁴ molariform, P⁴–M³ with four transverse lophs, longitudinally extending protocone, without conules and hypocone.

<==Sciuridae [Sciurida]
    |  i. s.: Palaeosciurus Vianey-Liaud 1974SM93
    |         Vulcanisciurus africanusL78
    |         Aepyosciurus Wang & Qiu 2003 [Aepyosciurinae]WQ03
    |           `--*A. orientalis Wang & Qiu 2003WQ03
    |         Baginia tenuisB49
    |           |--B. t. tenuisB49
    |           |--B. t. evidensB49
    |           `--B. t. lowii [=Sciurus lowii]B49
    |         Hesperopetes thoringtoniMJ11
    |--CedromurinaeWQ03
    |    |--CedromusB74
    |    `--Oligospermophilus Korth 1987R06, SM93
    |         `--O. douglassiOB13
    `--+--+--Sciurillus [Sciurillinae, Sciurillini]SSH04
       |  |    `--S. pusillusMR03
       |  |         |--S. p. pusillusMR03
       |  |         `--S. p. kuhliiMR03
       |  `--Ratufa [Ratufinae, Ratufini]SSH04
       |       |  i. s.: R. ephippium (Müller 1839) [=Sciurus ephippium, S. bicolor ephippium]B00
       |       |           |--R. e. ephippiumB00
       |       |           |--R. e. baramensis Bonhote 1900B00
       |       |           |--R. e. bunguranensis (Thos. & Hart 1894) [=Sciurus ephippium bunguranensis]B00
       |       |           |--R. e. nanogigas (Thos. & Hart 1895) [=Sciurus ephippium nanogigas]B00
       |       |           |--R. e. sandakanensis Bonhote 1900B00
       |       |           `--R. e. sirhassensis Bonhote 1900B00
       |       |         R. gigantea (MacClelland 1839) (see below for synonymy)B00
       |       |--R. macrouraFS15
       |       `--+--R. indicaFS15
       |          `--+--R. affinis (Raffles 1822)FS15, B00 [=Sciurus affinisB00]
       |             |    |--R. a. affinisB00
       |             |    `--R. a. aureiventer (Geoffr. 1832) [=Sciurus aureiventer]B00
       |             `--R. bicolor (Sparrman 1778)FS15, K92 (see below for synonymy)
       `--+--+--XerinaeSSH04
          |  `--CallosciurinaeSSH04
          |       |--CallosciuriniSSH04
          |       `--FunambulusMR03 [FunambuliniSSH04]
          |            |  i. s.: F. berdmoreiB01
          |            |--F. tristriatusFS15
          |            `--+--F. pennantiFS15
          |               `--+--F. palmarumFS15
          |                  `--+--F. layardiFS15
          |                     `--F. sublineatusFS15
          `--SciurinaeSSH04
               |  i. s.: Douglassciurus jeffersoni (Douglass 1901)R06, S96 [=Protosciurus jeffersoniSSH04]
               |--PteromyiniSSH04
               `--+--SciuriniSSH04
                  `--Tamiasciurus [Tamiasciurini]SSH04
                       |  i. s.: T. tenuidensGD61
                       |--T. hudsonicusFS15
                       |    |--T. h. hudsonicusB75
                       |    |--T. h. minnesota (Allen 1899) [=Sciurus hudsonicus minnesota]B75
                       |    `--T. h. richardsoniG56
                       `--+--T. douglasiiFS15
                          `--T. mearnsiFS15

Ratufa bicolor (Sparrman 1778)^{FS15, K92} [=Sciurus bicolor^B00; incl. S. albiceps Desm. 1817^B00, S. ephippium albiceps^B00, S. humeralis Coulon 1835^B00, S. hypoleucos Horsf. 1824^B00, S. javensis Zimm. 1780^B00, S. leschenaulti Desm. 1820^B00]

Ratufa gigantea (MacClelland 1839) [=Sciurus giganteus, S. bicolor var. giganteus; incl. S. macruroides Jerdon 1867]^B00

*Type species of generic name indicated

References

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[L78] Lavocat, L. 1978. Rodentia and Lagomorpha. In: Maglio, V. J., & H. B. S. Cooke (eds) Evolution of African Mammals pp. 69–89. Harvard University Press: Cambridge (Massachusetts).

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[R06] Rose, K. D. 2006. The Beginning of the Age of Mammals. John Hopkins University Press: Baltimore.

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[S96] Storer, J. E. 1996. Eocene-Oligocene faunas of the Cypress Hills Formation, Saskatchewan. In: Prothero, D. R., & R. J. Emry (eds) The Terrestrial Eocene–Oligocene Transition in North America pp. 240–261. Cambridge University Press.

[SM93] Stucky, R. K., & M. C. McKenna. 1993. Mammalia. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 739–771. Chapman & Hall: London.

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 3^rd ed. vol. 2 pp. 754–818. John Hopkins University Press: Baltimore.

[WQ03] Wang, B. Y., & Z. X. Qiu. 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).