Sciurini

      Sciurini
      Temporal range: Late Eocene[1] – Recent
      Eastern gray squirrel (Sciurus carolinensis)
      Scientific classification
      Kingdom: Animalia
      Phylum: Chordata
      Class: Mammalia
      Order: Rodentia
      Family: Sciuridae
      Subfamily: Sciurinae
      Tribe: Sciurini
      G. Fischer, 1817
      Type genus
      Sciurus
      Linnaeus, 1758
      Genera
      Diversity
      Five living genera and about forty species.[2]

      Sciurini is a tribe that includes about forty species of squirrels,[2] mostly from the Americas. It includes five living genera—the American dwarf squirrels, Microsciurus; the Bornean Rheithrosciurus; the widespread American and Eurasian tree squirrels of the genus Sciurus, which includes some of the best known squirrel species; the Central American Syntheosciurus; and the American red squirrels, Tamiasciurus. Like other arboreal squirrels, they are sometimes referred to as tree squirrels.[3]

      Taxonomy

      The name "Sciurini" was first employed by Hermann Burmeister in 1854, who used it for the entire squirrel family.[4] In his influential 1945 classification of mammals, George Gaylord Simpson included four genera of squirrels in Sciurini, which he recognized as one of eight tribes within the subfamily Sciurinae (including all squirrels except the flying squirrels): Sciurus, Syntheosciurus, Microsciurus, and Sciurillus. He also classified Rheithrosciurus as "?Sciurini incertae sedis" (of uncertain placement).[5] This grouping derives from Reginald Innes Pocock, who united these squirrels in 1923 as the subfamily Sciurinae.[6]

      In 1959, Joseph Curtis Moore published a review of the interrelationships of the squirrels. His definition of Sciurini was similar to Simpson's, but he no longer considered Rheithrosciurus to be incertae sedis. He noted that the members of Sciurini were united only by the possession of a special type of baculum (penis bone).[7] He also divided the tribe into subtribes, producing the following classification:[8]

      In their 1997 update to Simpson's classification, McKenna and Bell retained a similar definition for Sciurini, but also included several extinct genera, as follows:[10]

      • Tribe Sciurini
        • Freudenthalia (fossil, early Miocene of Europe; assignment to Sciurini "uncertain")[10]
        • Rheithrosciurus
        • Plesiosciurus (fossil, middle Miocene of Asia)
        • Subtribe Sciurina
        • Subtribe Sciurillina
      Sciuridae

      Sciurillus


      Ratufa

        

      Callosciurinae


      Xerinae

      Sciurinae

      Pteromyini

      Sciurini

      Tamiasciurus


      Sciurus[fn 1]





      Phylogeny of the squirrels.[12]

      In the early 2000s, several studies were published using DNA sequences to study the interrelationships of squirrels. Two, published in 2003 and 2004 and both based on several different genes, produced largely concordant results, concluding that Sciurillus is not related to other Sciurini, but rather forms one of the most distinctive lineages of all squirrels; that Tamiasciurus is the closest relative to the other Sciurini; and that the group of Tamiasciurus and the other Sciurini is most closely related to the flying squirrels.[12] The authors of the 2004 study formalized these results into a revised classification of squirrels. They removed Sciurillus from Sciurini, placed Tamiasciurus in it, and classified Sciurini with the flying squirrels (tribe Pteromyini) in a subfamily Sciurinae.[13] Their classification was adopted in the 2005 third edition of Mammal Species of the World[2] and remains current.

      The same studies also provided insights into the interrelationships of genera within Sciurini. Microsciurus, Syntheosciurus, and Rheithrosciurus all appear among the various species of Sciurus included, making the latter genus paraphyletic; additionally, the two species of Microsciurus included in Mercer and Roth's 2003 study did not cluster with each other.[12] A morphological study of Central American Sciurini also found that Microsciurus and Syntheosciurus are part of the Sciurus radiation, and suggested that Syntheosciurus be lumped into Sciurus while further work is needed on Microsciurus.[14] In a 2008 monograph on Brazilian rodents, Bonvicino and others considered Guerlinguetus and Urosciurus, conventionally placed in Sciurus, as separate genera.[15]

      Fossil history

      Douglassciurus, a fossil from the late Eocene (about 36 million years ago) of Wyoming, Montana, and Saskatchewan, is so similar to living Sciurus that the latter has been considered a living fossil.[16] but some exclude this animal from the squirrel family because of several primitive characters.[17] Emry and Korth, who re-described the animal in 1996, classified it within Sciurini[1] and speculated that other squirrels may have evolved from animals similar to Sciurini squirrels.[18] The Oligocene to early Miocene North American genera Protosciurus and Miosciurus are classified in Sciurini and may have given rise to the earliest known member of Sciurus, S. olsoni from the early late Miocene (about 10 million years ago) of Nevada. In Europe, Sciurus first appears early in the Pliocene. The 2005 discovery of S. olsoni provided evidence that the origin of the Sciurini lies in North America.[19]

      A Miocene squirrel from France and Spain, Freudenthalia, has been tentatively placed in Sciurini.[20]Plesiosciurus from the Miocene of China has been interpreted as a member of Sciurini, but is unlikely to belong to the tribe.[21]

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      Footnotes

      1. ^ Syntheosciurus, Rheithrosciurus, and Microsciurus appear among the species of Sciurus.[12]
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      References

      1. ^ a b Emry and Korth, 1996, p. 775
      2. ^ a b c Thorington and Hoffmann, 2005, p. 754
      3. ^ Thorington and Ferrell, 2006, p. 18
      4. ^ Burmeister, 1854, p. 145; McKenna and Bell, 1997, p. 122
      5. ^ Simpson, 1945, p. 78
      6. ^ Moore, 1959, p. 177
      7. ^ Moore, 1959, pp. 177–178
      8. ^ Moore, 1959, pp. 177–180
      9. ^ Thorington and Hoffmann, 2005, p. 758
      10. ^ a b McKenna and Bell, 1997, p. 122
      11. ^ Emry and Korth, 2001
      12. ^ a b c d Mercer and Roth, 2003; Steppan et al., 2004
      13. ^ Steppan et al., 2004, p. 715
      14. ^ Villalobos and Cervantes-Reza, 2007
      15. ^ Bonvicino et al., 2008, pp. 15–16, 18
      16. ^ Mercer and Roth, 2003, p. 1569; Emry and Korth, 1996, p. 775
      17. ^ Thorington and Ferrell, 2006, p. 23
      18. ^ Emry and Korth, 1996, p. 778
      19. ^ Emry et al., 2005, p. 235
      20. ^ Cuenca Bescós, 1988, p. 92; Aguilar, 2002, p. 388
      21. ^ Emry et al., 2005, p. 228
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      Literature cited

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      Last modified on 1 March 2013, at 09:53