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Sciurini () is a tribe that includes about forty species of ,Thorington and Hoffmann, 2005, p. 754 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 pine squirrels, Pine squirrel. Like other arboreal squirrels, they are sometimes referred to as .Thorington and Ferrell, 2006, p. 18
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).Moore, 1959, pp. 177–178 He also divided the tribe into , producing the following classification:Moore, 1959, pp. 177–180
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:McKenna and Bell, 1997, p. 122
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 . 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.Steppan et al., 2004, p. 715 Their classification was adopted in the 2005 third edition of Mammal Species of the World 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 paraphyly; additionally, the two species of Microsciurus included in Mercer and Roth's 2003 study did not cluster with each other. A morphological study of 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.Villalobos and Cervantes-Reza, 2007 In a 2008 monograph on rodents, Bonvicino and others considered Guerlinguetus and Urosciurus, conventionally placed in Sciurus, as separate genera.Bonvicino et al., 2008, pp. 15–16, 18
A Miocene squirrel from France and Spain, Freudenthalia, has been tentatively placed in Sciurini.Cuenca Bescós, 1988, p. 92; Aguilar, 2002, p. 388 Plesiosciurus from the Miocene of China has been interpreted as a member of Sciurini, but is unlikely to belong to the tribe.Emry et al., 2005, p. 228
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.Mercer and Roth, 2003, p. 1569; Emry and Korth, 1996, p. 775 but some exclude this animal from the squirrel family because of several primitive characters.Thorington and Ferrell, 2006, p. 23 Emry and Korth, who re-described the animal in 1996, classified it within SciuriniEmry and Korth, 1996, p. 775 and speculated that other squirrels may have evolved from animals similar to Sciurini squirrels.Emry and Korth, 1996, p. 778 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, Sciurus 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.Emry et al., 2005, p. 235
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