RODENTS OF LIBYA 57 



Today the genus GerbiUus is widely distributed throughout the 

 steppes and deserts of Southwest Asia and is practically ubiquitous 

 in the Sahara. Gerbils also occur sporadically throughout most of the 

 African continent wherever desert or steppe conditions prevail and 

 are represented by a broad spectrum of species. The greater number 

 of species occurring south of the Sahara may reflect the more disjunct 

 nature of suitable habitat in these less arid portions. 



Owing to the greater number of species in Africa, probably the 

 African continent has been the center of differentiation for members 

 of the genus GerbiUus. The occurrence of gerbils in Southwest Asia, 

 Saudi Arabia, and the Middle East therefore must be the result of 

 the northward and eastward dispersal of this group through the 

 Isthmus of Sinai sometime early in the Tertiary. This hypothesis of an 

 early northward and eastward dispersal for members of this genus is 

 supported by the discovery of fossil GerbiUus from the Pliocene of 

 Asia (Darlington, 1957). 



The genus Acomys is widely distributed in Africa but is limited in 

 Asia to the steppe and desert regions south of the Caucasus Moun- 

 tains and as far east as the Sind Desert of West Pakistan. Spiny mice 

 occur also in the intervening deserts of Saudi Arabia and the Middle 

 East. Because these mice are more restricted ecologically, their dis- 

 persal must have required long periods of time. As with GerbiUus, the 

 African continent probably has served as the center of origin for spiny 

 mice, and the populations of the Middle East and Southwest Asia 

 represent invasions of the African fauna into Asia. 



Although the genus Jaculus is ubiquitous in the Sahara, it clearly 

 represents an intrusion of the jerboan fauna of central and Southwest 

 Asia into North Africa. Jerboas are unknown from Africa south of the 

 Sahara. 



The four-toed jerboa, Allactaga tetradactyla, is restricted to the 

 coastal areas of northern Egypt and northeastern Libya and is the 

 only representative of the genus in North Africa. This genus occurs 

 widely in temperate Asia ranging throughout Southwest Asia and 

 extending north and east into the cold steppes of Russian and Chinese 

 Turkestan and is represented by several species. 



The genera Jacuius and Allactaga are the only African representa- 

 tives of the Dipodidae. Temperate Eurasia, however, has 10 additional 

 dipodid genera and a broad assemblage of species. Furthermore, the 

 fossil record of Asia contains four genera of dipodid rodents extending 

 as far back as the Miocene (Darlington, 1957). No fossil dipodids are 

 known from North Africa. 



The present distribution of the jerboan fauna of temperate Eurasia, 

 strengthened by the rather extensive fossil record, provides convincing 



285 134 () us — r. 



