. Biology of the Heteromyidae. Heteromyidae. PANDESERTIC RODENT ECOLOGY 661 gulates, and various ungulate groups (, G. G. Simpson, 1980). Rodents did not col- onize the continent until the Oligocene, and only the caviomorphs are known from that period (, Patterson and Pascual, 1972). Cricetids do not appear as fossils until the Late Pliocene (Marshall et al., 1982) and, although it has been argued that they may have colonized South America as early as the Miocene (Hershkovitz, 1972; Reig, 1981), there is strong evidence that they did not enter South America until the Central American l
. Biology of the Heteromyidae. Heteromyidae. PANDESERTIC RODENT ECOLOGY 661 gulates, and various ungulate groups (, G. G. Simpson, 1980). Rodents did not col- onize the continent until the Oligocene, and only the caviomorphs are known from that period (, Patterson and Pascual, 1972). Cricetids do not appear as fossils until the Late Pliocene (Marshall et al., 1982) and, although it has been argued that they may have colonized South America as early as the Miocene (Hershkovitz, 1972; Reig, 1981), there is strong evidence that they did not enter South America until the Central American land bridge was nearly completed (, Baskin, 1978, 1986; Mares, 1985; G. G. Simpson, 1980). Whenever they entered, it seems clear that they did not colonize the Monte/Patagonian desert before the Late Pliocene. In a series of papers, I have examined the degree of desert specialization of the small mammals of the Monte Desert (Blair et al., 1976; Mares, 1973, 1975^, 1975^, 1976, 1977a, \911b, 1977c, \911d, 1979, 1980, 1983; Mares and Hulse, 1977; Mares and Rosenzweig, 1978; Mares et al., 1977a, \911b, 1985fl; Orians et al., 1977; Solbrig et al., 1977; Williams and Mares, 1978). The Monte and Patagonian deserts share numerous taxa of small mammals (Appen- dix 1), particularly at the generic level. Ba- sically, the fauna is composed of the old inhabitants (the caviomorph rodents, mar- supials and xenarthrans) and the newer im- migrants (cricetids). Most of the old inhab- itants show pronounced bullar hypertrophy (Roig, 1969, 1972). Two small marsupials {Marmosa pusilla and Lestodelphis halli) and one small xenarthran {Chlamyphorus trim- catus) are found in this xeric region. The small mammal niches filled by these species are very different from those of the hetero- myids; all are small insectivores. Their physiology is poorly known (McNab, 1982), but the invertebrate food source does supply significant amounts of free water. A similar species of Marmosa (M. elegans) is known to
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