Evolution and systematics

The rodent family Muridae is the largest mammalian family. Murids occur on nearly all landmasses, except for some arctic and oceanic islands, New Zealand, Antarctica, and parts of the West Indies. Morphological, ecological, behavioral, dietary, and taxonomic diversity in murids is truly astonishing. Murids range from solitary animals to highly social animals and from very common to very rare. Murid fossils are known from the Oligocene to recent in North America and Eurasia, from the Pliocene until recent in South America, and more recently in Africa, Madagascar, and Australia.

The relationships among all subfamilies of Muridae (including those not covered here: Murinae, Sigmodontinae, Cricetinae, and Arvicolinae) are poorly understood. Careful

A white-throated wood rat (Neotoma albigula) foraging among the cactuses of Arizona, USA. (Photo by G. C. Kelley/Photo Researchers, Inc. Reproduced by permission.)

examination of morphological characters, especially cranial and dental characters, has not been able to elucidate patterns of relationship among the various subfamilies, although some affinities were identified. Carleton and Musser (1984), in their exhaustive morphological work, preferred to treat relationships among all murid subfamilies as unresolved.

Recent systematic work using molecular characters has helped to resolve patterns of evolutionary relationships among many murid subfamilies. Although some murid subfamilies (Lophiomyinae and Platacanthomyinae) have yet to be included in a molecular analysis, DNA sequences of representative members of all other subfamilies have been generated and analyzed within a systematic framework. Nearly all currently recognized murid subfamilies retain their rank as distinct lineages in these analyses. The fossorial lineages, Spalacinae and Rhizomyinae, appear to share a common ancestor with all of the remaining subfamilies. The remaining subfamilies form several groups of related lineages. A primarily African group (including Madagascar) includes Nesomyinae, Petromyscinae, Cricetomyinae, Dendromurinae, and Mystromyinae. Calomyscinae appears as its own lineage; the remaining subfamilies form two large lineages. A group made up mainly of Arvicolinae, Sigmodonti-nae, and Cricetinae includes the subfamily Myospalacinae. The final cluster is made up of the subfamilies Murinae (in-

Gerbils (Meriones unguiculatus) have a number of different coat colors. (Photo by Carolyn A. McKeone/Photo Researchers, Inc. Reproduced by permission.)
An ice rat or Sloggett's vlei rat, (Otomys sloggetti) basking in the sun outside its burrow. (Photo by Peter Chadwick/Photo Researchers, Inc. Reproduced by permission.)

cluding Otomyinae), Acomyinae (previously included within Murinae), and Gerbillinae.

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