Although a loose group concept of Muroidea emerged with Gill’s 1872 classification (as Myoidea), its contents and definition were most substantively advanced in Tullberg’s (1899) seminal monograph (as Muriformes) and its usage popularized in Miller and Gidley’s (1918) supergeneric arrangement of Rodentia (as Muroidae). The landmark classification of Simpson (1945) standardized the superfamily’s spelling, using the now familiar family-group suffix. With the exception of Gliridae, included by some early authors within Muroidea (e.g., Ellerman, 1941; Miller and Gidley, 1918), monophyly of the superfamily has been widely ratified from paleontological, morphological, and molecular perspectives (Bugge, 1970, 1971a; Emry, 1981; Flynn et al., 1985; Klingener, 1964; Luckett, 1985; Michaux et al., 2001b; Tullberg, 1899; Wilson, 1949; and see character summary and systematic review by Carleton and Musser, 1984:290-300). This same body of literature identifies Dipodoidea as sister group to Muroidea, a coordinate relationship first appreciated by Tullberg (1899), formalized by Schaub (1958) as the infraorder Myodonta (followed by McKenna and Bell, 1997), and sustained by recent molecular surveys of appropriate breadth (Adkins et al., 2001, 2003; DeBry and Sagel, 2001; Huchon et al., 1999, 2000, 2002; Jansa and Weksler, 2004; Martin et al., 2000; Michaux and Catzeflis, 2000; Michaux et al., 2001b; Montgelard et al., 2002; Nedbal et al., 1996; Sarich, 1985); see Hartenberger (1998) for a dissenting view of their close kinship. The geological range of the superfamily covers the middle Eocene to Recent in Asia, early Oligocene to Recent of Europe, early Miocene to Recent in Africa, early Eocene to Recent in North America, and early Pliocene to Recent in South America (McKenna and Bell, 1997).
Research on muroid systematics, from the demic to family-group level, has been dynamic and prodigious over the past decade … prodigious, and in some cases disconcertingly inconsistent when faced with integrating and explaining results across those publications in the context of a bare-bones systematic checklist. Below, we offer comments and caveats regarding the bases for taxonomies adopted, format of our accounts, and the family-group classification employed (Table 1).
Alpha-level classification and format.—Changes of taxa recognized at the generic and specific levels since 1993 generally reflect the authors’ decisions in recent systematic studies. Molecular contributions, especially genetic sequencing results, are conspicuous in the prolific 10-year output on muroid alpha systematics and have impelled many of the changes so recorded here. Unsurprisingly, the application of late 20th century investigative methodologies to taxonomies forged in the early to middle 1900s, when the biological species concept held sway over the prosecution of systematic revision, has uncovered greater diversity of muroid rodents, often labeled as "cryptic" species in titles and discussions. Our general impression is that systematists who have consulted morphological traits along with nucleotide sequences seldom construe their results as disclosing truly cryptic entities.
Understandably, not all muroid taxa have received equal attention in the last decade. As in 1993, we have emphasized the original descriptive and primary revisionary literature over secondary compilations and checklists in representing those many cases, sometimes as a basis for returning to earlier classifications where recent information is absent, ambiguous, or incorrect. Authorities sometimes disagree, of course, about the rank or application of a name, and we attempted to explain the bases for disagreement and offer a rationale for the course we adopted. Readers should consult the primary literature referenced and judge for themselves the worthiness of the authors’ data, analyses, and conclusions, not our interpretations of them. Where time and access to critical specimens allowed, we consulted type material and museum series to resolve, or at least understand, conflicting opinions and to otherwise aid taxonomic decisions. In certain instances, museum abbreviations (see Appendix I) and sometimes registration numbers, are given to substantiate a taxonomic interpretation or unpublished geographic occurrence.
Citations given within Distribution are understood to usually supply maps, sometimes noted to figure or page number, and/or collecting localities that explicitly vouch the distributions outlined. Users are urged to consult such sources rather than rely upon our necessarily cursory sketches of specific ranges. Throughout the text, we use the abbreviations M1-3 or m1-m3 to individually reference the upper (maxillary) and lower (dentary) molars, respectively.
Common names.—Early natural historians were regrettably unimaginative in coining vernacular names for small rodents, especially when compared with the rich and colorful appellations available for bird and butterfly species. The etymological archetypes of "rat" and "mouse" were seared into European vocabularies either as scurrilous purveyors of the Black Death and other pandemic miseries or as unrelenting predators on agricultural fields and food stores, well before the appearance of Linnaeus and the earliest glimmerings of natural history understanding. Western European explorers and naturalists of the latter 2nd millenium indiscriminately extended those commensal vernaculars to similarly appearing indigenous mammals around the world, foremost on the basis of size and to a lesser extent on pelage texture.
As a result, "rat" or "mouse" has become loosely and inconsistently applied to mammals that are non-murine or non-murid, commonly non-muroid or non-myomorph (spiny rats and pocket mice), and even non-rodent (moon rats and marsupial mice). Coupled with "rat" or "mouse" is some descriptive modifier that identifies collector or color, size or form, geographic place or habitat. Notwithstanding such modifiers, redundancies across Muroidea are commonplace (e.g., climbing rat or mouse, spiny or harvest mouse, short- or long-tailed rat or mouse, and so on) and are duplicated for muroid species in different genera and families. We believe that emulating the ornithological convention of adopting the genus as the vulgar name is helpful (e.g., as was done by Osgood  and is already accepted for many Moluccan, New Guinean, and Australian mammals [Flannery, 1995a, b; Strahan, 1995]); or contracting dual standard names to create a new and unique vernacular (e.g., deermouse a la woodrat); or impressing regional vernaculars from languages other than English (e.g., as suggested by Thomas , a practice which may occasionally impart logical redundancies for speakers of those languages). Even so, we sometimes fell back on Wilson and Cole’s (2000) dictionary of common names for expediency.
Common names, like the languages to which they belong, are regionally fluid and continually evolving, and the need to prescribe "standard" names seems an ever elusive and fruitless goal. While at times an entertaining diversion, this exercise pointedly reminded us why a stable system of scientific nomenclature, founded on a language long dead, was conceived in the first place. Thank you Linnaeus.
Synonyms.—The International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature, 1999) is explicit about the coordinate status of family-group taxa and identification of authorship for such names (Article 36), whatever their subsequent usage at other ranks. Although other rodent classifiers have inconsistently adhered to these guidelines (e.g., Simpson, 1945; Reig, 1980, 1981), we observe them here. Family-group synonyms, therefore, are listed in the orthography given by the original describer, that is, a generic root in plural construction which provides the formal availability and authorship of the name; in parentheses following that original form are spelling amendments that effected the standard suffix, homonymous spellings for first subsequent usage at different ranks, and other irregularly formed variants. A full and proper family-group synonymy is beyond the scope of the present work; additional citations for subsequent employment at various ranks may be found in McKenna and Bell (1997). While we include genus-group synonyms of extinct muroids, where confidently known, we did not do so at the family-group level, although many are mentioned in the introductory discussions to families and subfamilies. Readers are again directed to McKenna and Bell (1997) for such family-group synonymies, especially the many extinct groups of Cricetidae.
In specific accounts (with one exception), we alphabetically list all species-group synonyms rather than delineate formally recognized subspecies (in bold-face). Even assuming consensus among mammalogists in the meaning and usage of subspecies, their recognition is impractical for most of the world’s muroid species, given the unrefined alpha level comprehension over vast geographic regions and the usual dearth of modern infraspecific studies to objectively delimit races and to vouch their distributions. Even within the exhaustively reported rodent faunas of North America and Europe, the recent upheaval in definitions and distributions of supposedly well known species of Apodemus, Microtus, Neotoma, Peromyscus, and Sigmodon renders perpetuation of subspecific classifications in such cases as superficial, if not actually misleading taxonomically. Users who wish conventional arrangements of subspecies, mostly for the North American and European faunas, should refer to compendia such as Hall (1981), Wilson and Ruff (1997), and Mitchell-Jones et al. (1999), unless another authority is deliberately referenced under Comments. The sole exception is Mus musculus, for which the plethora of old names and their inconsistent usage persuaded us to group synonyms under five "subspecies" as a means to improve understanding of their application.
Family-group classification.—In 1984 (p. 299), we posed the question: "Should rats and mice be assigned to just one family, to the two families Muridae and Cricetidae, or to more than two?" The systematic research over the last two decades has resoundingly answered the last. Still, exactly how many more is less clear, and we have steered a conservative course in recognizing just six: Platacanthomyidae, Spalacidae, Calomyscidae, Nesomyidae, Cricetidae, and Muridae. Such a primary subdivision of muroid evolutionary diversity draws upon numerous studies generated over the past century, but most notably blends elements of certain influential classifications (Chaline et al., 1977; Ellerman, 1941; Lavocat, 1978; Reig, 1981, 1984; Simpson, 1945; Thomas, 1896; Tullberg, 1899) with the phylogenetic perspective lent by recent molecular investigations, particularly those that have incorporated broad taxon representation in addressing questions of suprageneric relationships (e.g., Ducroz et al., 2001; Engel et al., 1998; Jansa and Weksler, 2004; Jansa et al., 1999; Martin et al., 2000; Michaux and Catzeflis, 2000; Michaux et al., 2001b; Watts and Baverstock, 1995b).
Increased employment of family-group ranks, from subtribe to family, will concomitantly issue from firmer understanding of genealogical hierarchies among some 1500 species (terminal taxa) of Muroidea and the desire to represent those patterns of evolutionary descent in our classification. As such comprehension continues to unfold, we note two areas of systematic investigation that might improve the dialogue concerning muroid intrarelationships. First, explicit diagnoses of newly revealed (or rediscovered) supraspecific associations has lagged, especially at the family-group level but even for genus-group ranks. Taxa such as Acomyinae, Arvicanthini, and Zygodontomyini have appeared in narrative discussion without any intent to differentiate or awareness of older available names. Although older descriptive habits were sufficiently loose to permit such casual anointing of family-group names, the International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature, 1999) has stipulated progressively more rigorous standards for creation of such names (particularly Articles 13 and 16). Perhaps a differential diagnosis in this implicitly morphological sense is meaningless when considering the distribution of molecular traits on maximum parsimony and likelihood trees or consensus renditions thereof. The effort nonetheless seems worthwhile, e.g., as Smith and Patton (1999) attempted for nucleotide changes of cytochrome b and amino acid synapomorphies for tribal clades of Sigmodontinae. At the genus-group level, it is well to remember that the subgenus serves as a legitimate rank for expression of interspecific relationships and could be employed more regularly where diverse character data substantially concur. Second, there is comparable need for syntheses of paleontological and neontological results that bear on family-group classification. As investigators continue to probe deep muroid phylogeny, one anticipates that certain Miocene, perhaps even Oligocene, assemblages should be implicated as ancestral or cognate groups, but integrative work by those facile with both paleontological and neontological evidentiary sources is wanting. Certain studies on Arvicolinae (Chaline et al., 1999) and Otomyinae (Chevret et al., 1993b) stand apart as notable and salutary exceptions, but more syncretic initiatives of this kind are called for.