Chapter 3 Review of the Oryzomys couesi Complex (Rodentia: Cricetidae: Sigmodontinae) in Western Mexico MICHAEL D. CARLETON1 AND JOAQUIN ARROYO-CABRALES2 ABSTRACT The status and distribution of eight species-group taxa of the Oryzomys couesi group (Rodentia: Cricetidae: Sigmodontinae) described from western Mexico are evaluated based on morphological and morphometric comparisons. Four of these are recognized as valid species within the region: Oryzomys albiventer Merriam, 1901 (including molestus Elliot, 1903), from inland plateau of the Mesa de Ana?huac; O. couesi mexicanus J.A. Allen, 1897 (including bulleri J.A. Allen, 1897; lambi Burt, 1934; rufus Merriam, 1901), from Pacific coastal plain, contiguous lower mountain slopes, and interior valleys along the R??os Tepalcatepec?Balsas; O. nelsoni Merriam, 1898, from Isla Mar??a Madre; and O. peninsulae Thomas, 1897, from the southern tip of the Baja California Peninsula. Three other taxa named from uplands in interior Mexico? aztecus Merriam, 1901; crinitus Merriam; 1901; and regillus Goldman, 1915?are provisionally retained within O. couesi, but further study of their specific stature and relationships is required. The recommended taxonomic changes within western Mexico serve to discuss directions for future revisionary research that will refine the definition and distribution of O. couesi sensu stricto in Middle America. INTRODUCTION North and Middle American populations of the Oryzomys palustris group (sensu Merriam, 1901; Goldman, 1918) have been allocated to as many as 18 species (Merriam, 1901) or to as few as three (Hall, 1981). The systematic contributions of Goldman (1918) and Hall (1960) have played historically influential roles in reversing the early view of specific diversity within the O. palustris group. In his revision of the genus, Goldman (1918), perhaps emulating the revisionary model of Peromyscus as set forth by his colleague Osgood (1909), demoted many of Merriam?s species to intergrading geographic races of the widely ranging, polytypic forms O. couesi and O. palustris. Within the O. palustris group, Goldman designated two sections, the O. palustris section, which contained the single North American species, and the O. couesi section, which contained the nominotypical species and six geograph- ically isolated specific allies within Middle America (i.e., O. antillarum, O. cozulmelae, O. fulgens, O. gatunensis, O. nelsoni, and O. peninsulae). Goldman?s specific classification was essentially maintained through the mid- dle 1900s (Miller and Kellogg, 1955; Hall and Kelson, 1959). Subsequently, Hall (1960) considered couesi to be synonymous with O. palustris based on perceived intergradation of populations along coastal Texas in certain cranial and chromatic traits. Although ar- gued within a very localized geographic context, Hall swept all of Goldman?s (1918) subspecies of couesi, including those in western Mexico and Central America, under his expansive definition of O. palustris. His action set a broad-brushed precedent for assessing and taxonomically conveying mor- phological variation among populations of the Oryzomys palustris group. Soon 1 Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560- 0111 (carletonm@si.edu). 2 Laboratorio de Arqueozoolog??a ??M. en C. Ticul A?lvarez Solo?rzano,?? Subdireccio?n de Laboratorios y Apoyo Acade?mico, INAH, Moneda # 16, Col. Centro, 06060 Me?xico, D. F., Me?xico (arromatu5@yahoo.com.mx). 94 afterward, other Middle American forms retained as species by Goldman (1918) or Hall and Kelson (1959) were pro forma brought into O. palustris as well (cozumelae by Jones and Lawlor 1965; azuerensis and gatunensis by Handley, 1966; fulgens, nelsoni, and peninsulae by Hershkovitz, 1971; antil- larum by Hall, 1981). Judged against today?s standards of sys- tematic documentation, presentation of data and analysis to support these sweeping taxonomic changes has been at best perfunc- tory, at worst nonexistent. Reexaminations of the purported zone of intergradation between couesi (aquaticus) and O. palustris (texensis) in Texas tellingly exposed the fragile underpinnings of such an inclusive definition of O. palustris (Benson and Gehl- bach, 1979; Schmidt and Engstrom, 1994). As a result of these regional studies, O. couesi (Alston) was resurrected as a species distinct from North American O. palustris (Harlan), but most other Middle American taxa have remained as synonyms of a broadly ranging O. couesi (e.g., Honacki et al., 1982; Corbet and Hill, 1991; Musser and Carleton, 1993, 2005), an arrangement based on classificatory inertia rather than on actual documentation of consanguinity. Herein, we review morphological and morphometric evidence for intergradation among forms of the Oryzomys couesi complex whose distribution lies in western Mexico. The region encloses striking topographic and biotic diversity and accordingly has proven to be strategic for unraveling specific diversity in other taxonomically problematic rodent gen- era, such as Peromyscus (Carleton, 1977; Carleton et al., 1982; Bradley et al., 1989, 1996) and Sigmodon (Zimmerman, 1970; Carleton et al., 1999; Peppers et al., 2002). Eight type specimens of species-group epi- thets currently allocated to Goldman?s (1918) O. couesi section originate from this area (see fig. 1). In particular, we address the status and relationships of taxonomically recog- nized populations of Oryzomys that occur at the southernmost tip of Baja California (O. couesi peninsulae), on the Tres Mar??as Islands (O. nelsoni), along coastal lowlands in Sonora (O. c. lambi) and from Sinaloa through Oaxaca (O. c. mexicanus), and on the inland plateau region of Jalisco (O. c. albiventer). In addition, we discuss possible systematic af- finities of other taxa named from uplands in interior Mexico?O. c. regillus (Michoaca?n), O. c. aztecus (Morelos), and O. c. crinitus (Distrito Federal). MATERIALS AND METHODS Specimens that form the basis of this report principally consist of skins with their associated skulls and are contained in the following institutional and museum collec- tions (abbreviation in parentheses): American Museum of Natural History, New York (AMNH); Carnegie Museum of Natural History, Pittsburgh (CM); Dickey Collection, University of California, Los Angeles (UCLA); Museum of Natural History, Uni- versity of Kansas, Lawrence (KU); Field Museum of Natural History, Chicago (FMNH); National Museum of Natural History, Smithsonian Institution, Washing- ton, D.C. (USNM, formerly the U.S. Na- tional Museum); the Natural History Muse- um, London (BMNH, formerly the British Museum of Natural History), and the Uni- versity of Michigan Museum of Zoology, Ann Arbor (UMMZ). Type specimens and original series of most taxa described for populations of Oryzomys in western Mexico were examined firsthand, namely those of albiventer Merriam, 1901; aztecus Merriam, 1901; bulleri Allen, 1897; crinitus Merriam, 1901; mexicanus Allen, 1897; nelsoni Mer- riam, 1898; peninsulae Thomas, 1897; regillus Goldman, 1915; and rufus Merriam, 1901. Original topotypes of lambi Burt, 1934, were consulted, but we did not personally examine the holotype; William T. Stanley examined and measured the FMNH type of molestus Elliot, 1903, for us. Approximately 600 specimens were exam- ined (see registration numbers and full locality information under Taxonomic Sum- mary) and measured, of which 284 adults were grouped into 11 geographically cohesive samples for univariate and multivariate analyses. According to current taxonomic understanding, these operational taxonomic units (OTUs), their abbreviations as em- ployed in figures, specific localities, and sample sizes are recognized as follows: 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 95 Oryzomys couesi albiventer: (J3) Jalisco, Ameca, N 5 12. Oryzomys couesi mexicanus: (C1) Colima, vicinity of Santiago, N 5 33; (C2) Colima, 5 mi NW Manzanillo, N 5 20; (J1) Jalisco, localities to the N and E of Barra de Navidad, N 5 60; (J2) Jalisco, Cuitzamala, N 5 20; (N1) Nayarit, 1 mi S Cuautla, N 5 62; (N2) Nayarit, San Blas and vicinity, N 5 23; (N3) Nayarit, Chacala and Las Varas, N 5 16; (N4) Nayarit, 2 mi E San Pedro Lagunillas, N 5 20. Oryzomys couesi peninsulae: (BC) Baja California Sur, San Jose? del Cabo, N 5 14. Oryzomys nelsoni: (TM) Nayarit, Tres Mar??as Islands, Isla Mar??a Madre, N 5 4. Four external dimensions were transcribed from skin tags to the nearest whole millimeter (mm): total length (TOTL); tail length (TL); hindfoot length (HFL); and ear (pinna) length (EL). 17 cranial and three dental variables were measured to 0.01 mm (most are defined and illustrated in Carleton and Musser, 1995, and Musser et al., 1998), using Fig. 1. Type localities of 11 taxa of the Oryzomys couesi species group described from western and central Mexico and discussed in the present study. Taxa and localities include: albiventer Merriam (1901) from Ameca, Jalisco; aztecus Merriam (1901) from Yautepec, Morelos; bulleri Allen (1897) from Valle de Banderas, Nayarit; crinitus Merriam (1901) from Tlalpan, Distrito Federal; lambi Burt (1934) from San Jose? de Guaymas, Sonora; mexicanus Allen (1897) from Tonila, Jalisco; molestus Elliot (1903) from Ocotla?n, Jalisco; nelsoni Merriam (1898) from Isla Mar??a Madre, Nayarit; peninsulae Thomas (1897) from Santa Anita, Baja California; regillus Goldman (1915) from Los Reyes, Michoaca?n; rufus Merriam (1901) from Santiago, Nayarit. Topography of Mexican sierras is indicated by 1000 m elevational bands. 96 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 p-x- ; / \. famb/ (^^ / \ ID \"b ^{ \ \ x^v (^ \ ^ L^_ MEXICO /^^ \ 1 ^ KX \ y &. J ^ \ m r \' % k& \-/ ^^-^/ \ ( \^^ \ 1 \ w { iJ \ ?^p*n/n#wf## I rufwa /^ ' L/ 7 /^ r fh ^"v *> nelsoni ? \ * \ y"^ 3000 m o ?1 2000-3000 m @ e mexicanus] r-v 1000-2000 m %> I I < 1000 m handheld digital calipers while viewing crania under a stereomicroscope. These measure- ments, and their abbreviations as used in text and tables, are: occipitonasal length (ONL); greatest zygomatic breadth (ZB); breadth of braincase (BBC); depth of braincase (DBC); breadth across occipital condyles (BOC); interorbital breadth (IOB); length of rostrum (LR); breadth of rostrum (BR); depth of rostrum (DR); length of incisive foramen (LIF); breadth across incisive foramina (BIF); length of diastema (LD); postpalatal length (PPL); length of bony palate (LBP); breadth of bony palate (BBP); breadth of zygomatic plate (BZP); length of auditory bulla (LAB); coronal length of maxillary tooth row (CLM); width of the upper first molar (WM1); depth of the upper incisor (DI). Five age classes were defined according to stage of eruption of the upper third molar (M3) and relative wear of the upper molars (M1?3): (1) juvenile?M3 not erupted, or if erupted not reaching the occlusal plane; (2) immature?all molars fully erupted, little wear on M1?2 but no wear on M3; (3) young adult?upper molars, including M3, lightly worn, enamel pattern retaining well defined cusps and ridges, dentinal connections nar- row; (4) full adult?M1?3 moderately to heavily worn with blunt cusps, wide dentinal connections, and enamel pattern variously obscured; (5) old adult?occlusal wear so advanced that the enamel pattern is largely obliterated, dentinal lakes of cusps are broadly continuous, and molars basinlike. We use the abbreviations M1?3 or m1?3 to individually reference the upper (maxillary) and lower (dentary) molars, respectively. Standard descriptive statistics (mean, range, standard deviation) were derived for the 11 OTUs using only those specimens judged to be young, full, or old adult. External data are provided as general guid- ance to identification (table 1) but were not subjected to morphometric comparisons. Canonical variates (CVs) derived from mul- tigroup discriminant-function classification and principal components (PCs) were com- puted using only the 20 craniodental vari- ables, all of which were first transformed to natural logarithms. With only three speci- mens, the topotypes of lambi were entered as unknowns in the discriminant-function anal- ysis and their probabilities of group mem- bership calculated a posteriori, as were the holotypes of aztecus, bulleri, crinitus, molestus (as measured by W.T. Stanley, FMNH), regillus, and rufus. Principal components were extracted from the variance-covariance matrix, and variable loadings are expressed as Pearson product-moment correlation co- efficients of the extracted components or canonical variates with the original cranial measurements. All analytical procedures were implemented using statistical routines contained in Systat for Windows, Version 10.2 (2002). RESULTS AND COMPARISONS INTRASAMPLE AGE AND SEX VARIATION: Superficial perusal of skulls in series of western Mexican Oryzomys reveals substan- tial range in size, shape, and pronouncement of cranial ridges, an impression remarked on by others who have worked with examples of Oryzomys (Merriam, 1901; Goldman, 1918; Paradiso, 1960; Park et al., 1974; Humphrey and Setzer, 1989). Some of this variation plausibly corresponds to the age of individ- uals, as suggested by occlusal wear of molars, but consistent size differences between the sexes are also evident and seemingly vary independently of age. Such casual visual impressions are reinforced by statistical comparisons among age and sex cohorts within our largest OTU of O. couesi mex- icanus (N1, N 5 62). Many variables display regular, incremen- tal increases in mean size across the three adult age classes defined, producing age- correlated differences that contribute sub- stantially to nongeographic variation within our locality samples (table 1). The magnitude of F values and their significance levels differ according to major anatomical regions of the skull: most measurements taken on the facial region (LR, BR, DR, LIF, BIF, LD), which experiences substantial postweaning growth, disclose clear mean differences with greater age and achieve moderate significance levels, whereas those dimensions measured on the neurocranium (BBC, BOC, DBC, IOB), whose maturation plateaus earlier in postna- tal development, exhibit slight or no increases 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 97 with age and generally insignificant F values. Dimensions of the molars (CLM, WM1), which decrease in crown height with occlusal abrasion after eruption, do not grow in length and width, unlike osseous components of the skull; in contrast, the incisors substan- tially increase in girth with age (table 1). In nearly all of the age-sensitive variables, the significant differences reside between the young adult and full adult or old adult age classes, but not between the full adult and old adult age classes (according to Bonferroni post hoc multiple comparison tests). Such a pattern is predicated by sample sizes avail- able for the three age groups and corre- sponding mean differences. We suspect that the underrepresentation of the old adult cohort in OTU N1 accounts for the fewer variables that exhibit significant age effects and the generally smaller F values. The imbalance among age classes and predomi- nance of young adults in OTU N1 typify age representation across all our analytical sam- ples (young adults comprise ca. 70%, full adults 25%, and old adults only 5% of all measured crania, respectively). Moderate to strong differences between the sexes were derived for most cranial measurements of Oryzomys couesi, males averaging 4%?8% larger than females de- pending upon the variable used as an index of size (table 1). Interestingly, molar dimensions (CLM, WM1) are among those few variables that yielded no significant differences, a finding that indicates that females possess more robust dentitions relative to the head skeleton. Humphrey and Setzer (1989), on the other hand, did obtain significant differ- TABLE 1 Arithmetic Means of External and Craniodental Variables and Results of One-way ANOVAs for Sex and Age Cohorts in Adult Oryzomys couesi mexicanus from Nayarit (OTU N1, n 5 62; Y, A, and O 5 young, full, or old adult age class, respectively; see Materials and Methods for variable abbreviations.) Sex Age M F Y A O Variable (N 5 31) (N 5 31) f(sex) (N 5 45) (N 5 13) (N 5 4) f(age) TOTL 254.3 235.3 19.2*** 241.7 248.5 268.5 4.1* TL 129.9 120.3 15.1*** 123.2 128.5 136.0 3.7* HFL 31.0 30.0 15.5*** 30.5 30.4 31.0 0.4 ONL 30.1 28.8 12.5** 29.2 29.8 31.8 6.9** ZB 16.0 15.2 17.7*** 15.5 15.7 16.5 2.9 BBC 12.6 12.4 4.4* 12.5 12.4 12.6 0.4 BOC 6.8 6.7 3.8 6.7 6.7 6.8 0.5 DBC 9.5 9.3 7.7** 9.4 9.4 9.6 0.6 IOB 4.8 4.6 18.4*** 4.7 4.7 4.8 0.7 LR 9.5 8.9 11.8** 9.1 9.3 10.3 7.2** BR 5.8 5.5 11.6** 5.6 5.7 6.2 4.6* DR 6.6 6.3 9.6** 6.4 6.5 7.1 6.5** LIF 6.0 5.7 10.2** 5.8 6.0 6.2 3.7* BIF 2.3 2.3 1.9 2.2 2.3 2.5 7.3** LD 7.9 7.3 18.5*** 7.5 7.8 8.3 5.6** PPL 10.0 9.5 11.6** 9.6 9.8 10.7 5.0** LBP 5.5 5.1 16.7*** 5.3 5.2 5.8 4.5* BBP 5.5 5.4 3.8 5.5 5.5 5.6 1.5 BZP 3.3 3.1 5.9* 3.1 3.3 3.5 6.8** LAB 4.1 4.0 1.1 4.0 3.9 4.2 3.8* CLM 4.56 4.54 0.2 4.55 4.50 4.67 1.6 WM1 1.29 1.30 0.3 1.30 1.29 1.32 0.2 DI 1.85 1.79 3.8 1.77 1.89 1.99 8.7*** * 5 P # 0.05; ** 5 P # 0.01; *** 5 P # 0.001. 98 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 ences for molar measurements, but their variable definition was an alveolar length that would reflect changes in the bony palate (ANOVAs of LBP were significant for age and sex effects; table 1). Our next largest sample of O. couesi disclosed the same pattern of sexual and age variation, variable differences according to anatomical region, and attained significance levels (J1, N 5 60 or 58; results not tabulated). The larger size of male Oryzomys was qualitatively noted by early researchers (Merriam, 1901; Goldman, 1918; Paradiso, 1960) and quantitatively verified by Humphrey and Setzer (1989) in subspecies of Oryzomys palustris. Further- more, growth studies of laboratory Oryzomys palustris have demonstrated that well-marked differences in weight and head-and-body length of males and females become apparent by approximately 3?4 weeks in age (Park and Nowosielski-Slepowron, 1972), or shortly after weaning when dispersing subadults are entering the trappable population. The oc- currence of strong sexual dimorphism in Oryzomys contrasts with the pattern of nongeographic variation reported for other oryzomyines, in which conspicuous age- related size differences are demonstrable, but size disparity between the sexes is statistically trivial (e.g., Microryzomys?Carle- ton and Musser, 1989; Oligoryzomys?Myers and Carleton, 1981, Carleton and Musser, 1995; Zygodontomys?Voss, 1991). On aver- age, there is a slight female bias (56%) among all adult skulls suitable for measurement, but the proportion varies appreciably among individual OTUs (33%?85% females). Humphrey and Setzer (1989) procedurally circumvented the problematic variation in- troduced by consistent sexual size dimor- phism in O. palustris by confining their analyses to adult males, an approach achiev- able with the large museum series available for a common North American species. Such selectivity proved impractical for several taxa of interest here and would severely limit their sample sizes for morphometric comparisons. Only four specimens of nelsoni and 21 of peninsulae will ever be available for study; the 26 examples of albiventer that we managed to assemble exemplify the relative rarity of this taxon in North American collections of Mexican Oryzomys. To explore possible biases in morphometric comparisons associ- ated with imbalanced sex cohorts, we ran three iterations of discriminant-function analysis (using only females, only males, and both sexes) on three morphologically distinctive subspecies of O. couesi in western Mexico that vary in proportions of the sexes (albiventer, OTU J3 [4U, 8-]; mexicanus, OTUs C2 [17U, 3-], J2 [5U, 15-], N1 [31U, 31-]; peninsulae, OTU BC [10U, 4-]). Comparable and unambiguous phenetic structure among these five OTUs was dis- closed in bivariate plots of the first two CVs extracted in each iteration (fig. 2), and the percent variation explained by the first two CVs is very similar in all three trials (81.1%? 82.4%). Moreover, the hierarchy of cranio- dental phenetic resemblance among the five OTUs, based on Mahalanobis? distances between group centroids, is fully congruent. The exact clustering distances and relative disposition of OTUs on the first two CVs do differ according to the sexual subset ana- lyzed, but such minor departures would be expected as a result of the different numbers of males and females available for each OTU and corresponding effects on covariance matrices. In summary, we combined the sexes and three adult age classes in the following morphometric comparisons in order to am- plify specimen numbers of the rarer taxa. Although the contributions of secondary sexual dimorphism and postweaning growth (age) to within-sample variation are statisti- cally appreciable, those sources of variation are typically negligible relative to the extract- ed factors that emerged as taxonomically informative among samples. Indeed, the discriminant-function trials using one or the other or both sexes largely previewed the results we obtained with the full data set. MORPHOMETRIC COMPARISONS: Pronounced disparity in size and pelage color was especially evident among populations in Jalisco, where key species-group taxa had been named (albiventer Merriam, 1901; mexicanus Allen, 1897), and initially sparked our suspicion about the possible existence of two species. Two well-defined morphologies were visually apparent in sorting locality samples: large- bodied, brightly colored specimens with broad skulls and robust molar rows versus 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 99 Fig. 2. Three iterations of discriminant-function analysis of select OTUs using only females, only males, and the sexes combined. Left column, projection of specimen scores onto the first two canonical variates extracted; right column, phenograms based on Mahalanobis distances among group centroids. See text for discussion. 100 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 Females CV1 1C0 a: 00 40 Mahalanobis D2 20 Males eo 40 Mahalanobis D2 J3 J3 CV1 Females + Males 80 40 Mahalanobis D2 J3 CV1 smaller individuals with narrower skulls and more delicate molars (compare measure- ments for OTUs J1 and J3, table 2). Princi- pal component ordination of all Jaliscan adults with intact skulls supplies additional evidence of substantial craniodental differen- tiation over small geographic distances. Projection of individual scores onto PCs I and II reveals two elliptical densities (fig. 3), one corresponding to specimens from the coastal plain and adjoining foothills in western Jalisco (5 mexicanus, ca. sea level to 1000 m) and the other to those from uplands in central and eastern Jalisco (5 albiventer, ca. 1200?1800 m). Although they marginally overlap, the major axes of these elliptical spreads are phenetically dis- crete (fig. 3), their Y-intercepts being signif- icantly different between taxa (0.46 versus ? 1.68; F 5 25.2, P , 0.001) while their slopes are comparable (0.65 versus 0.41; F 5 1.4, P 5 0.245). This covariation pattern among the specimens of albiventer and mexicanus is a familiar one that recalls multivariate con- trasts recovered for other closely related, congeneric species of Muroidea (Voss et al., 1990; Voss and Marcus, 1992; Carleton and Musser, 1995; Carleton et al., 1999). Predict- ably, all variables load positively on the first component and most correlations are mod- erate to large (r 5 0.55?0.98; table 3), indicating a general size factor. Although juveniles were omitted from our analyses, some postweaning growth still accounts for much of the elongate scatter observed within the species samples and for the oblique orientation of the group constellations on the first and second principal components. Those variables that load heavily on PC II, more or less orthogonal to the major axes of the taxon samples, emphasize the robust molars (CLM, WM1) of albiventer and, to a lesser extent, the relatively broader expanse of the incisive foramina (BIF) of mexicanus (tables 2, 3). The type specimen of albiventer (USNM 82236) nests comfortably among other large-bodied specimens from inland Jalisco (fig. 3). That of mexicanus (AMNH 2650/2128) was too damaged to include in the PCA, but its small molars (CLM 5 4.63 mm, WM1 5 1.26 mm) conform to the smaller Oryzomys found commonly throughout the coastal lowlands of western Jalisco. We expanded our geographic perspective by including other samples of the broadly ranging mexicanus and two forms with localized distributions in western Mexico, nelsoni from the Tres Mar??as Islands and peninsulae from the Baja Peninsula. The only analytically suitable locality series of albiven- ter remains Merriam?s (1901) original type series (N 5 12) from Ameca, Jalisco. Discriminant-function analysis of the 11 OTUs representing these taxa disclosed three well-defined clusters in the first three canon- ical variates extracted (fig. 4), which collec- tively summarize about 75% of the between- sample variation. The largest cluster consists of the eight samples that represent mexicanus. Their centroids are closely approximate and they broadly overlap one another in canon- ical scores; a posteriori misclassifications among the eight is commonplace (24%?92% incorrect depending upon the OTU). The mexicanus aggregation is entirely separate from OTU J3 of albiventer along the first canonical variate, a hiatus mainly reflecting the uniformly larger size of the latter (tables 2, 4), a distinction also emphasized in the first component derived from PCA of all Jaliscan specimens. Although the sample of peninsulae is intermediate to those of mexicanus and albiventer in external size (table 2), cranial size and shape differences contribute to its sharp isolation from the latter OTUs along canonical variates 1?3 (fig. 4). Larger size in nearly all variables sets apart peninsulae from the various OTUs of mexicanus along the first canonical variate (correlations positive, coefficients moderately large; table 4). A subset of dimensions conveys certain shape differences that affect separation of peninsu- lae along the next two latent variates, including a deeper braincase (DBC), broader rostrum (BR), and expansive bony palate (LBP, BBP). Noteworthy are the absolutely and relatively broad incisive foramina (BIF) that characterize specimens of peninsulae, features that also contribute to the morpho- metric isolation of the Baja sample, especially on the second canonical variate (table 4). The incisive foramina in peninsulae even average wider than the large-bodied taxa albiventris 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 101 TABLE 2 External and Craniodental Measurements (in mm) of Samples of the Oryzomys couesi Complex in Western Mexico (Sample statistics include the mean, 6 1 SD, and the observed range; variable abbreviations are defined in Materials and Methods; column headings reflect our taxonomic recommendations.) O. couesi mexicanus O. peninsulae O. albiventer O. nelsoni Jalisco Nayarit Baja California Jalisco Mar??a Madre Variable (J1, N 5 58) (N1, N 5 62) (BC, N 5 14) (J3, N 5 12) (TM, N 5 4) TOTL 245.7 6 25.1 244.8 6 19.5 265.6 6 22.3 285.4 6 22.2 322 6 28.8 195?301 210?288 227?305 245?314 282?344 TL 132.3 6 13.2 125.1 6 10.7 136.8 6 10.8 155.4 6 14.3 181.5 6 14.6 102?160 105?150 114?156 129?173 160?191 HBL 113.3 6 12.8 119.7 6 11.0 128.9 6 12.3 130.0 6 8.2 140.5 6 14.8 91?141 96?144 113?152 116?142 122?153 HFL 30.2 6 1.7 30.5 6 1.2 32.0 6 1.2 36.1 6 1.9 37.3 6 1.7 26?34 27?33 29?34 33?40 35?39 ONL 29.4 6 1.4 29.5 6 1.5 31.5 6 1.7 32.9 6 1.3 34.8 6 2.5 26.4?32.5 26.7?33.2 27.8?34.3 30.0?34.5 31.7?37.8 ZB 15.8 6 0.9 15.6 6 0.8 17.1 6 0.9 17.6 6 0.6 18.0 6 1.0 13.8?18.0 13.9?17.9 15.1?18.7 16.6?18.3 16.6?18.9 BBC 12.5 6 0.4 12.5 6 0.4 12.9 6 0.2 13.2 6 0.3 13.4 6 0.3 11.7?13.5 11.7?13.3 12.4?13.3 12.8?13.8 13.0?13.6 BOC 6.7 6 0.3 6.7 6 0.2 6.8 6 0.2 7.1 6 0.2 7.2 6 0.3 6.1?7.3 6.3?7.2 6.2?7.0 6.9?7.6 6.8?7.6 DBC 9.1 6 0.3 9.4 6 0.4 9.9 6 0.4 9.7 6 0.3 10.0 6 0.2 8.3?9.8 8.4?10.5 9.4?10.8 9.3?10.1 9.9?10.2 IOB 4.7 6 0.2 4.7 6 0.2 4.9 6 0.2 4.8 6 0.2 5.2 6 0.3 4.1?5.1 4.3?5.3 4.7?5.3 4.5?5.1 4.8?5.6 LR 9.2 6 0.6 9.3 6 0.7 10.0 6 0.7 10.3 6 0.7 11.3 6 0.9 8.3?10.7 8.2?11.1 8.4?11.1 9.0?11.4 9.9?11.9 BR 5.8 6 0.4 5.6 6 0.4 6.4 6 0.5 6.1 6 0.3 7.0 6 0.6 5.0?6.7 4.8?6.9 5.4?7.4 5.4?6.5 6.2?7.5 DR 6.6 6 0.4 6.5 6 0.4 6.8 6 0.5 7.1 6 0.5 8.1 6 0.7 5.8?7.5 5.7?7.4 5 .7?7.7 6.2?7.7 7.0?8.7 LIF 5.8 6 0.3 5.9 6 0.4 6.4 6 0.5 6.5 6 0.4 6.4 6 0.3 5.0?6.5 5.0?6.7 5.5?7.3 5.7?6.9 6.1?6.9 BIF 2.2 6 0.2 2.3 6 0.1 2.6 6 0.2 2.4 6 0.2 2.5 6 0.1 1.9?2.7 2.0?2.7 2.3?3.2 1.9?2.6 2.4?2.6 LD 7.4 6 0.5 7.6 6 0.6 8.3 6 0.7 8.3 6 0.7 8.8 6 0.7 6.1?8.6 6.4?9.1 7.1?9.7 7.2?9.5 8.0?9.6 PPL 9.8 6 0.7 9.7 6 0.7 10.3 6 1.0 11.0 6 0.6 11.5 6 1.0 8.0?11.4 8.3?11.3 8.6?12.5 9.5?11.7 10.5?12.7 LBP 5.5 6 0.3 5.3 6 0.3 5.9 6 0.3 6.2 6 0.3 6.5 6 0.6 4.9?6.2 4.6?6.2 5.2?6.3 5.6?6.7 5.7?7.2 BBP 5.6 6 0.2 5.5 6 0.2 6.1 6 0.2 6.0 6 0.2 6.2 6 0.2 5.2?5.9 5.1?6.0 5.5?6.6 5.7?6.2 5.9?6.4 BZP 3.1 6 0.2 3.2 6 0.3 3.3 6 0.3 3.7 6 0.2 4.2 6 0.3 2.6?3.6 2.6?3.8 2.7?3.9 3.5?4.0 3.7?4.5 LAB 4.0 6 0.1 4.0 6 0.2 4.1 6 0.2 4.5 6 0.2 4.4 6 0.3 3.7?4.4 3.5?4.4 3.9?4.5 4.2?4.8 4.1?4.6 CLM 4.60 6 0.14 4.56 6 0.17 4.72 6 0.11 5.24 6 0.19 4.92 6 0.12 4.23?4.94 4.22?4.91 4.53?4.94 4.74?5.51 4.75?5.00 WM1 1.35 6 0.05 1.30 6 0.07 1.36 6 0.05 1.53 6 0.06 1.52 6 0.04 1.21?1.45 1.04?1.40 1.26?1.44 1.45?1.64 1.47?1.56 DI 1.75 6 0.12 1.81 6 0.14 2.18 6 0.21 2.06 6 0.13 2.31 6 0.18 1.53?1.98 1.54?2.19 1.83?2.59 1.83?2.26 2.06?2.47 102 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 and nelsoni (table 2). Clustering (UPGMA) based on Mahalanobis? distances between centroids does indicate a closer phenetic affinity between peninsulae and OTUs of mexicanus, compared with albiventris and nelsoni, and simultaneously its strong differ- entiation from any one of them (fig. 5). In principal-component and discriminant-func- tion analyses confined to samples of peninsu- lae and mexicanus (not illustrated), contrast in incisor girth (DI) emerges as an important variable for separation of the two taxa (table 4), in addition to those already men- tioned. No specimen of peninsulae was mistakenly associated with another OTU according to post hoc probabilities of group membership. Variable loadings that influence the isola- tion of albiventris similarly apply to the OTU of nelsoni, insofar as the first three variates extracted are concerned (fig. 4). Compared with examples of mexicanus and peninsulae, specimens of nelsoni and albiventris are large Oryzomys that possess more robust skulls and notably heavier molars (table 2), differ- ences evidenced in the loadings on canonical variates one and two (table 4). Clear-cut circumscription of the four specimens of nelsoni from those of albiventer, however, is captured on the fourth and fifth variates (not plotted), which principally emphasize the massive development of the rostrum (LR, WR, DR, BZP) and upper incisors (DI) in nelsoni (tables 2, 4). Given the scope of taxa Fig. 3. Projection of specimen scores onto first two axes extracted from principal component (PC) ordination of 20 log-transformed variables as measured on intact skulls (N 5 108) representing Oryzomys couesi albiventer and O. c. mexicanus from Jalisco, Mexico. The position of the type specimen (holotype) of albiventer Merriam (USNM 82236) is indicated by ??ht.?? See table 3 for variable correlations and variance explained. 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 103 A albiventer ? mexicanus 1 - CO O -2 - Y = 0.65X + 0.456 Y = 0.41X-1.678 -2 0 1 PC I (72.2%) under consideration, covariation among these variables emerged as minor compared with the overarching patterns of between- group variation summarized by the first three canonical variates. Nonetheless, their unique and consistent expression in nelsoni accounts for its strong differentiation in the pheno- gram generated from Mahalanobis? distances between OTU centroids (fig. 5). The weak clustering association obtained between nel- soni and albiventer approximates that derived for peninsulae and OTUs of mexicanus. Those type specimens with intact skulls were entered as unknowns in a 10-group discrimi- nant-function analysis for classification ac- cording to a posteriori probabilities of group membership. The OTU of O. nelsoni was excluded as a classificatory option; further- more, DI was omitted in calculating discrim- inant coefficients because the incisors in the type specimen of aztecus (USNM 51173) could not be measured. Type material of the three species-group taxa that originate within coastal plain were all definitely assigned to mexicanus (fig. 6), including the holotypes of bulleri Allen (AMNH 6663 to N3) and rufus Merriam (USNM 91404 to N2), as well as the three original topotypes of O. couesi lambi Burt (UCLA 51201 to N2, UCLA 51026 to C2, UCLA 51038 to J1); none demonstrated even a low level of affinity for the OTUs of albiventer or peninsulae. Those taxa described from upland localities in central Mexico were variously affiliated (fig. 6). The specimen of molestus Elliot (FMNH 8667), described from Ocotla?n, Jalisco, was dispersed just peripheral to the type series of albiventer (Ameca, Jalisco) but definitely classified with that OTU (P 5 1.000); the type of albiventer Merriam (USNM 82236) naturally clustered among the original type series. Type speci- mens of aztecus Merriam (USNM 51173, Yautepec, Morelos) and regillus Goldman (USNM 125945, Los Reyes, Michoaca?n) plotted at the margin of our large represen- tation of mexicanus, but their canonical scores statistically aligned them with those OTUs (the greatest affinity of both with J2), not with albiventer. Finally, the holotype of crinitus Merriam (USNM 50182, Tlalpan, Distrito Federal) was associated with the series of peninsulae, at an unexpectedly strong level of probability (P 5 0.996). The large values measured for the incisive foramen width (BIF), rostral breadth (BR), and braincase depth (DBC) of this specimen (table 5), variables that importantly contribute to the discrimination of mexicanus and peninsulae (see above), presumably influenced its multi- variate disposition and a posteriori classifica- tion in peninsulae. DISCUSSION TAXONOMIC RECOMMENDATIONS AND FUTURE STUDY: We recognize four species among our study samples from western Mex- ico: Oryzomys albiventer Merriam, O. couesi mexicanus J.A. Allen, O. nelsoni Merriam, and O. peninsulae Thomas (figs. 7?9). These forms can be objectively defined by a combination of qualitative morphological traits, demonstrate cohesive patterns of morphometric identity, TABLE 3 Correlations (Loadings) of 20 Log-transformed Craniodental Variables with Derived Principal Components (PC) Extracted from Ordination Using Intact Specimens (N = 108) of Oryzomys couesi albiventer and O. c. mexicanus in Jalisco, Mexico (See fig. 3; variable abbreviations are defined in Materials and Methods.) Variable PC I PC II ONL 0.98*** 0.03 ZB 0.94*** 20.02 BBC 0.75*** 20.09 BOC 0.71*** 20.05 DBC 0.84*** 20.03 IOB 0.59*** 0.19 LR 0.93*** 0.15 BR 0.87*** 0.28 DR 0.92*** 0.18 LIF 0.87*** 0.12 BIF 0.65*** 0.40** LD 0.94*** 0.15 PPL 0.93*** 0.18 LBP 0.81*** 20.25 BBP 0.84*** 20.22 BZP 0.91*** 20.18 LAB 0.68*** 20.26 CLM 0.64*** 20.59*** WM1 0.55*** 20.65*** DI 0.87*** 20.11 Eigenvalues 0.069 0.006 % variance 72.2 6.3 *** P # 0.001; ** P # 0.01. 104 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 Fig. 4. Results of 11-group discriminant function analysis based on 20 log-transformed measurements of intact specimens (N 5 246) representing the Oryzomys couesi group from western Mexico: top, projection of specimen scores on canonical variates (CV) 1 and 2; bottom, projection of specimen scores on canonical variates 2 and 3. Individual scores are plotted for the four known specimens of O. nelsoni; minimum convex polygons enclose individual scores around the centroid for OTUs of albiventer and peninsulae. To avoid visual clutter only centroids are indicated and a single bounding polygon encloses maximum dispersion for all specimens of mexicanus. See table 4 for variable correlations and variance explained. 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 105 \mexicanus openinsulae xnelsoni Aalbiventer CV 1 (34.5%) 2 - CM c, o CO > O -2 - I */ I I 1 \ J3 N2 \~ X ? . 2> N4 N3 C1. N1 * - I I l BC O 1 CV2(26.1%) and occupy discrete geographic ranges (see emended diagnoses, distributions, and re- marks under Taxonomic Summary). Such well-marked contrasts stand at odds with the conventional notion of intergrading subspe- cies as conveyed in past studies and influential classifications (Goldman, 1918; Hall, 1981). The genesis of the widely distributed, polytyp- ic view of O. palustris, encompassing couesi and most other Middle American taxa, hinged on a geographically and taxonomically limited study (Hall, 1960) that was uncritically expanded in the latter 1900s (Handley, 1966; Hershkovitz, 1971; Hall, 1981), compelled more by the revisionary habit of the era than by fresh analyses of character data. Consid- ered in this historical context, the sharp differentiation we encountered among western Mexican taxa, unexamined in any systematic detail since Goldman?s (1918) revision, is unsurprising. These taxa have received mini- mal study since their descriptions; they are not ??cryptic?? species. As now understood (i.e., Weksler et al., 2006, plus species recognized herein), Oryz- omys is a moderately speciose genus (8 species to date considered valid). The distri- bution of most species is confined to Neo- tropical environments of Middle America, but one inhabits temperate settings in south- eastern North America (O. palustris) and another occupies the northernmost margin of South America (O. gorgasi). Even so recon- stituted, Oryzomys still contains several species-level uncertainties. The widely rang- ing, polytypic definition of Oryzomys palus- tris, as classically formulated in Hall (1981), reminds us of other Middle American species whose composite nature has been emphati- cally documented by recent researches. For example, Oryzomys rostratus has been sepa- rated from O. ??melanotis?? (Engstrom, 1984); TABLE 4 Correlations (Loadings) of 20 Log-tranformed Craniodental Variables with Derived Canonical Variates (CV) (Based on 11-group discriminant function analysis of intact specimens, N 5 246, representing Oryzomys in western Mexico; see fig. 4 for scatter plots of CVs 1?3. Variable abbreviations are defined in Materials and Methods.) Variable CV 1 CV 2 CV 3 CV 4 CV 5 ONL 0.69*** 20.07 20.09 20.22 0.27** ZB 0.65*** 20.09 20.22 20.19 0.15 BBC 0.61*** 20.07 20.05 0.09 0.25** BOC 0.24* 20.11 20.01 0.13 0.20 DBC 0.65*** 0.29*** 0.04 0.05 0.13 IOB 0.46*** 0.17 20.03 20.33*** 0.27** LR 0.59*** 20.11 20.14 20.16 0.28** BR 0.46*** 20.05 20.38*** 20.42*** 0.29*** DR 0.55*** 20.16 20.06 20.33*** 0.43*** LIF 0.62*** 0.05 20.15 20.25** 0.01 BIF 0.56* 0.35*** 20.22* 20.23* 0.16 LD 0.62*** 0.05 20.12 20.23* 0.12 PPL 0.51*** 20.11 20.04 20.35*** 0.15 LBP 0.65*** 20.29*** 20.31*** 0.00 0.25** BBP 0.70*** 20.05 20.37*** 20.28** 0.16 BZP 0.62** 20.08 0.12 20.44*** 0.13 LAB 0.62*** 20.21 0.03 20.21 0.03 CLM 0.76*** 20.40*** 20.09 20.11 20.12 WM1 0.62*** 20.56*** 20.23* 20.07 0.00 DI 0.75*** 0.20 20.15 20.16 0.24** Eigenvalues 2.19 1.66 0.91 0.73 0.41 Canonical Correlations 0.83 0.79 0.69 0.65 0.54 % variance 34.5 26.1 14.2 12.4 6.4 *** P # 0.001; ** P # 0.01; * P # 0.05. 106 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 Sigmodon arizonae, S. hirsutus, S. mascoten- sis, and S. toltecus have been recovered from S. ??hispidus?? (Zimmerman, 1970; Carleton et al., 1999; Peppers and Bradley, 2000; Peppers et al., 2002; Carroll et al., 2005); and Peromyscus aztecus, P. beatae, P. levipes, P. madrensis, P. sagax, P. simulus, and P. spicilegus have been resurrected as valid species, all of them formerly masked under P. ??boylii?? (Carleton, 1977, 1979; Carleton et al., 1982; Rennert and Kilpatrick, 1987; Schmidly et al., 1988; Bradley et al., 1989, 1996; Castro-Campillo et al., 1999). In similar fashion, future taxonomic research upon Oryzomys, in particular the O. couesi section, will undoubtedly disclose greater systematic diversity, and some of those research avenues might explore or solidify the following points: (1) The remoteness of the Oryzomys population that once existed at the tip of Baja California, far removed from the core generic distribution in mainland Mexico, prompted Nelson (1922: 124) to remark that it ??may readily have been introduced in shipments of bananas or other farm products from Mazatlan or elsewhere along the main- land coast, where a related species [i.e., O. couesi mexicanus] is plentiful.?? His conjecture on the nonnative status of the peninsular form may have influenced Hall (1981) to arrange it within a polytypic O. palustris. However, the morphological distinctiveness and morphometric separation of peninsulae that we observed are substantial and instead portray a species that evolved in situ, isolated appreciably longer from the mainland distri- bution of Oryzomys than a recent anthropo- genic introduction would suggest. The phenetic linkage of O. couesi mexica- nus and O. peninsulae offers a preliminary kinship hypothesis to be tested with other data, and two biogeographic scenarios de- serve consideration in reconstructing the latter?s relict distribution. Oryzomys peninsu- lae may have differentiated from an ancestral stock whose distribution once covered the Baja Peninsula, extreme southwestern United States, and western Mexico. Several rodent species or species complexes today exhibit a similar distributional pattern (e.g., see ranges in Lawlor, 1983: appendix 10), suggesting that Pleistocene climatic changes and subse- quent range contraction could have isolated Fig. 5. Pattern of phenetic similarity among the 11 OTUs of Oryzomys from western Mexico based on UPGMA clustering of the squared Mahalanobis distances among group centroids as derived from discriminant-function analysis (see fig. 4). Currently recognized species-group taxa are shown above the appropriate stem subtending terminal OTUs. 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 107 peninsulae mexicanus albiventer nelsoni Baja California Colima 1 Collma 2 Jalisco 1 Jalisco 2 Nayarit 4 Nayarit 1 Nayarit 2 Nayarit 3 Jalisco 3 Tres Marias 60 50 40 30 20 10 Mahalanobis D an Oryzomys population at the tip of the peninsula. The extension of O. couesi mex- icanus along the coastal plain in western Mexico, as far north as west-central Sonora (fig. 7), further intimates the former continu- ity of Oryzomys populations on the mainland surrounding the Sea of Cortez. Alternatively, O. peninsulae may have originated when the southernmost cape region of the Baja Peninsula was positioned as an island approximate to the Nayarit- Jalisco coast in the late Miocene (Carren?o and Helenes, 2002). Geologic and tectonic evidence indicates that northwestward rifting (beginning approximately 5.5 million years ago) and accelerated seafloor spreading (beginning approximately 3.5 million years ago) eventually brought the insular cape block to its present peninsular terminus, a geological nonconformity demarcated by the Isthmus of La Paz. The biotic legacy of the former juxtaposition of the southern cape block and west-central Mexico is suggested by the current distributions of certain plants and birds (Cody and Velarde, 2002; Cody et al., 2002). Phylogeographic analysis, using ancient DNA, may disclose sufficiently fine-scale genetic affinities to choose among these or other biographic scenarios. Fig. 6. Dispersion of type specimens of Oryzomys named from western Mexico based on a posteriori classification using discriminant coefficients extracted from 10-group discriminant function analysis of albiventer, couesi mexicanus, and peninsulae. Minimum convex polygons enclose individual scores around the group centroid (or grand centroid in the case of couesi mexicanus). See text for discussion. 108 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 mexicanus O peninsulae ? albiventer m 2 CM CM 5 0 molestus CV1 (31.5%) T A B L E 5 M ea su re m en ts a (m m ), S ex , an d A ge of T yp e S pe ci m en s of th e O ry zo m ys co ue si G ro up in W es te rn M ex ic o (Y , A , an d O 5 yo u n g, fu ll , o r o ld ad u lt ag e cl as s, re sp ec ti ve ly . V ar ia b le ab b re vi at io n s ar e d ef in ed in M at er ia ls an d M et h o d s. ) O . al bi ve nt er O . ne ls on i O . pe ni ns ul ae O . co ue si m ex ic an us O . c. f. co ue si al bi ve nt er m ol es tu s b ne ls on i pe ni ns ul ae m ex ic an us bu ll er i ru fu s la m bi c cr in it us az te cu s re gi ll us U S N M F M N H U S N M B M N H A M N H A M N H U S N M U C L A U S N M U S N M U S N M 82 23 6 86 67 89 20 0 98 .3 .1 .1 07 26 50 /2 12 8 83 59 /6 66 3 91 40 4 to p o ty p es 50 18 2 51 17 3 12 59 45 S ex M M M M M M F ? M M M A ge A O A A A Y O ? Y Y A T O T L 28 8 32 5 34 2 29 8 24 9 24 2 25 0 22 6? 22 9 30 7 29 0 30 5 T L 15 3 17 0 19 0 19 0 14 2 12 7 13 6 11 1? 11 6 16 1 15 4 16 9 H F L 37 .5 38 38 34 30 27 28 29 ?3 0 37 35 36 E L ? 15 ? 18 13 11 ? 17 ? ? ? O N L 32 .7 36 .1 37 .7 33 .8 ? 28 .0 28 .1 30 .2 ?3 0. 4 32 .6 32 .3 33 .5 Z B 17 .9 19 .4 18 .6 18 .3 ? 14 .9 15 .9 15 .9 ?1 6. 5 17 .0 16 .9 18 .2 B B C 13 .7 14 .1 13 .3 13 .2 12 .6 12 .7 12 .2 12 .1 ?1 2. 4 13 .4 13 .0 13 .6 B O C 7. 2 7. 7 7. 2 ? ? 6. 4 6. 4 6. 7? 6. 8 6. 9 7. 2 7. 2 D B C 10 .2 10 .6 9. 9 ? ? 8. 7 9. 3 8. 8? 8. 9 10 .6 9. 9 10 .1 IO B 5. 0 5. 5 5. 5 5. 2 4. 7 4. 5 4. 8 4. 6? 4. 7 5. 0 5. 0 5. 0 L R 10 .6 11 .8 12 .4 9. 7 ? 8. 9 8. 6 9. 6? 9. 8 10 .3 10 .4 10 .9 B R 6. 4 7. 2 7. 2 7. 1 6. 3 5. 1 5. 7 5. 8? 6. 1 6. 5 6. 3 6. 9 D R 7. 3 8. 1 8. 7 ? ? 5. 9 6. 5 6. 5? 6. 6 7. 3 7. 2 7. 5 L IF 6. 9 7. 8 6. 9 7. 0 6. 4 5. 3 6. 3 6. 0? 6. 3 7. 0 6. 7 6. 7 B IF 2. 6 2. 2 2. 4 2. 9 1. 5 1. 8 2. 3 2. 1? 2. 4 2. 8 2. 6 2. 5 L D 8. 8 10 .1 9. 6 8. 8 8. 2 6. 8 7. 5 8. 1? 8. 5 9. 0 8. 5 9. 0 P P L 11 .1 12 .9 12 .7 12 .1 ? 8. 9 9. 3 10 .1 ?1 0. 3 11 .3 11 .2 11 .6 L B P 5. 9 6. 6 7. 2 6. 0 5. 7 4. 8 4. 9 5. 1? 5. 4 5. 6 6. 0 6. 3 B B P 6. 2 ? 6. 0 6. 5 5. 6 5. 4 5. 3 5. 6? 5. 8 5. 9 5. 9 6. 2 B Z P 3. 5 3. 9 4. 3 3. 6 3. 5 2. 7 3. 2 3. 2? 3. 3 3. 1 3. 2 3. 7 L A B 4. 5 4. 9 4. 4 4. 4 4. 2 3. 9 4. 3 4. 2? 4. 6 4. 8 4. 3 4. 5 C L M 5. 17 5. 60 4. 96 5. 22 4. 63 4. 46 4. 24 4. 32 ?4 .7 7 4. 87 4. 97 4. 77 W M 1 1. 58 1. 63 1. 56 1. 53 1. 26 1. 28 1. 26 1. 40 ?1 .4 6 1. 44 1. 44 1. 44 D I 2. 01 2. 02 2. 43 ? 2. 07 1. 76 1. 74 1. 76 ?1 .8 9 1. 84 ? 1. 95 a E xt er n al m ea su re m en ts as o b ta in ed fr o m sk in ta gs o r gi ve n in o ri gi n al d es cr ip ti o n s; cr an io d en ta l m ea su re m en ts ta k en b y th e au th o rs . b C ra n io d en ta l m ea su re m en ts su p p li ed b y W .T . S ta n le y, F M N H . c B as ed o n th re e o ri gi n al to p o ty p es id en ti fi ed in B u rt ?s (1 93 4) su b sp ec ie s d es cr ip ti o n (U C L A 51 02 1, M , Y A ; U C L A 51 02 6, F , Y A ; U C L A 51 03 8, F , A ). 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 109 (2) The Oryzomys population that once inhabited Isla Mar??a Madre, the largest of the Tres Mar??as archipelago off Nayarit, has been accepted as a species by most authors since Merriam (1898) first described it as O. nelsoni. In spite of the small sample size (only four specimens known), the differentiation of O. nelsoni is sharply circumscribed from mainland forms and sustains its continued recognition as a species, following Merriam (1901) and Goldman (1918) and contra Hershkovitz (1971). In retaining nelsoni as a species, Goldman (1918) considered it to be closely allied to the nearest mainland form, O. couesi mexicanus. However, by its size, O. nelsoni invites careful comparison with O. albiventer, a species whose distribution is currently confined to the interior plateau region of west-central Mexico (fig. 7), re- moved from the coastal lowlands where the progenitor of the island species must have once occurred. The two were paired as the phenetically most similar OTUs in all multi- variate analyses of craniodental variables (figs. 4, 5), and this linkage provides a working hypothesis of relationship to be tested with other data. Peromyscus madrensis, another rodent endemic of the Tres Mar??as Islands, was apparently isolated early during the radiation of the P. boylii species group (Tiemann-Boege et al., 2000). In like manner, perhaps O. nelsoni represents an old lineage within the diversification of western Mexican Oryzomys, diverging from the common ancestor that also gave rise to O. albiventer. (3) We elected a conservative course and retained mexicanus Allen as a subspecies of O. couesi in lieu of the detailed analyses and Fig. 7. Distribution of four species of Oryzomys here recognized in western Mexico, based on specimens examined by us. Gray shading represents the same elevation zones depicted in figure 1. 110 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 Species of Oryzomys A 0. o/bfyenfr/s # OLcouM/mex/conus + O. ne/son/ ^ O.pen/n^u/oe -, _ # jy F ig . 8. D o rs al (u p p er ro w ) an d ve n tr al (l o w er ro w ) cr an ia l vi ew s (c a. 2 3 ) o f a d u lt O ry zo m ys re p re se n ti n g sp ec ie s h er e re co gn iz ed fr o m w es te rn M ex ic o : A ,O .a lb iv en te r (U S N M 82 24 0, O N L 5 33 .8 m m ), a yo u n g ad u lt m al e fr o m Ja lis co ,A m ec a, 40 00 ft ; B ,O .n el so ni (U S N M 89 20 1, O N L 5 34 .6 m m ), an ad u lt fe m al e fr o m N ay ar it , Is la M ar ??a M ad re , 80 0 ft ; C , O . pe ni ns ul ae (U S N M 14 66 10 , O N L 5 32 .3 m m ), a yo u n g ad u lt m al e fr o m B aj a C al if o rn ia S u r, S an Jo se? d el C ab o ; D , O . c ou es im ex ic an us (U S N M 50 94 18 , O N L 5 32 .2 m m ), a yo u n g ad u lt m al e fr o m N ay ar it , 1 m iS C u au tl a, n ea r se a le ve l. 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 111 broader geographic canvas required to clarify its status. Visual comparisons of skins and skulls suggested that, among the four species in western Mexico, examples of mexicanus most closely resemble those of O. couesi proper, whose type locality is Coban, Gua- temala. Recent studies of Sigmodon ??hispi- dus?? and Peromyscus ??boylii?? cast suspicion on such a conclusion: subspecies distributed along the western versus eastern versants of Mexico in those so-called species have been demonstrated to be distinct from one another (see above references). In particular, the distribution of O. couesi mexicanus is exten- sively congruent with that of Sigmodon mascotensis (Carleton et al., 1999), its range penetrating the interior along the basin of the R??o Balsas and following the coastal plain to the Isthmus of Tehuantepec. In this case, however, examples of the large-bodied S. mascotensis are readily distinguishable from the smaller S. toltecus where their ranges approach one another in the region of the isthmus (Carleton et al., 1999; Peppers and Bradley, 2000). No such morphological dichotomy is apparent to us based on inspection of O. couesi specimens from the eastern lowlands of Mexico, the Isthmus of Tehuantepec, and Guatemala, including those that belong to the currently recognized subspecies couesi and zygomaticus (e.g., Hall, 1981). Other data sources and a broader geographic perspective are needed, one that stresses additional sampling in the Isthmus of Tehuantepec where couesi mexicanus and O. c. couesi are thought to intergrade (Goldman, 1918; Hall, 1981). Acquisition of topotypic material from the type localities of the several forms in eastern and southern Mexico placed in full synonymy under O. c. couesi?jalapae Allen and Chapman (1897), jalapae rufinus Merriam (1901), jalapae apatelius Elliot (1904), goldmani Merriam (1901), teapensis Merriam (1901)?may prove critical. Farther south, the status and relationships of several other epithets will bear on delim- Fig. 9. Lateral cranial views (ca. 2 3) of the same specimens adult Oryzomys illustrated in figure 8: A, O. albiventer; B, O. nelsoni; C, O. peninsulae; D, O. couesi mexicanus. 112 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 iting the southern distributional occurrence of O. couesi proper. Jones and Engstrom (1986) considered richmondi Merriam (1901), whose type locality is in southeastern Nicar- agua (Zelaya, 50 mi above Bluefields), to be another synonym that is inseparable from O. couesi couesi. Our examination of that type (USNM 36340/48705) indicates that it is an example of O. couesi, but evaluation based on denser geographic sampling is necessary before one might comment on subspecific divisions. Another Oryzomys described from southeastern Nicaragua (Zelaya, 7 mi below Rama) is the enigmatic O. dimidiatus Thomas (1905), known by only two specimens, but investigators who have examined the taxon have retained it as a valid species (Genoways and Jones, 1971; Jones and Engstrom, 1986; Musser and Carleton, 1993; Sa?nchez et al., 2001). In reporting the rediscovery of O. gorgasi Hershkovitz (1971), Sa?nchez et al. (2001) confirmed its distinction from other forms described from southern Cen- tral America, including dimidiatus Thomas (1905), gatunensis Goldman (1912), and richmondi Merriam (1901), the latter two taxa now placed under O. couesi. The distribution of O. gorgasi plausibly extends into southern Central America, and museum series from this region should be reexamined with the fresh perspective supplied by Sa?n- chez et al. (2001). (4) Within his arrangement of O. couesi, Goldman (1918: 37) identified a complex of related subspecies from central Mexico: ??It [O. c. crinitus] is closely allied to the other forms inhabiting river valleys of the plateau region of Mexico?O. c. aztecus, O. c. albiventer, and O. c. regillus.?? As argued above, we acknowledge O. albiventer to be specifically distinct from O. couesi mexicanus, the form distributed along coastal plain and lower mountain slopes in western Mexico (fig. 7). Of the four taxa described from interior uplands, O. albiventer Merriam (1901: 279) is an older name than regillus Goldman (1915) and has page priority over crinitus Merriam (1901: 281) and aztecus Merriam (1901: 282). Evidence for the synonymy of these forms with O. albiventer is so far unpersuasive, however. Post hoc classification of the type speci- mens of the latter three taxa, based on 19 craniodental variables, associates them with O. couesi mexicanus (aztecus and regillus) or with O. peninsulae (crinitus), not with O. albiventer. The holotypes and type series of aztecus and regillus consist of large individ- uals whose dimensions measure at the upper limits of variation observed within typical O. couesi mexicanus (compare tables 2 and 5), and it is zoogeographically plausible that these names simply apply to more robust populations of the same species where they occupy higher elevations. Whether they represent yet another species within the O. couesi group must await improved sample sizes and additional data sources to discern broader patterns of variation and critically decide their status. We do not accept that populations represented by the type of crinitus, described from over 2000 m in the Valley of Mexico, and the series of peninsu- lae, described from near sea level at the tip of Baja California, form one and the same biological species. As in the case of aztecus and regillus, we view these discriminant- function results as indicative of taxonomic problems in need of resolution and the probable existence of greater species diversity than offered by our predefined OTUs. The recent photograph of a large, long-tailed Oryzomys from the Distrito Federal (Medel- l??n and Medell??n, 2006: 710), verifies that populations referable to crinitus Merriam still persist in the Valley of Mexico and invite elucidation of their systematic relationships. (5) Oryzomys fulgens Thomas (1893) is a little-known and overlooked taxon, but clarification of its status will prove pivotal in deciding nomenclatural issues that concern albiventer and the other upland taxa, espe- cially crinitus. Thomas described fulgens based on one of the three specimens origi- nally used by Alston (1876) in his character- ization of Hesperomys couesi. In doing so, Thomas selected a lectotype for H. couesi (specimen ??a?? 5 BMNH 75.2.26.15) and fixed its type locality (Coban, Guatemala). The type locality of the specimen that he designated as the type of O. fulgens (??b?? 5 BMNH 70.6.20.3), collected by the French naturalist Adolphe Boucard (1839?1905), was given only as ??Mexico, exact locality unknown.?? Thomas supposed that it origi- nated from ??southern Mexico, probably in or 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 113 near the Valley of Mexico,?? and his suppo- sition about the source of Alston?s specimen ??b?? has been repeated in the subsequent taxonomic literature on Oryzomys (Merriam, 1901; Goldman, 1918; Miller and Kellogg, 1955; Hall, 1981). No one has since offered a formal restriction of the type locality, how- ever. In spite of the possible origin of fulgens from the Valley of Mexico, Merriam (1901) proceeded to name another species, O. crinitus, from the same region (Distrito Federal, Tlalpan) and suspected that it ??will prove only a subspecies of fulgens.?? Merriam had not examined the type of fulgens but based his impression of their close similarity on Thomas? description, which he quoted in full. Apparently the indeterminate type lo- cality of fulgens persuaded Merriam to name crinitus as a full species based on a well- prepared specimen newly collected by Nelson and Goldman, conveniently at hand, and with undoubted provenience. When nearly all Middle American forms of Oryzomys were viewed as races of the older taxa O. couesi (Alston, 1876) or O. palustris (Harlan, 1837), the imprecise geo- graphic source of the junior name fulgens Thomas (1893) was nomenclaturally incon- sequential. Its possible senior status among Oryzomys taxa named from uplands in interior Mexico now invites careful archival research on Boucard?s travels to see whether a definite type locality can be established and the name can be critically applied to Oryz- omys populations. Having satisfied that crucial step, the relationship of fulgens to albiventer, crinitus, and aztecus can be addressed and synonymies allocated if ap- propriate. (6) Individuals of Oryzomys commonly inhabit coastal wetlands (Sharp, 1967; Wolfe, 1982, 1985), are adept swimmers (Svihla, 1931; Hamilton, 1946; Esher et al., 1978), and are known to emigrate between near-shore islands (Forys and Dueser, 1993). Their semiaquatic habits and ecological preference readily ex- plain the common presence of Oryzomys on continental-shelf islands and perhaps the one oceanic occurrence (O. antillarum on Ja- maica). Several of these peripheral isolates acquired species-group epithets, all of which were hastily reallocated as subspecies of the all-inclusive O. palustris (Hershkovitz, 1971; Hall, 1981). As demonstrated for O. antillarum by Morgan (1993) and documented herein for O. nelsoni and O. peninsulae, however, the differentiation of these insular and peninsular forms is appreciable and contradicts their continued maintenance under the one main- land species O. couesi. Other insular and peninsular forms (azuerensis, cozumelae, and gatunensis) now retained within O. couesi similarly deserve objective assessment of their status. The generic assignment of the subfossil form described from Curac?ao (McFarlane and Debrot, 2001), a continental-shelf island in the Netherlands Antilles, and its possible synon- ymy with O. gorgasi also warrant attention (see comments in Musser and Carleton, 2005: 1148, and Weksler et al., 2006: 4). CONSERVATION NOTES: Oryzomys nelsoni is known by only four specimens of the original type series (Merriam, 1898), ob- tained by Nelson and Goldman from Mar??a Madre Island in May 1897, and is considered extinct (Ceballos et al., 2002; Alvarez-Casta- n?eda and Me?ndez, 2003; IUCN, 2006). The specimens were collected in moist herbaceous vegetation surrounding springs located on the upper slopes (, 245 m) of Mar??a Madre (Nelson, 1899a, 1899b), the largest of the Tres Mar??as Islands. No other dedicated inventory of small mammals occurred until Don E. Wilson led a Biological Survey field team to the Tres Mar??as in March 1976. Wilson and colleagues located the area described by Nelson where O. nelsoni had been discovered but collected only Rattus rattus; Wilson (1991) inferred that the abun- dant presence of the commensal may have contributed to the extinction of O. nelsoni. Another Tres Mar??as endemic rodent, Per- omyscus madrensis, was recovered in moder- ate numbers on Isla Mar??a Madre along with Rattus rattus (Carleton et al., 1982; Wilson, 1991). Current evidence holds little promise that populations of O. peninsulae persist. The form is documented from only two localities, Santa Anita and San Jose? del Cabo, in southernmost Baja California. The original series (6 specimens) from Santa Anita, the type locality (Thomas, 1897), was collected by Dane Coolidge (1873?1940), a freelance field collector and writer, in May 1896. The 15 USNM specimens from San Jose? del Cabo 114 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 were obtained by Nelson and Goldman in January 1906; the companions afterward traveled to Santa Anita but failed to collect any topotypic examples of O. peninsulae. The localities are about 13 km apart and both were situated along the R??o San Jose, a permanent river around the turn of the century that flowed through the predomi- nantly arid landscapes in southern Baja California (Nelson, 1922; Goldman, 1951). Goldman (1918: 46) reported that ??The range of the species seems to be centered in the marshes near the mouth of the San Jose? River at San Jose? del Cabo, extending a few miles inland along the course of the stream,?? and that ??The neighboring coast is extremely arid and unsuitable for habitation by an Oryzomys.?? Subsequent habitat changes over the latter half of the 20th century in southern Baja California have been dramatic and would be inimical for a semiaquatic rodent tied to such a precarious ecological niche. The river no longer exists, having been sapped dry for irrigation projects, and the marshy lagoon at its mouth in San Jose? del Cabo is polluted commensurate with devel- opment of the shoreline for tourism (Alvarez- Castan?eda, 1994). Two deliberate attempts to rediscover peninsulae in the late 1900s proved unsuccessful: Michael A. Bogan and a Bio- logical Survey field team visited San Jose? del Cabo for one intensive night of collecting in 1979, and Ticul Alvarez-Castan?eda (1994) surveyed habitats in and around both Santa Anita and San Jose? del Cabo several times over 1991?1993. The vulnerability of O. peninsulae has been obscured by its confusion under O. couesi, a species viewed as broadly distributed and accordingly classified as Lower Risk?least concern (IUCN, 2006). The conservation status of O. peninsulae must be considered critically endangered, if not extinct, in view of its lack of documentation in over a century, its restricted geographic occurrence, and the environmental deteriora- tion that has befallen its preferred habitat. Of the two remaining species we identify within western Mexico, populations of O. couesi mexicanus appear to deserve the IUCN designation of ??Lower Risk?least concern.?? Large series of this species have been documented within the latter 1900s, e.g., from lowlands of Nayarit, Colima, and Jalisco. On the other hand, we encountered few examples of O. albiventer in North American collections, only 26 specimens representing 11 localities. Most of these represent material collected by Nelson and Goldman in Jalisco over the years 1897? 1903, supplemented by scattered specimens obtained by KU field crews over the years 1949?1969. The distribution of O. albiventer largely conforms to the sloping plateau, ca. 1200?1900 m, surrounding Lago de Chapala and its greater drainage basin. Given its hydrographic advantages and more mesic climate, the region has long attracted large- scale agricultural development, and Goldman (1951) described the dramatic environmental changes that had transpired between his early (1902?1903) and later visits (1926, 1935) to the area (see especially accounts for La Barca and Ocotla?n). Although populations must be highly fragmented, we believe that the species survives in pockets of appropriate habitat. Efforts should be devoted to surveying the current status of O. albiventer, including refinement of its distribution based on vouchered specimens contained in Mexican collections. This refinement necessarily in- volves resolution of its relationship to the other forms described from central Mexico (see topics 4 and 5 above). TAXONOMIC SUMMARY Synonymies are presented below for the four species of Oryzomys recognized in western Mexico and trace their earliest identification and first subsequent usage in other name combinations. Emended diagno- ses are presented for those two (O. albiventer and O. peninsulae) that have long been confused with O. palustris or O. couesi (e.g., fide Goldman, 1918, and Hall, 1981). We regard the status of aztecus Merriam, crinitus Merriam, and regillus Goldman from central Mexico as unresolved and provisionally retain them within O. couesi sensu lato (appendix 1). Specimens examined include all individuals personally seen and identified by us. An emended diagnosis is also given for the phylogenetically restricted construct of Oryzomys. See Weksler et al. (2006) for character traits and diagnoses of the new genera that have been removed from the 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 115 polyphyletic shell that so long characterized ??Oryzomys?? in its traditional usage (Mer- riam, 1901; Goldman, 1918; Hall, 1981). Oryzomys Baird, 1857 Mus, Harlan, 1837: 385 (part, description of palustris). Arvicola, Bachman, 1854 [in Audubon and Bach- man, 1846?1854]: 214 (part, description of oryzivora). Hesperomys (Oryzomys) Baird, 1857: 482. Oryzomys, Allen, 1890: 187 (first usage as genus). TYPE SPECIES: Mus palustris Harlan, 1837, by original designation. EMENDED DIAGNOSIS: A clade of medi- um-sized oryzomyine species (Cricetidae: Sigmodontinae: Oryzomyini) characterized, in combination, by: supraorbital ridging well developed and interorbital shape cuneate; incisive foramina long, extending to or between the alveoli of the M1s; bony palate long, projecting behind the posterior margins of M3s, and posterolateral palatal pits conspicuous and internally perforated; ecto- tympanic bulla medium-sized, revealing ven- tral-medial wedge of the periotic; zygomatic plate broad, forming a moderately deep notch, and zygomatic arches sturdy, with small but consistently formed jugal; interpa- rietal small and narrow, about half as wide as the caudal border of parietals; alisphenoid strut absent; carotid arterial circulatory pattern derived, lacking supraorbital and infraorbital branches (sphenofrontal foramen and squamosal-alisphenoid groove absent, stapedial foramen absent or minute, posteri- or opening to alisphenoid canal irregularly formed or occluded). Molars pentalophodont (mesolophs/ids well developed) and cuspi- date, cusps bunodont with lingual and labial series unevenly terraced; anterocone/id broad, anteromedian flexus/id absent, ante- roloph discrete and well formed; satellite rootlets present on M1 (4-rooted) and m1 (3? 4-rooted). Eight mammae present (pectoral, postaxial, abdominal, and inguinal pairs); pedal digits 1 and 5 notably shorter than central three; ungual tuft of hairs sparsely developed, covering less than half of the nail; plantar surface squamate and pads small, hypothenar pad minute or absent. DISTRIBUTION: Palustrine, lacustrine, and riverine habitats in the southeastern United States, throughout much of Mexico and Central America, to northwestern South America (western Colombia and northwest- ern Venezulela), including many continental- shelf islands. CONTENTS: albiventer Merriam, 1901 (in- cluding molestus Elliot, 1903); antillarum Thomas, 1898; couesi Alston, 1876 (including apatelius Elliot, 1904; aquaticus J.A. Allen, 1891; aztecus Merriam, 1901; azuerensis Bole, 1937; bulleri J.A. Allen, 1897; cozumelae Merriam, 1901; crinitus Merriam, 1901; ful- gens Thomas, 1893; gatunensis Goldman, 1912; goldmani Merriam, 1901; jalapae J.A. Allen and Chapman, 1897; lambi Burt, 1934; mexicanus J.A. Allen, 1897; peragrus Mer- riam, 1901; pinicola A. Murie, 1932; regillus Goldman, 1915; richardsoni J.A. Allen, 1910; richmondi Merriam, 1901; rufinus Merriam, 1901; rufus Merriam, 1901; teapensis Mer- riam, 1901; zygomaticus Merriam 1901); dimidiatus Thomas, 1905 ; gorgasi Hershko- vitz, 1971; nelsoni Merriam, 1898; palustris Harlan, 1837 (including argentatus Spitzer and Lazell, 1978; coloratus Bangs, 1898; floridanus Merriam, 1901; natator Chapman, 1893; oryzivora Bachman, 1854 [in Audubon and Bachman, 1854]; planirostris Hamilton, 1955; sanibeli Hamilton, 1955; texensis J.A. Allen, 1894); peninsulae Thomas, 1897. REMARKS: In the early taxonomic litera- ture on North American forms (Merriam, 1901; Goldman, 1918), Baird?s Oryzomys was broadly conceived and included species groups and subgenera now removed to separate genera (Gardner and Patton, 1976; Carleton and Musser, 1989; Weksler, 2003, 2006; Weksler et al., 2006). The phylogenet- ically reified understanding of Oryzomys corresponds to the association of species placed in the palustris-mexicanus or palustris group, as designated, respectively, by Mer- riam (1901) and Goldman (1918). Thus, O. albiventer, O. couesi mexicanus, O. nelsoni, and O. peninsulae share with O. palustris those derived conditions that characterize the strict concept of Oryzomys (Carleton and Musser, 1989; Carleton and Olson, 1999; Sa?nchez et al., 2001) and that were amplified to infer the phylogenetic position of the genus within the oryzomyine radiation (Weksler, 2006). 116 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 The four western Mexican species further exhibit the trenchant characteristics of the couesi section within the O. palustris group, as denoted by Goldman (1918: 20) based on several pelage and cranial traits. In the palustris section, the pelage color is predom- inantly grayish brown, with long glistening guard hairs conspicuously overtopping the body fur; the pelage color is generally reddish brown, with the guard hairs only slightly longer than the body fur in the couesi section. Spacious, oval sphenopalatine vacu- ities perforate the walls of the mesopterygoid fossa, exposing much of the presphenoid bone in specimens of O. palustris (e.g., see character 36: state 0, as per Weksler, 2006); in forms of the couesi section, sphenopala- tine vacuities are absent or weakly expressed as short, narrow slits near the basisphenoid- presphenoid suture (character 36: states 1 and 2, as per Weksler, 2006). Other differ- ences reinforce Goldman?s perception of a division. Oryzomys couesi and the western Mexican species all possess a small, circular sphenopalatine foramen in contrast to the larger, ovate shape of the foramen as observed in skulls of O. palustris. Schmidt and Engstrom (1994) drew attention to the utility of this contrast in separating crania of O. palustris texensis and O. couesi aquaticus where their populations overlap along coast- al Texas, but the contrast holds between O. palustris and the four species in western Mexico as well. We noticed that the two sections also differ in the development of the anterolabial cingulum on the lower third molar: consistently present and forming a distinct ledge in the couesi section; absent or represented as a short spur in members of the palustris section (see Carleton and Musser, 1989: 44?45, for description and illustration of the condition in O. palustris). Plantar pads of the hindfoot are small in species of Oryzomys compared with many other oryzomyines, and the hypothenar pad is typically absent in O. palustris or tiny if present (Carleton and Musser, 1989; Weks- ler, 2006). A hypothenar pad is consistently present in all members of the couesi section for which we could ascertain its condition, its size slightly smaller than interdigital pads 1 and 4. Weksler (2006) scored incipient webbing as present between pedal digits 2?3 and 3?4 in his sample of O. palustris but absent in that of O. couesi, and Sa?nchez et al. (2001) indicated that such webbing also characterized O. dimidiatus and O. gorgasi. Based on examinations of the hind feet on dried skins and even fluid-preserved speci- mens of the taxa in western Mexico, we are uncertain about the condition of this trait. Certainly, webbing is not so obviously developed as in O. palustris, but slight webbing seemed to be present between digits 3?4 in some specimens. This character definition and variation should be critically reviewed within Oryzomys to better under- stand its taxonomic significance. In summary, the palustris and couesi ??sections?? of Goldman?s palustris group are sufficiently characterized morphological- ly to maintain them as the palustris and couesi species groups within the genus Oryzomys as now phylogenetically diag- nosed. For the character variation summa- rized above, the palustris group displays more derived conditions and may possess more semiaquatic adaptations than the couesi group. Only O. palustris is currently recognized as a valid species within the palustris group; the principal alpha system- atic uncertainty involves the status of argentatus Spitzer and Lazell (1978), a form described from the Florida Keys that has been treated as species or subspecies (see Musser and Carleton, 2005, for review). The other seven species fit with the couesi group, but as underscored by our findings in western Mexico, comprehension of species limits warrants additional research. Oryzomys albiventer Merriam, 1901 Oryzomys albiventer Merriam, 1901: 279. Type locality: Mexico, Jalisco, Ameca, 4000 ft; holotype: USNM 82236 (skin and skull), a male collected 6 February 1897 by E.W. Nelson and E.A. Goldman. Oryzomys couesi albiventer, Goldman, 1918: 38 (name combination, allocated as valid subspe- cies). Oryzomys palustris albiventer, Hall, 1960: 173 (name combination, listed as valid subspecies). Oryzomys molestus Elliot, 1903: 145. Type locality: Mexico, Jalisco, Ocotla?n, 5000 ft; holotype: FMNH 8667 (skin and skull), a male collected June 1901 by F.E. Lutz (not examined). 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 117 Oryzomys couesi albiventer, Goldman, 1918: 38 (molestus allocated as full synonym, invalid as subspecies). Oryzomys palustris albiventer, Hall, 1981: 609 (molestus listed as full synonym, invalid as subspecies). EMENDED DIAGNOSIS: A species of the O. couesi group characterized by a combination of large size (HFL < 34?38 mm), tail relatively and absolutely long (TL < 140? 170 mm), skull heavily constructed (ONL < 31?34 mm), and molars correspondingly robust (CLM < 5.0?5.4 mm). DISTRIBUTION: Central and eastern Ja- lisco, southern Guanajuato, and northern Michoaca?n at intermediate elevations, 1200? 1800 m (fig. 7). REMARKS: Oryzomys albiventer is a large, brightly colored species with a long tail. Merriam?s (1901) key diagnostic feature, ??underparts white,?? and consequent choice of the specific name are somewhat misleading. The basal portion of ventral hairs is pale gray and the distal half a pure white in the type series from Ameca. The white tips do not obscure the basal gray and, to us, create a general effect of pale gray to grayish-white underparts. In the specimens from Ocotla?n, the gray bases are more prominent and the underparts appear medium gray. Other aspects of Merriam?s description are accurate, notably the large skull and long and wide molar rows which he emphasized in his comparisons with nearby species. Goldman (1918) repeated these salient contrasts in his rearrangement of albiventer, mexicanus, and regillus as subspe- cies of O. couesi in western Mexico. Goldman (1918) reallocated molestus El- liot (1903) as a full synonym of Merriam?s (1901) albiventer (then a subspecies of O. couesi), and we concur with their synonymy. Elliot based his new species on a single specimen, but four other individuals in USNM from Ocotla?n, the type locality of molestus, conform to his description. The type specimen of molestus is a strikingly large individual (table 5), but it is, as noted by Goldman, an old adult and a male. Mea- surements of the type fit within the size range of O. albiventer as we understand the species, albeit at or toward the upper limits for most variables; still, discriminant-function analysis decidedly assigned the type to the albiventer OTU. Two specimens from localities in northern Michoaca?n (Zamora and Quere?n- daro), which Goldman (1918) referred to O. c. regillus, are here considered examples of O. albiventer proper. Although the distributional limits of O. albiventer require further documentation, the species so far appears to be endemic to the Mesa de Ana?huac, a small inland plateau (1200?1900 m) that is enclosed by southern ranges of the Sierra Madre Occidental and by the Transverse Volcanic Range. Character- ized by a subhumid climate and subtropical vegetation, the region corresponds to one of the five zoogeographic zones that Guerrero Va?zquez et al. (1995) identified within Jalisco. SPECIMENS EXAMINED: Mexico, Guana- juato: Salvatierra, 5775 ft (KU 66597). Jalisco: Ameca, 4000 ft (USNM 82235? 82246); 6 km NE Contla, 1310 m (KU 120611, 120612); 2 mi N and 0.5 mi W Guadalajara, 5200 ft (KU 31782, 31783); 27 mi S and 12 mi W Guadalajara (KU 31784); La Barca (USNM 129038); 2 mi SE Mascota, 5200 ft (KU 100509); Ocotla?n, 5000 ft (USNM 120160?120163); 4 km W Tuxpan, 1380 m (KU 120613). Michoaca?n: Quere?n- daro (USNM 50180); Zamora, 4000 ft (USNM 120308). Oryzomys couesi mexicanus J.A. Allen, 1897 Oryzomys mexicanus J.A. Allen, 1897: 52. Type locality: Mexico, Jalisco, Tonila, Hacienda San Marcos, 3500 ft; holotype: AMNH 2650/2128 (skin and skull), a male collected 30 December 1889 by Audley C. Buller. Oryzomys couesi mexicanus, Goldman, 1918: 33 (name combination, allocated as valid subspe- cies). Oryzomys palustris mexicanus, Hall, 1960: 173 (name combination, listed as valid subspecies). Oryzomys bulleri J.A. Allen, 1897: 53. Type locality: Mexico, Tepic [Nayarit], Valle de Banderas; holotype: 8329/6663 (skin and skull), a male collected 2 February 1893 by Audley C. Buller. Oryzomys couesi mexicanus, Goldman, 1918: 33 (bulleri allocated as full synonym, invalid as subspecies). Oryzomys palustris mexicanus, Hall, 1981: 610 (bulleri listed as full synonym, invalid as subspecies). Oryzomys rufus Merriam, 1901: 287. Type locality: Mexico, Tepic [Nayarit], Santiago, 200 ft; 118 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 holotype: USNM 91404 (skin and skull), a female collected 20 June 1897 by E.W. Nelson and E.A. Goldman. Oryzomys couesi mexicanus, Goldman, 1918: 33 (rufus allocated as full synonym, invalid as subspecies). Oryzomys palustris mexicanus, Hall, 1981: 610 (rufus listed as full synonym, invalid as subspe- cies). Oryzomys couesi lambi Burt, 1934: 107. Type locality: Mexico, Sonora, San Jose? de Guaymas; holotype: California Academy of Sciences 51024 (skin and skull), a female collected 20 January 1933 by C.C. Lamb (not examined). Oryzomys palustris lambi, Hall, 1960: 173 (name combination, listed as valid subspecies). DISTRIBUTION: Coastal plain and adjoin- ing lower Pacific-facing slopes from central Sonora to southeastern Oaxaca; interior river valleys to central Michoaca?n, southern Morelos, southern Puebla, and northwestern Oaxaca (fig. 7); most localities from sea level to 1000 m, exceptional occurrence at 1525 m in the Sierra de Autla?n, Jalisco. REMARKS: J.A. Allen (1897) named mex- icanus and bulleri as new species based on one specimen of the former and two of the latter. Although their type localities are relatively near one another in western Mexico, he differentiated mexicanus with respect to O. palustris and bulleri to O. couesi rather than to one another. Working with larger series and better geographic representation, Gold- man (1918) recognized the fundamental similarity of Allen?s taxa and adopted mex- icanus as the subspecific epithet, with bulleri as full synonym, for the populations of smaller Oryzomys widely distributed along Pacific coastal plain. Although the damaged Fig. 10. Scatter plot of coronal length of molar rows versus length bony palate (in mm) for specimens of O. albiventer and O. couesi mexicanus from Jalisco. Values for type specimens of taxa described from Jalisco (albiventer Merriam, mexicanus Allen, molestus Elliot) and nearby Nayarit (bulleri Allen, rufus Merriam) are indicated. 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 119 6.0 5.5 CO < fe Mi x h- z LU 4.5 4.0 1 1 1 1 A 1 ? 0. albiventer 0 Holotypes molestus # 0. mexicanus A 0 1 A 4* albiventer . A ? s A A A ? A 0 A A A A * * ?? * ? ?*" ? J - % ? * ? 1 ? rufus 1 ? 1 # # i 4.5 5.0 55 6.0 LENGTH OF BONY PALATE 6.5 7.0 skull of the type of mexicanus prevented its use in our morphometric analyses, bivariate scatterplots of measurable variables depict both mexicanus and bulleri as falling within the range of samples drawn from coastal plain (fig. 10), in contrast to those originating from the inland plateau region (i.e., albiventer and molestus). Merriam (1901: 287) considered his new species rufus, based on a single specimen, to be ??similar in general to bulleri but smaller and redder.?? The type specimen is a small individual (table 5) but falls within the variational sphere of the larger series from Nayarit and Jalisco, whether viewed from a univariate or multivariate perspective (figs. 6, 10). The contrasts that impressed Merriam can be largely attributed to the age and gender differences of the types of bulleri (young adult male) and rufus (old adult female) and the tendency of the pelage of aged Oryzomys to become more reddish. Again, we agree with Goldman?s (1918) synonymy of rufus under O. couesi mexicanus. The three topotypes of lambi Burt (1934) at hand generally resemble other examples of the Oryzomys species that inhabits coastal low- lands in western Mexico (e.g., fig. 6, table 5), here called O. couesi mexicanus, and we include it among the synonyms of this ??subspecies.?? Our maintenance of a trinomial is pro forma in this instance because we cannot here resolve the larger issue of the taxonomic and distributional limits of O. couesi sensu stricto. The subspecific range of lambi is depicted as restricted to its type locality in west-central Sonora, whether as a race of O. couesi (Hall and Kelson, 1959; Alvarez-Castan?eda and Corte?s-Calva, 1999) or O. palustris (Hall, 1981). We note that certain specimens from northern Sinaloa (e.g., 2 mi E Costa Rica, 3 mi N Higueras) are larger than those from southern Sinaloa and farther south along coastal plain in Nayarit, Jalisco, and Colima, and may belong with Burt?s lambi. Denser sampling in central Sinaloa and refined analyses are needed to determine the relationships of these popula- tions and whether lambi deserves formal subspecific retention. Goldman (1918) viewed the distribution of the subspecies mexicanus to adhere to Pacific coastal plain and contiguous foothills, a range conforming to his Arid Lower Tropical Zone (and Goldman, 1951) and essentially repeated in later taxonomic compendia of mammals (Hall and Kelson, 1959; Hall, 1981). Howev- er, specimens from certain localities that Goldman allocated to aztecus also represent O. couesi mexicanus proper in our judgment, and these reallocations extend its range into interior Mexico along the valleys of the R??os Tepalcatepec and Balsas. A geographic range that covers coastal plain, Pacific-facing lower slopes of western sierras, and the Balsas Depression resembles other cricetid rodents found in western Mexico (e.g., Sigmodon mascotensis, Hodomys alleni, Peromyscus perfulvus, Osgoodomys banderanus) and pre- sumably reflects a shared biogeographic history. The pattern is nearly exactly congru- ent to the biotic zone distilled from the distributions of the Mexican herpetofauna (Flores-Villela, 1993) but is not so precisely mirrored in those recognized in biogeographic syntheses of birds and mammals (Escalante Pliego et al., 1993; Fa and Morales, 1993). SPECIMENS EXAMINED: Mexico, Colima: Armer??a, 200 ft (USNM 33284/45312?33285/ 45313, 33292/45320?33297/45325); 4 km S and 2 km W El Chavar??n (CM 79555, 79556); Hacienda Magdalena, 1500 ft (USNM 33299/ 45327, 33300/45328); 5 mi N Manzanillo (KU 87515?87534); Paso del R??o, 200 ft (UMMZ 100656?100669, 100689); 3 mi N Santiago (KU 87490?87492, 87653); 1 mi NE Santiago, 10 ft (KU 36867?36873); 2 km E Santiago (KU 87498); 3 km E Santiago (KU 87499? 87514); 1 mi S and 4 mi W Santiago, 10 ft (KU 36874); 12 km W Santiago (KU 87493? 87497). Guerrero: 5 km N Agua del Obispo, 3250 ft (KU 99899); 5.5 km N Agua del Obispo, 3400 ft (KU 99898); near Ometepec, 200 ft (USNM 71400?71405, 71419); Zihua- tanejo Bay (UMMZ 80926, 80927, 80929, 80931); Petacalco, sea level (UMMZ 113850? 113852); Tlalixtaquilla, 4200 ft (USNM 70277); 2 km SSE Zihuatanejo, Las Gatas (KU 35371?35379). Jalisco: 11 mi SW Autla?n, 2000 ft (KU 105742); Sierra de Autla?n, 5000 ft (UMMZ 95476, 95477); Cihuatla?n, 15 ft (KU 109262?109265); 15 KM NW Cihuatla?n (87454); Cuitzamala, 25 ft (KU 105743? 105763); 2 km NW Emiliano Zapata, 20 m (KU 120615); Jilotla?n de Los Dolores, 2400 ft (KU 111830, 111831); 8 mi E Jilotla?n de Los 120 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 Dolores, 2000 ft (KU 111832?111837); 6 mi E Limo?n, 2700 ft (KU 103799); 2 mi S La Cuesta, 2400 ft (KU 111829); 2 mi N La Resolana, 1500 ft (UMMZ 95478, 95479); 2 mi SW La Resolana, 1100 ft (KU 40125, 40126?40129, 40138, 40139); 1 mi N Barra de Navidad (KU 97242?97244, 97246?97248); 3 mi NW Barra de Navidad, 30 ft (KU 97239? 97241); 5 km NNW Barra de Navidad (KU 87455?87465, 87467?87472, 87474?87489); 0.5 mi N Navidad, 25 ft (UMMZ 100654, 100655, 101268?101270); 3 mi E Navidad, 50 ft (UMMZ 100641?100653); Navidad Bay (UMMZ 80912?80914, 80924); 14.5 mi S Pihuamo, 1100 ft (KU 111820?111827); 4 mi NNE Puerto Vallarta, 50 ft (KU 64494? 64498); 4 mi SW Puerto Vallarta, 20 ft (KU 111828); 20 km WNW Purificacio?n, 1400 ft (KU 93021, 93652); 30 km N and 10 km E Santiago (KU 87489); El Tabaco, 200 ft (KU 107669?107674); Tenacatita Bay (UMMZ 80897, 80906, 80911). Michoaca?n: 16 mi S Arteaga, 800 ft (KU 40132?40137, 40140); 0.5 mi SE Coalcoma?n, 3600 ft (UMMZ 100670? 100672); near La Huacana (USNM 126497? 126501); 1 mi E La Mira, 300 ft (KU 40141? 40144); 9 mi S Lombard??a, 1500 ft (KU 38652); 7 mi S Tumbiscatio, 2700 ft (KU 40130); Parque Nacional Uruapan (UMMZ 110560, 110561). Morelos: Puente de Ixtle (USNM 126943, 127496, 127497). Nayarit: Arroyo de Jiguite, R??o Santiago, 100 m (USNM 523637, 523638); Aticama, 10 ft (KU 36860?36864); Chacala, 30 m (USNM 509467?509473, 523631?523636); 1 mi S Cuautla, near sea level (USNM 509389? 509450, 510551?510560); 1.2 mi S (by road) El Casco, R??o Chilte, 480 ft (USNM 511645? 511652, 511751, 511752, 511884?511889); El Refilion, 850 m (USNM 509463?509466); 3.5 mi E (by road) El Venado, 100 m (USNM 509455, 510562); Estanzuela, 1380 m (USNM 509474); 1 mi E Ixtlan, 4000 ft (UMMZ 95475); 2 mi E Jalcocotan, 500 m (USNM 523624?523630, 524358, 524359); 1 mi NE Las Piedras, R??o Tigrera (USNM 523639, 523640); 5 mi S Las Varas, 150 ft (KU 40124); 8 mi SSW Las Varas (KU 64480?64482); 4 mi NE San Blas, 100 ft (UMMZ 103184? 103187); 8.8 mi E San Blas, Paso de Soquilpa, 100 m (USNM 509456?509462, 510563); 10 mi E Ruiz, Platanares (KU 85830, 85831, 85837); 4 mi E Playa Los Corchos (USNM 553966, 554089); San Blas, 75 ft (USNM 89206?89208, 89240); 0.5 E San Blas, 10 ft (KU 36852?36859); 1 mi S San Francisco, 50 ft (KU 64490?64493); 2 mi SW San Juan de Abajo (USNM 512558?512561, 512759, 512760); 2 mi E San Pedro Lagunillas, E side lake, 1300 m (USNM 509474?509493, 510564?510567); 15 km S Santa Cruz, Playa Colorado (USNM 553963?553965, 554088); Santiago, 200 ft (USNM 91403, 91404); Teponahuaxtla (USNM 509451?509454, 510561); 2 mi WNW Valle de Banderas (KU 64483?64489). Oaxaca: Cuicatla?n, 1800?1900 ft (USNM 69654, 69660); 1 km N Cuicatla?n, 560 m (KU 29490?29492); 3 km NNE Cuicatla?n, 600 m (KU 29487?29489); 1 km S Cuicatla?n, 590 m (KU 29493); 2 km SSW Cuicatla?n, 570 m (KU 29494); Huilotepec, 100 ft (USNM 73408, 73726); Llano Grande, 200 ft (USNM 71406, 71407, 71921, 73727, 73730); Plumas, 3000 ft (USNM 71415, 71417, 71418); Puerto Angel, 100?200 ft (USNM 71408?71414, 73745); Tehuantepec (USNM 73407). Puebla: Piaxtla, 3900 ft (USNM 70273?70276). Sinaloa: 2 mi E Costa Rica, 100 ft (KU 100502?100508); 24 km S Gua- save, 20 ft (KU 90316); 3 mi N Higueras, 30 ft (KU 96669?96674); 6 mi NNW Teacpan, Isla Palmito del Verde (KU 98960, 98961); 6 mi W La Concha, 10 ft (KU 98962, 99156); 5 mi NW Mazatla?n (KU 85825?85829); Rosario, 100 ft (USNM 91405); 8 km N Villa Unio?n, 450 ft (KU 96099, 96100); 1 mi N Zaragoza, 30 ft (KU 90314, 90315). Sonora: San Jose? de Guaymas (UCLA 51021, 51026, 51038). Oryzomys nelsoni Merriam, 1898 Oryzomys nelsoni Merriam, 1898: 15. Type locality: Mexico, Nayarit, Tres Mar??as Islands, Mar??a Madre Island, 800 ft; holotype: USNM 89200 (skin and skull), an adult male collected 13 May 1897 by E.W. Nelson and E.A. Goldman. Oryzomys palustris nelsoni, Hershkovitz, 1971: 704 (name combination, listed as valid subspecies). DISTRIBUTION: Known only from Isla Mar??a Madre, the largest of the Tres Mar??as, Nayarit. REMARKS: Merriam (1898, 1901) aptly captured the distinguishing characteristics of his new form, in particular its large skull, massive and strongly decurved rostrum, and large, broad incisors (figs. 8, 9). Dimensions 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 121 that reflect Merriam?s qualitative descrip- tions, particularly those measured on the rostrum, figure prominently among the several variables that account for isolation of the four specimens of O. nelsoni in discriminant-function analyses (table 4). Merriam also mentioned the island form?s large body size, long tail, and big hind feet; in relative and absolute size, nelsoni possesses the longest tail among the species of the O. couesi group in western Mexico (table 2). Examples of O. albiventer lack such a robust rostrum and upper incisors as found in O. nelsoni but possess heavier molars (table 2). We must accept that the morphology evident in the small museum series of the Tres Mar??as Oryzomys is typical of the species. Sadly, O. nelsoni represents an instance where the investigator?s sample statistics and popula- tion parameters are one and the same. SPECIMENS EXAMINED: Mexico, Nayarit: Tres Mar??as Islands, Isla Mar??a Madre (USNM 89200?89203). Oryzomys peninsulae Thomas, 1897 Oryzomys peninsulae Thomas, 1897: 548. Type locality: Mexico, Lower California [Baja Cali- fornia Sur], Santa Anita; holotype: BMNH 98.3.1.107 (skin and skull), a male collected on 23 May 1896 by Dane Coolidge. Oryzomys palustris peninsulae, Hershkovitz, 1971: 704 (name combination, listed as valid subspe- cies). Oryzomys couesi peninsularis, Alvarez-Castan?eda, 1994: 99 (incorrect subsequent spelling, assess- ment of conservation status). Oryzomys couesi peninsulae, Alvarez-Castan?eda and Corte?s-Calva, 1999: 448 (name combina- tion, treated as valid subspecies). EMENDED DIAGNOSIS: A species of the O. couesi group characterized by a combination of moderate size (larger than O. couesi mexicanus, smaller than O. albiventer) and dorsal pelage of head and forequarters predominantly gray toned. Cranium with deep braincase, zygomatic arches broadly spreading and squarely shaped, and incisive foramina long and exceptionally broad. Upper incisors relatively deep and curvature more nearly orthodont. DISTRIBUTION: Known only from two localities in southernmost Baja California Sur, Nayarit. REMARKS: Merriam (1901) and Goldman (1918) were clearly impressed by the differenti- ating traits of O. peninsulae, as emphasized by Thomas (1897), and many of their qualitative characterizations of its distinctly shaped skull contributed to its morphometric separation (e.g., ??high braincase,?? ??large and broadly open palatal slits,?? ??rather large molars??). The pelage contrast between the grayish-brown forequarters and fulvous brown hindquarters is still appreciable in the century-old, now slightly faded skins that Nelson and Goldman collected at San Jose?del Cabo and is unmatched for its constancy in other western Mexican Oryzomys. The orthodont shape of the upper incisors (fig. 9) impresses us as also diagnostic of the species, but the feature was not mentioned by the aforementioned authors. The Oryzomys population isolated at the southernmost tip of the Baja Peninsula was maintained as a valid species until grandiosely synonymized under O. palustris by Hershko- vitz (1971). Hall (1981: 610?611) later pro- vided some post hoc rationale for its subspe- cific rank within O. palustris and considered ??the degree of difference between O. peninsu- lae and O. palustris mexicanus of the eastern coast of the Gulf of California to be less than between some intergrading subspecies of O. palustris (e.g., O. palustris texensis and O. p. aquaticus).?? Hall?s precept for the synonymy of peninsulae, the purported intergradation of texensis and aquaticus, has proven to be erroneous, the latter forms instead represent- ing nonintrogressing races of the species O. palustris and O. couesi, respectively (Benson and Gehlbach, 1979; Schmidt and Engstrom, 1994). As we document above, the morpho- logical identity of O. peninsulae is sharply and uniquely circumscribed relative to popula- tions of O. couesi mexicanus and other Oryzomys in western Mexico. Such a ??degree of difference?? clearly advises specific recog- nition until controverted by other evidence. SPECIMENS EXAMINED: Mexico, Baja Cal- ifornia Sur: Santa Anita (BMNH 98.3.1.107, USNM 93398); San Jose? del Cabo (USNM 146607?146620, 147340). ACKNOWLEDGMENTS Foremost, we appreciate the access to specimens, through loans or museum visits, 122 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 331 as facilitated by the following museum staff: Guy G. Musser (AMNH), Paula Jenkins and Daphne Hill (BMNH), Sue McLaren (CM), William T. Stanley (FMNH), Thor Holmes and Bob Timm (KU), Fritz Hertel (UCLA), and Steve Hinshaw and Phil Myers (UMMZ). Bill Stanley (FMNH) graciously accommodated our request to provide mea- surements of the type specimen of Oryzomys molestus. John Steiner (Senior Photographer, NMNH Branch, Smithsonian Photographic Services) photographed the skulls illustrated in figures 8 and 9, and Dan Cole (Office of Information Technology, NMNH) offered assistance and advice on GIS mapping. 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Oryzomys couesi aztecus: Mexico, Morelos, Yautepec, 4100 ft (USNM 51171, 51172, 51173 [holotype], 51174, 51175); 10 mi S Cuernavaca (USNM 20907/ 36019). Oryzomyscouesicrinitus:Mexico,DistritoFederal,Tlal- pan,7600ft(USNM50181,50182[holotype],50183). Oryzomys couesi regillus: Mexico, Michoaca?n, Los Reyes (USNM 125936?125940, 125943, 125944, 125945 [holotype], 125946?125948); 1.5 N Los Reyes, 4400 ft (KU 96101?96103); 1.5 mi S Los Reyes, 4500 ft (KU 96104). 2009 CARLETON AND ARROYO-CABRALES: ORYZOMYS COUESI COMPLEX 127