Character 1

Four mammae present in inguinal and abdominal pairs (0); or six mammae in inguinal, abdominal, and postaxial pairs (1); or eight mammae in inguinal, abdominal, postaxial, and pectoral pairs (2). Almost all oryzomyines have eight mammary glands, and the presence of pectoral mammae was considered one of the synapomorphies for the tribe (Voss and Carleton, 1993; Steppan, 1995). However, specimens of Scolomys and Handleyomys lack the pectoral pair.2 Among non-oryzomyine taxa, Nyctomys has only four mammae, Peromyscus and Thomasomys have six, and Delomys and Wiedomys have eight. Lacking suitable female material, I could not determine the number of mammae in Amphinectomys, Holochilus chacarius, Nesoryzomys narboroughi, and N. swarthi.3

My scoring of several taxa is at odds with some observations in the literature. Steppan (1995: table 6; also Gyldenstolpe, 1932), for example, stated that Neacomys has only six mammae (lacking the pectoral pair); however, all exemplars of the three included species of Neacomys that I analyzed have eight mammae, including the pectoral pair (see also Voss et al., 2001). The same is true for Wiedomys, which Steppan (1995) reported as having six mammae, whereas I observed eight (see also Pacheco, 2003). More problematic is the positional identification of the “axillarie” pair (Arvy, 1974) of Scolomys. Patton and da Silva (1995), using the positional chart of muroid mammary loci of Voss and Carleton (1993), described Scolomys as having a thoracic pair of mammae rather than a postaxial pair. Indeed, the anteriormost pair of mammae in Scolomys is situated more posteriorly than are the postaxial mammae of most oryzomyines. Because there seems to be a continuous variation in the position of this pair among other oryzomyines, however, it is impossible to falsify the hypothesis that the thoracic mammae of Scolomys and the postaxial mammae of other oryzomyines are homologous using topographical criteria. Lacking other (e.g., ontogenetic) information, it seems more parsimonious to recognize thoracic mammae only when postaxial mammae are also present. This character was treated as ordered (0 ↔ 1 ↔ 2).4

Character 2

Claws of manus small, not extending much beyond digital pads, not keeled (0); or claws long, extending conspicuously beyond digit pads and ventrally, keeled for about half their length (1). Although claw size is a continuous trait, the two described states can be unambiguously differentiated by the presence or absence of keels on the ventral surface of the unguis. Long keeled claws were observed only in Lundomys. The remaining ingroup and outgroup taxa have small claws that are ventrally open, or keeled only in proximity of contact with the digit. No information is currently available about the morphology of the claws in Amphinectomys (scored as missing, “?”, in table 5).

Character 3

Ungual tufts at base of manual claws present and long (0); or reduced or absent (1). Ungual tufts, consisting of hairs rooted at the base of claws, have been extensively cited in sigmodontine phylogenetic studies (see character 7), but the presence or absence of the tufts on the manus has only been recently used (Pacheco, 2003). Although ungual tufts on the manus and pes are serially homologous, they exhibit distinct patterns of occurrence among oryzomyines. Most examined taxa have ungual tufts on the manual claws, but Holochilus, Lundomys, Nectomys apicalis, and Oryzomys palustris lack them. No information is currently available about the occurrence of manual ungual tufts in Amphinectomys (coded “?”).

Character 4

Hypothenar pad of pes present (0); or absent or vestigial (1). Plantar foot pads are epidermal and dermal thickening of discrete areas on the surface of feet, often containing eccrine glands (Brown and Yalden, 1973; Haffner, 1998). In rodents, pads are usually most developed in arboreal taxa, whereas pads of semiaquatic or fossorial rodents are the least developed (Hershkovitz, 1944, 1960; Brown and Yalden, 1973). Most oryzomyines (and sigmodontines in general) have two metatarsal pads on the hindfoot, the thenar and hypothenar pads (fig. 9A, B). In contrast, the hindfoot of Holochilus, Lundomys, Nectomys, Scolomys, Sigmodontomys, Pseudoryzomys, Oryzomys palustris, and O. couesi have only a conspicuous thenar pad, with the hypothenar pad being absent or vestigial5 (fig. 9C). Oryzomys yunganus is polymorphic for this trait (see Musser et al., 1998: 58). No information is currently available about morphology of pads in Amphinectomys (coded “?”).

Character 5

Plantar pads on hindfeet large and fleshy, interdigitals 1–4 set close together, often in contact (0); or pads smaller but still fleshy, interdigitals 1 and 4 displaced proximally relative to 2 and 3 (1); or interdigital pads distributed as in state 1 but extremely small and with low relief (2). Oecomys is the only oryzomyine with highly developed interdigital pads (fig. 9A), sharing this condition with all outgroup taxa except Delomys. Holochilus, Lundomys, Nectomys, Sigmodontomys, Pseudoryzomys, Oryzomys palustris, and O. couesi have reduced interdigital pads (fig. 9C), and the remaining taxa have fleshy, but not highly developed, interdigital pads (fig. 9B). With the exception of Scolomys, the same taxa that lack the hypothenar pad also have reduced interdigitals pads, suggesting a correlation between this and the last character; previous studies have described both features in a single multistate character (e.g., Carleton, 1980: char. 77; Carleton and Musser, 1989: char. 2; Voss and Carleton, 1993: char. 3). Nevertheless, I preferred to recognize the configuration of Scolomys as conflictual, rather than intermediary, and kept each trait as separated characters. This character was treated as ordered (0 ↔ 1 ↔ 2) to reflect the morphoclinal aspect of the development of pads. No information is currently available about pad morphology in Amphinectomys (coded “?”).

Character 6

Plantar surface of hindfeet smooth, without well-developed squamae (0); or plantar surface covered with squamae (1). The sole of the hindfoot of most oryzomyines is continuously covered with small scalelike irregularities that Voss et al. (2002) called “squamae” (fig. 9C). In contrast, the squamae are absent (fig. 9A), or present as inconspicuous irregularities along the margins of the sole or at the proximal region of the digits (but never at the central sole region around the plantar pads; fig. 9B), in Handleyomys, all Oecomys species except Oe. mamorae, several Oryzomys species (O. albigularis, O. alfaroi, O. chapmani, O. hammondi, O. lamia, O. levipes, O. macconnelli, O. megacephalus [contra Voss et al., 2001: fig. 53; see Musser et al., 1998: fig. 17], O. russatus, O. talamancae, and O. yunganus), Sigmodontomys aphrastus, and Zygodontomys cherriei. Among outgroups, Nyctomys and Thomasomys lack squamae. No information is currently available about the plantar surface morphology in Amphinectomys (coded “?”).

Character 7

Ungual tufts present on all claws of hindfoot as a uniform thick sheath extending to or beyond claw tip (0); or tufts absent on digit I (dI), present on dII–dV as a uniform thick sheath extending to or beyond claw tip (1); or tufts absent on dI, present as sparse cover and with few hairs extending beyond the claw tip on dII–dV (2); or tufts extremely reduced or absent on all claws (3). Most oryzomyines have thick ungual tufts on dII–dV of the hindfeet, but lack the tufts on dI (state 1). Oryzomys angouya and O. polius are the only taxa with thick tufts on all pedal claws (state 0). Holochilus, Lundomys, Nectomys apicalis, and Sigmodontomys aphrastus lack, or have extremely reduced, ungual tufts on all pedal digits (state 3), whereas Pseudoryzomys, Melanomys, Nectomys squamipes, Oryzomys couesi, O. hammondi, O. palustris, and Sigmodontomys alfari have sparse ungual tufts on dII–dV (state 2). Descriptions of taxonomic variation in the expression of this trait have usually noted only presence or absence of the tufts on dII–dV (e.g., Patton and Hafner, 1983; Carleton and Musser, 1989; Voss, 1993; Voss and Carleton, 1993; Musser et al., 1998; Carleton and Olson, 1999). However, I included the condition of tufts on dI (see also Pacheco, 2003) and introduced an intermediary state of sparse tufts (equivalent to the description of Oryzomys yunganus by Musser et al., 1998: 320). Sparse and thick ungual tufts can be distinguished by comparing the proximal and distal portions of the claw, which are uniformly covered in states 0 and 1 (fig. 10A), whereas an unequivocal difference in density between proximal and distal portions exists in state 2, with very few hairs extending onto the terminal portion of the claws and extending beyond it (fig. 10B). The intermediate state avoids ambiguous characterizations, such as the different codings for Pseudoryzomys in Voss and Carleton (1993) and Carleton and Olson (1999). All outgroups show state 0, except Thomasomys, which exhibits state 1. This character was treated as ordered (0 ↔ 1 ↔ 2 ↔ 3). No information is currently available about ungual tufts in Amphinectomys (coded “?”).

Character 8

Natatory fringes on hindfeet absent (0); or present (1). Natatory fringes are continuous combs of stiff hairs along the plantar margins and sometimes between the digits of the hindfoot (Voss, 1988: fig. 7). The semiaquatic taxa Amphinectomys, Holochilus, Lundomys, and Nectomys are the only oryzomyines to exhibit natatory fringes. Among other sigmodontines, natatory fringes are also present in ichthyomyines, but these were not included in this study.

Character 9

Interdigital webbing on hindfeet absent (0); or present but small, not extending to first interphalangeal joint of any digit (1); or present and long, extending to or beyond first interphalangeal joints of digits II, III, and IV (2). Interdigital webbings are present between digits I and IV in Oryzomys palustris, Pseudoryzomys simplex, and Sigmodontomys alfari (small membrane, fig. 10B), and in Amphinectomys, Holochilus, Lundomys, and Nectomys (large membrane, fig. 10C). All of the remaining oryzomyines and outgroups lack interdigital webbing (fig. 10A). This character was treated as ordered (0 ↔ 1 ↔ 2) to reflect the hypothesis of a morphoclinal change in web development (see Voss and Carleton, 1993).

Character 10

Dorsal surface of hindfeet densely covered with white hairs, feet appear solid white (0); or dorsal surface sparsely covered with short silvery hairs, feet appear grayish white or pale tan (1); or dorsal surface covered with dark hairs, feet appear brown (2). The overall color of the dorsal surface of the hindfoot depends on the color and density of the covering hairs. Although white, gray, or brown hindfeet include a range of subtly different tones, they can be unambiguously recognized in side-by-side comparisons. White feet are observed only in Peromyscus, Nesoryzomys, and Oryzomys polius, whereas Melanomys and Sigmodontomys have brown feet. All of the remaining taxa have grayish dorsal feet surfaces.

Character 11

Ventral surface of tail covered with dark hairs (0); or covered with hairs with dark basal band and white distal band (1); or covered with white hairs (2). In oryzomyines (and muroid rodents in general) the ventral surface of the tail can be paler than the dorsal surface because of the color of the tail scales and the color of the hairs that emerge from them. These two features combine in various ways to create the impression of a bicolored or unicolored tail, with intermediate grayish tones (variously referred to by authors as paler, mottled, or indistinctly bicolored tails). Although caudal scale coloration varies continuously, hair coloration displays clear-cut variation suitable for cladistic character coding. I coded this character from the base (proximal half) of the tail because there is considerable variation in hair color (both within species and among taxa) on the distal half of the tail. The distribution of character-states among ingroup and outgroup taxa is recorded in table 5. In general, but not always, the coding I used is correlated with the bicolored/indistinctly bicolored/distinctly bicolored description of some authors (e.g., Steppan, 1995). This character was treated as ordered (0 ↔ 1 ↔ 2). No information is currently available about this character for Amphinectomys (coded “?”).

Character 12

Tail densely furred, scales not visible even at higher magnification (0); or tail sparsely furred, scales macroscopically obvious (1). The tails of muroid rodents always have hair, usually consisting of at least three bristles emerging from the posterior margin of each scale. Short hairs give the tail a naked, scaly appearance, which is the pattern observed in most oryzomyines. In contrast, Nesoryzomys and all outgroups except Delomys and Thomasomys have longer bristles, which provide the tail with a densely furred and scale-free appearance (even when viewed under a stereomicroscope). I could not distinguish an intermediate slightly furred state that has sometimes been recognized in sigmodontine phylogenetics (e.g., Patton and Hafner, 1983; Steppan, 1995).

Character 13

Dorsal and ventral fur without grooved spines (0); or with grooved spines (1).Scolomys and Neacomys have dorsal and ventral guard hairs modified into grooved spines with a broad base, as opposed to the conventional guard hairs with a slender base and soft distal portion present in the remaining taxa. Scolomys and Neacomys, together with Rhagomys longilingua and Abrawayaomys (taxa not included in this study), are the only sigmodontines with spiny pelage; however, the presence of dorsal spines is recurrent among other muroid genera (e.g., in Acomys, Echiothrix, Maxomys, Platacanthomys, and Tokudaia) and among non-muroid rodents (e.g., Heteromyinae, Echimyidae, Hystricidae, and Erethizontidae).

Character 14

Ventral fur with plumbeous or dark gray base (0); or ventral fur entirely white, without dark base (1). The ventral fur of most oryzomyines consists of banded hairs that are dark gray basally and white, cream, or ochraceous distally. In contrast, the ventral fur of some species of Neacomys, Oecomys, and Nyctomys is entirely white.

Character 15

Dorsal and ventral colors sharply delimited, dorsum much darker than pale ventral surface, resulting in conspicuous countershading (0); or dorsal and ventral colors subtly delimited, dorsum slightly darker than ventral surface, resulting in weak countershading (1); or limits of dorsal and ventral colors indistinct, ventral surface dark, countershading absent (2). Conspicuous countershading is observed in most oryzomyines as the result of contrast between the darker grizzled-brown dorsal fur and the lighter (white or whitish gray) ventral surface. In several taxa, however, the dividing line between dorsal and ventral color zones is less distinct because of gradual fading of the lateral pigments into a creamy or grayish ventral color, creating a weak countershading effect; ingroup taxa that exhibit this condition include Handleyomys, Holochilus, Lundomys, Nectomys, Nesoryzomys, Oecomys concolor, Oe. mamorae, Oligoryzomys flavescens, Ol. fornesi, Ol. fulvescens, Oryzomys couesi, O. hammondi, O. polius, Scolomys, and Zygodontomys. Two outgroup taxa also exhibit weak countershading (Thomasomys and Wiedomys). In Melanomys, Microryzomys, and Sigmodontomys aphrastus, however, the ventral surface is almost as dark as the dorsum, resulting in the absence of obvious countershading. This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 16

Subauricular patches absent (0); or present (1). Subauricular patches, being whitish areas immediately ventral to the base of each pinna, are rare among oryzomyines, occurring only in exemplars of Nesoryzomys and Oryzomys albigularis. Among analyzed outgroups, subauricular patches were observed only in specimens of Thomasomys. Braun (1993) also reported subauricular patches in both species of Andalgalomys, a phyllotine genus not included in the present analysis.

Character 17

Rostral tube absent (0); or present (1).Voss (1993) described the rostral tube in Delomys as a substantial anterior projection of the premaxillae and nasals beyond the upper incisive plane, with the bony margins approximated to extend the nasal cavity anteriorly to the rest of the snout. Although there is a continuous variation in the anterior projection of these bones relative to the incisors, the rostral projections of Peromyscus and Delomys are noticeably longer than in the other species.

Character 18

Nasals with blunt posterior margin (0); or nasals with acutely pointed terminus, forming a sharp angle (1). The posterior margins of the nasal bones in oryzomyines are usually bluntly rounded or squared (fig. 11A), although some specimens exhibit small pointed introgressions of the nasal into the frontal (fig. 11B). The lateral nasal borders are slightly convergent (sometimes parallel) throughout most of their length. In contrast, the posterior margins of the nasals of Nectomys, Scolomys, and Sigmodontomys aphrastus always terminate in a sharp angle (fig. 11C), and the lateral nasal borders of these taxa are strongly convergent.

Character 19

Nasals short, not extending posteriorly beyond the triple-point suture between the maxillary, frontal, and lacrimal (0); or long, extending posteriorly well beyond the maxillary-frontal-lacrimal suture (1). The posterior extent of the nasals is a continuous trait, but an unambiguous distinction can be made between species with nasals that barely extend beyond the maxillary-frontal suture at its contact with the lacrimal bone (fig. 11B) and species with a substantial penetration of the nasals into the frontals (fig. 11A, C). A small number of taxa, however, could not be placed unequivocally in one or the other state, and they are scored as polymorphic: Thomasomys, Wiedomys, Oligoryzomys nigripes, Oryzomys angouya, O. xanthaeolus, Sigmodontomys alfari, and Zygodontomys. The distribution of character-states among the remaining taxa is recorded in table 5. Because of ontogenetic variation in nasal extension, only adult specimens were used in coding this character (see also Steppan, 1995: chars. 9S, 46P).

Character 20

Premaxillaries long, extending posteriorly beyond the nasals (0); or shorter, extending posteriorly to about the same level as the nasals (1); or very short, terminating anterior to the nasals (2). As for the previous character, the posterior extension of the premaxillaries varies continuously among taxa, but three states can be unequivocally identified by the relative position of other topographical elements of the cranial dorsum. In state 0, observed only in Nyctomys, the nasals are bordered by much longer premaxillaries. In state 2, the premaxillaries terminate anterior to the nasal end (fig. 11A, C). This condition is observed in specimens of Thomasomys, Wiedomys, Amphinectomys, Handleyomys, Neacomys, Nectomys, Oryzomys balneator, O. lamia, Scolomys, and Sigmodontomys aphrastus. The remaining taxa have premaxillaries that terminate at about the same level as the nasals (state 1, fig. 11B). A few species that could not be unambiguously placed in a single state were scored as polymorphic (i.e., 12): Melanomys caliginosus, Oryzomys alfaroi, O. xanthaeolus, and Sigmodontomys alfari. Some correlation exists among this and the last two (18 and 19) characters, and it is possible that a single character describing the nasal-premaxillary-frontal contact area might be preferred, especially if it decreases the number of taxa scored as polymorphic. This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 21

Lacrimal equally contacting maxillary and frontal bones (0); or lacrimal contacting mainly maxillary (1). In most oryzomyines, and in all outgroups, each lacrimal is wedged between the maxillary and the frontal, and the sutures that the lacrimal shares with these bones have similar lengths (fig. 12A). In several other oryzomyine species, however, the lacrimal is largely enclosed by the maxillary, and its contact suture with the frontal is much reduced (fig. 12B). This condition occurs in Handleyomys, Holochilus, Melanomys, Nectomys squamipes, Nesoryzomys, Oecomys bicolor, Oe. trinitatis, Oryzomys couesi, O. levipes, O. megacephalus, O. palustris, O. xanthaeolus, Pseudoryzomys, Scolomys, and Sigmodontomys. I observed both conditions among specimens of Oryzomys angouya, which was scored as polymorphic.

Character 22

Interorbital region symmetrically constricted (hourglass-shaped or amphoral), with rounded supraorbital margins (0); or interorbital region symmetrically constricted with squared supraorbital margins (1); or interorbital region symmetrically constricted with conspicuously beaded supraorbital margins (2); or interorbital region convergent anteriorly (cuneate) with weakly beaded supraorbital margins (3); or interorbital region convergent anteriorly with well-developed supraorbital crests (4). Three features of the interorbital region are summarized in this character: the overall shape of the interorbital region, the shape of the supraorbital margins (formed by the dorsal and lateral surfaces of the frontal bone), and the development of supraorbital beads and crests (fig. 13). In the first three states (0, 1, and 2), the interorbital region is symmetrically constricted in the morphology described by authors as amphoral or hourglass-shaped, whereas in the latter two states (3 and 4), the interorbital region is convergent anteriorly. The first three states differ in the contact between the lateral and dorsal surfaces of the frontal bone and in the development of supraorbital beads: species with state 0 have smooth, almost rounded supraorbital margins (fig. 13A); species with state 1 have sharp, more-or-less squared supraorbital margins that sometimes develop slight beads in older specimens (fig. 13B); and species with state 2 have supraorbital margins with strongly developed beads (fig. 13C). States 3 and 4 are differentiated by the degree of development of supraorbital ornaments and by the degree of anterior interorbital convergence. In state 3, supraorbital beads are always weakly developed and dorsally oriented (fig. 13D), whereas state 4 is characterized by strongly dorsolaterally expanded supraorbital crests (fig. 13E). Most states of this character are extensively distributed among oryzomyines (see table 5), with the exception of state 2, which is observed only in Holochilus. This character is treated as additive, but a step matrix was employed to account for the hypothesized nonlinear transformation between states (fig. 8A).

Converting taxonomic variation of interorbit shape into cladistic characters is difficult, and every analysis to date has presented a new coding scheme (Carleton, 1980: char. 24; Patton and Hafner, 1983: chars. 13, 14; Carleton and Musser, 1989: char. 6; Voss, 1993: char. 6; Voss and Carleton, 1993: char. 7; Steppan, 1995: chars. 11S, 49P, 50P, and 51P); the present study is therefore no exception. Nevertheless, the character-states delineated here are based on those of Carleton and Olson (1999: char. 4), with an additional intermediate condition (state 3) to accommodate Pseudoryzomys (scored as state 1 by Carleton and Olson, 1999); another difference is that the development of temporal ridges (considered here strictly as the ridges at the parietal-squamosal suture) was not taken into account. The rationale for considering interorbital shape and supraorbital ornamentation as a single character, instead of delineating two or more characters, is based on my assumption that anterior interorbital convergence is a morphological consequence of crest development (cf. Carleton, 1980; Carleton and Olson, 1999).

Character 23

Postorbital ridge absent, posterior orbital wall without conspicuous relief, frontosquamosal suture exposed (0); or postorbital ridge present and concealing frontosquamosal suture in most old specimens (1). The postorbital ridge is a vertical bony ridge extending from the anterior contact between the frontal and parietal to above the zygomatic root of the squamosal, following the frontosquamosal suture (Voss and Carleton, 1993). Holochilus is the only taxon that displays this feature among the analyzed terminals of this study. No information is currently available about this character for Amphinectomys (coded “?” in table 5).

Character 24

Frontosquamosal suture continuous with the frontoparietal suture, dorsal facet of frontal never in contact with squamosal (0); or frontosquamosal suture anterior to frontoparietal suture, leading to an area of contact between dorsal facet of frontal and squamosal (1). The sutures of the frontal with the squamosal and parietal are collinear in most oryzomyines (fig. 14A, B). In Holochilus,Melanomys,Nectomys, Oryzomys hammondi, and Sigmodontomys, however, the frontosquamosal suture is situated anterior to the frontoparietal suture, creating an area of contact between the dorsal facet of frontal and the squamosal. (fig. 14C).

Character 25

Parietals restricted to the dorsal surface of the braincase, or slightly expanded below the lateral edges of the dorsal at about the squamosal root of the zygomatic arch (0); or parietals deeply expanded onto lateral surface of the braincase (1). The parietals of Amphinectomys, Microryzomys, Neacomys, Oligoryzomys, Oryzomys balneator, O. lamia, O. megacephalus, O. rostratus, O. russatus, O. yunganus, Scolomys, and Zygodontomys are restricted to the dorsal surface of the braincase, or they extend only slightly onto the lateral surface on the squamosal, forming a shallow deflection at the level of the squamosal zygomatic root (fig. 15A). In contrast, the parietals of Holochilus, Lundomys, Melanomys, Nectomys, Nesoryzomys, Oecomys, Oryzomys angouya, O. couesi, O. hammondi, O. palustris, O. polius, O. subflavus, O. xanthaeolus, Pseudoryzomys, and Sigmodontomys have an extensive expansion onto the lateral surface of the braincase (fig. 15B). I observed both conditions among specimens of Handleyomys and several Oryzomys species (O. albigularis, O. alfaroi, O. chapmani, O. levipes, O. macconnelli, and O. talamancae; see also Musser et al., 1998: fig. 62). Thomasomys is the only outgroup with deep lateral expansions of the parietals.

Character 26

Interparietal wider than posterior border of frontals, in contact with squamosal (0); interparietal strap-shaped, nearly as wide as posterior border of frontals, but not in contact with squamosal (1); or interparietal wedge-shaped, about half as wide as posterior border of frontals, not in contact with squamosal (2). Taxonomic variation in the relative breadth of the interparietal is continuous, but the three described states can be differentiated by taking into account the overall shape of the interparietal and the contact area between parietal and occipital. Strap-shaped interparietals (state 1) extend laterally, almost in contact with the squamosal, leaving a diminutive area of contact between parietal and occipital (fig. 16A). Most oryzomyine taxa display this pattern. In the wedge-shaped pattern (state 2), the interparietal does not extend laterally, leading to a large parietal-occipital suture line that is visible even without magnification (fig. 16B). Amphinectomys, Handleyomys, Melanomys, Nectomys apicalis, Oryzomys couesi, O. palustris, O. polius, Scolomys, and Zygodontomys display this pattern. State 0 is observed only in Nyctomys, which has an interparietal that contacts the squamosal on either side. Five species of Oryzomys display intraspecific variation for this character (O. angouya, O. levipes, O. macconnelli, O. subflavus, and O. xanthaeolus) and were scored as polymorphic (i.e., 12). This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 27

Basicranial flexion weakly pronounced, foramen magnum oriented mostly caudad (0); or strongly pronounced, foramen magnum oriented mostly posteroventrally (1). A weakly pronounced posteroventrad basicranial flexion is observed in all taxa except Microryzomys, which has a strongly pronounced basicranial flexion. This feature was described by Carleton and Musser (1989) as an apomorphy for Microryzomys.

Character 28

Zygomatic plate narrow and zygomatic notch indistinct; anterior border of plate flat, below or slightly in front of anterior margin of superior maxillary root of zygoma (0); or plate broad with moderate or deep notch; anterodorsal margin smoothly rounded, conspicuously anterior to superior maxillary root of zygoma (1); or plate broad and notch conspicuous; anterodorsal margin produced as a sharp corner or spinous process, conspicuously anterior to superior maxillary root of zygoma (2). The anterodorsal margin of the zygomatic plate in most oryzomyines is rounded and projects anteriorly to the superior maxillary root of zygoma, forming the zygomatic notch (state 1; fig. 17B). Pseudoryzomys, Lundomys, and Holochilus, however, have deeper zygomatic notches because the anterodorsal margin of the zygomatic plate projects rostrally as a spinous process (state 2; fig. 17C). The zygomatic plate of five oryzomyine taxa (Microryzomys, Oryzomys balneator, Oecomys, Scolomys, and Sigmodontomys aphrastus) and three non-oryzomyines (Nyctomys, Peromyscus, and Thomasomys) have squared or flat anterodorsal margins situated at the same plane of the anterior margin of the antorbital bridge; consequently, the zygomatic notch, albeit present, is indistinct (state 0; fig. 17A). Because the development of the zygomatic notch is an anatomical consequence of the morphology and position of the anterior border of the zygomatic plate, I treated these two features as a single character (contra Steppan, 1995: chars. 8S, 43P; see also Patton and Hafner, 1983: chars. 11, 12). This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 29

Posterior margin of zygomatic plate situated anterior to the alveolus of M1 (0); or approximately even with the alveolus of M1 (1). Although the position of the posterior margin of the zygomatic plate relative to the alveolus of M1 appears to vary continuously among analyzed taxa, an unequivocal gap was detected between species with posterior margins unambiguously anterior to the M1 alveolus (fig. 18A) and those with the margins at about the same level as the alveolus (fig. 18B). Only two species—Oryzomys chapmani and O. xanthaeolus—appear to exhibit intermediate conditions, and they were consequently scored as polymorphic for this character. The distribution of character-states among remaining taxa is recorded in table 5. Character definition is taken from Carleton and Olson (1999: char. 2), which is similar to Steppan (1995: chars. 6S, 42P). My scoring for Zygodontomys brevicauda and Oryzomys megacephalus, however, differed from that assigned by Steppan (1995).

Character 30

Jugal present and large, maxillary and squamosal processes of the zygoma not overlapping (0); or jugal present and small, maxillary and squamosal processes overlapping, but not in contact (1); or jugal absent, or reduced to slivers of bones, maxillary and squamosal processes in contact (2). The jugal of most oryzomyine species is small, and the maxillary and squamosal roots of the zygomatic arch are overlapping (in lateral view) but are not in contact (state 1; fig. 19A). In contrast, the jugal of most outgroups (except Thomasomys) is robust, and the squamosal and maxillary processes of the zygoma do not overlap in lateral view (state 0). In Oligoryzomys, Microryzomys, Oryzomys balneator, and O. lamia, the jugal is usually absent (state 2; fig. 19B); when present, the jugal is reduced to diminutive bony slivers (fig. 19C). This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 31

Posterior margins of incisive foramina conspicuously projecting between first molars (0); or terminating anteriorly or at the front of first molar alveoli (1). The incisive foramina of most oryzomyines are short and do not project between the molar rows (fig. 18B). On the other hand, the foramina of Lundomys, Nesoryzomys, Pseudoryzomys, Zygodontomys, and several Oryzomys species (O. angouya, O. couesi, O. palustris, O. polius, O. subflavus, and O. xanthaeolus) are much longer and extend posteriorly between the procingula of the left and right M1s (fig. 18A). All outgroups were scored with the latter condition, with the exception of Nyctomys. Because younger specimens usually have relatively longer foramina that reach more posteriorly than those in older specimens, this character was assessed only in adult individuals.

Character 32

Palate short, mesopterygoid fossa extends anteriorly between the molar rows (0); or palate of intermediate length, mesopterygoid fossa extends anteriorly between the maxillary bones but not between M3s (1); or palate long, mesopterygoid fossa does not extend anteriorly between the maxillary bones (2). This description of taxonomic variation in palatal length essentially corresponds to the classical definition of short versus long palates (e.g., Thomas, 1906; Hershkovitz, 1962) with the addition of an intermediate state (also employed by Carleton and Olson, 1999). Although taxonomic variation in palatal length is continuous, the three character-states can be unambiguously identified by the anterior extension of the mesopterygoid fossa relative to the third molars and the maxillary bones. Most oryzomyines have long palates, with the mesopterygoid fossa not extending anteriorly between the maxillary bones (state 2; fig. 20A). An intermediate-length palate, with the mesopterygoid extending anteriorly between the maxillae (fig. 20B, D), is observed in specimens of Holochilus brasiliensis, Melanomys, Microryzomys, Nectomys, Nesoryzomys swarthi, Oryzomys albigularis, O. angouya, O. balneator, O. hammondi, O. polius, O. russatus, O. subflavus, and Sigmodontomys. All outgroups, with the exception of Wiedomys, have short palates; Oryzomys levipes is the only oryzomyine with such pattern (fig. 20C). The character is treated as ordered (0 ↔ 1 ↔ 2).

Character 33

Bony palate flat, or with shallow lateral excavations, never with median longitudinal ridge (0); or with deep lateral troughs separated by median longitudinal ridge (1). The bony palate of almost all oryzomyines is either flat or moderately corrugated. Conversely, the bony palates of specimens of Holochilus and Lundomys have conspicuous median longitudinal ridges separating two deep lateral troughs. I included flat and moderately corrugated palates in a single state instead of two (contra Carleton and Olson, 1999: char. 8) because several taxa display both flat and slightly corrugated conditions within analyzed samples. Carleton and Olson (1999) also considered the width of the bony palate in their character definition, but this feature varies continuously among the larger array of taxa treated in this study.

Character 34

Posterolateral palatal pits absent (0); or one simple small foramen present at each side of the posterior palate (1); or posterolateral palatal pits always present as conspicuous perforations, usually more than one foramen, not recessed in fossae or recessed in shallow depression (2); or posterolateral palatal pits always present as perforations within deeply recessed fossa, generally with three foramina, one directed posteriorly, one anteriorly, and one dorsally (3). The lateral surface of the posterior palate is perforated by cavities in all analyzed sigmodontines. A small simple pit is observed on each side of the palate lateral to the mesopterygoid fossa in Oryzomys hammondi and Sigmodontomys aphrastus (fig. 20B), and in all outgroups except Peromyscus (which lacks the pits). The pits of the remaining oryzomyines are large and conspicuous, and more than one is usually present on each side of the palate. In Amphinectomys, Nectomys, Nesoryzomys, and several species of Oryzomys (O. albigularis, O. angouya, O. chapmani, O. couesi, O. levipes, O. rostratus, O. palustris, O. polius, O. subflavus, and O. xanthaeolus), the pits, usually three on each side, are recessed in a deep fossa (state 3; fig. 20C, D). In other oryzomyines, the pits, usually one or two on each side, are at the same level of the palate or situated slightly dorsally but not recessed in deep fossae (state 2; fig. 20A). Holochilus brasiliensis, Oryzomys alfaroi, O. megacephalus, and O. talamancae are variable for this character, with exemplars with and without fossae, and thus they were scored as polymorphic (i.e., 23). This character was treated as ordered (0 ↔ 1 ↔ 2 ↔ 3).

Character 35

Parapterygoid fossae at same level as palate (0); or parapterygoid fossae dorsally excavated but not reaching level of mesopterygoid roof (1); or parapterygoid fossae deeply excavated, reaching level of mesopterygoid roof (2). The parapterygoid fossae of most oryzomyines are dorsally excavated above the level of the bony palate (state 1; fig. 20B, C), but they do not reach the level of the mesopterygoid roof. Handleyomys and Oligoryzomys are the only oryzomyines with flat parapterygoid fossae forming a continuous surface with the palate (fig. 20A); although the parapterygoid fossae of some specimens of Oligoryzomys are positioned slightly dorsally relative to the palate, the surfaces between the two bones still form a continuous slope without a sharp edge. The only oryzomyines with deeply excavated parapterygoid fossae that reach the level of the mesopterygoid roof are Holochilus, Lundomys, and Oryzomys polius (fig. 20D). Among the outgroups, Peromyscus and Nyctomys have flat parapterygoid fossae, whereas Delomys, Thomasomys, and Wiedomys display the intermediate state. The character is treated as ordered (0 ↔ 1 ↔ 2). My scoring for several taxa (Holochilus brasiliensis, Nectomys squamipes, Oryzomys megacephalus, O. palustris, Oligoryzomys fulvescens, Pseudoryzomys, and Zygodontomys brevicauda) differed from that assigned by Carleton (1980: char. 23) and Steppan (1995: char. 67P), but I cannot explain the discrepancies. No information is currently available about this character for Amphinectomys (coded “?” in table 5).

Character 36

Sphenopalatine vacuities present as large apertures along the presphenoid, reaching basisphenoid (0); or vacuities present but reduced, generally as narrow openings, anterior to basisphenoid-presphenoid suture (1); or vacuities absent, mesopterygoid roof totally ossified (2); or vacuities present but reduced, situated posterior to basisphenoid-presphenoid suture (3). Most oryzomyines have either large or small sphenopalatine vacuities. Large sphenopalatine vacuities, characterized as openings that reach the basisphenoid and that are wider than the posterior expansion of the presphenoid bone (fig. 20A, D), are observed in Holochilus, Nesoryzomys, Oligoryzomys, Pseudoryzomys, Zygodontomys cherriei, and several species of Oryzomys (O. alfaroi, O. angouya, O. chapmani, O. rostratus, O. palustris, O. polius, O. subflavus, O. xanthaeolus). Reduced vacuities, characterized as openings that do not reach the basisphenoid and are narrower or of the same width of the posterior expansion of the presphenoid (fig. 20C), are observed in Amphinectomys, Lundomys, Microryzomys, Neacomys, O. albigularis, Oryzomys balneator, O. levipes, O. talamancae, and Sigmodontomys. The mesopterygoid roof is totally ossified (fig. 20B) in the remaining oryzomyines (table 5), except for three polymorphic species (Nectomys squamipes, Oryzomys megacephalus, and Zygodontomys brevicauda), which were scored accordingly (i.e., 12). Nyctomys is the only taxon with vacuities restricted to the region posterior to the basisphenoid-presphenoid suture, so it was scored as a distinct state (3). This character was treated as additive, but a step matrix was employed to account for the hypothesized nonlinear pattern of transformation among states (fig. 8A). The present character description is similar to Carleton's (1980: char. 20), but my coding of certain taxa differs from his (Oryzomys palustris is scored here as having large vacuities, instead of intermediate; Nectomys squamipes and Oryzomys megacephalus are scored as polymorphic). The condition of the sphenopalatine vacuities was also analyzed by Steppan (1995: chars. 20S, 68P), but his scoring differs from mine for Holochilus, Thomasomys, and Oligoryzomys fulvescens.

Character 37

Stapedial foramen and posterior opening of alisphenoid canal large, squamosal-alisphenoid groove and sphenofrontal foramen present (0); or stapedial foramen and posterior opening of alisphenoid canal large, squamosal-alisphenoid groove and sphenofrontal foramen absent (1); or stapedial foramen and posterior opening of alisphenoid canal small, squamosal-alisphenoid groove and sphenofrontal foramen absent, secondary branch crosses dorsal surface of pterygoid plate (2). Different patterns of the carotid arterial circulation have been extensively discussed in the muroid literature (e.g., Bugge, 1970, 1971; Carleton, 1980; Musser and Williams, 1985; Voss, 1988, 1993), and this is one of the few morphological characters that has been interpreted consistently in sigmodontine cladistic analysis (Carleton, 1980: char. 16; Voss, 1988: char. 12; Carleton and Musser, 1989; Voss and Carleton, 1993: char. 11; Steppan, 1995: chars. 22S, 76P; Carleton and Olson, 1999: char. 14). The primitive pattern (pattern 1 of Voss, 1988; state 0 herein) is observed in Handleyomys, Microryzomys, Neacomys minutus, N. spinosus, Zygodontomys cherriei, Oecomys bicolor, Oe. catherinae, Oe. trinitatis, several species of Oryzomys (O. albigularis, O. alfaroi, O. balneator, O. hammondi, O. lamia, O. levipes, O. macconnelli, O. rostratus, O. russatus, and O. talamancae), and in all outgroups except Thomasomys. The intermediate pattern (pattern 2 of Voss, 1988; state 1 herein) is observed only in Neacomys musseri, Oligoryzomys, Oryzomys megacephalus, and O. yunganus. The last pattern (pattern 3 of Voss, 1988; state 2 herein) is observed in Holochilus, Lundomys, Melanomys, Nectomys, Nesoryzomys, Oecomys concolor, Oe. mamorae, several Oryzomys species (O. angouya, O. chapmani, O. couesi, O. palustris, O. polius, O. subflavus, O. xanthaeolus), Pseudoryzomys, Scolomys, Sigmodontomys, and Zygodontomys brevicauda. My observation for this character in Nyctomys (scored here with state 0) is at odds with those of Carleton (1980; scored with state 2) and Steppan (1995; scored as polymorphic with states 1 and 2).6 All specimens of Nyctomys examined by me have a conspicuous stapedial foramen, with associated posterior opening of alisphenoid canal; almost all specimens have squamosal-alisphenoid grooves and associated sphenofrontal foramina. Two specimens of Nyctomys do not have obvious squamosal-alisphenoid grooves and sphenofrontal foramina, but careful inspections of the internal wall of the basicranium reveal the presence of the groove in this taxon. The character is treated as ordered (0 ↔ 1 ↔ 2). No information is currently available about this character for Amphinectomys (coded “?”).

Character 38

Alisphenoid strut present, buccinator-masticatory and accessory foramen ovale separate (0); or strut absent, buccinator-masticatory and foramen ovale confluent (1). Most oryzomyines lack the alisphenoid strut (fig. 21B, C; Voss and Carleton, 1993), but well-developed struts are observed in Oryzomys polius, Oryzomys levipes, and Holochilus (fig. 21A). The alisphenoid strut is present in all outgroups except Delomys and Wiedomys. The condition of the strut is variable in Pseudoryzomys, Oecomys bicolor, Oe. concolor, Oe. trinitatis, Oryzomys albigularis, O. lamia, and O. russatus, which were scored as polymorphic. Polymorphic taxa were those having a robust strut present on both sides of the skull in at least 20% or more of examined specimens. A few (<10%) specimens of Oligoryzomys, Sigmodontomys alfari, and Melanomys have thin struts present on one side of the skull; these taxa were coded as lacking the strut (state 1). No information is currently available about this character for Amphinectomys (coded “?”).

Character 39

Anterior opening of alisphenoid canal present, large (0); or absent (1). The alisphenoid canal has a large anterior opening in all analyzed taxa (fig. 21A, B), with the exception of Scolomys (fig. 21C), specimens of which either lack the aperture or have a diminutive opening in its place (Patton and da Silva, 1995: 323). No information is currently available about this character for Amphinectomys (coded “?”).

Character 40

Subsquamosal fenestra present (0); or fenestra vestigial or absent (1). The subsquamosal fenestra of most oryzomyines varies from small to medium-sized, but it is never larger than the postglenoid foramen (fig. 22A, B). Departing from this condition, the subsquamosal fenestra of Amphinectomys, Melanomys, Nectomys, Nesoryzomys narboroughi, Oryzomys hammondi, O. xanthaeolus, Sigmodontomys, and Zygodontomys cherriei is vestigial, obscured internally by the lateral border of the tegmen tympani, or it is absent (fig. 22C, D). Among outgroups, the fenestra is absent in specimens of Nyctomys. I scored Holochilus brasiliensis as having a normal oryzomyine subsquamosal fenestra (state 0) contra Carleton (1980) and Steppan (1995), who considered the opening as small or reduced to a slit in this taxon.

Character 41

Ectotympanic bullae small, exposed flange of periotic extends to internal carotid canal (0); or ectotympanic bullae intermediate, exposed wedge of periotic smaller and not contributing to wall of carotid canal (1); or ectotympanic bullae large, periotic bone mostly masked in ventral view (2). The periotic of most oryzomyines does not reach the internal carotid canal because the ectotympanic bulla extends over and obstructs it (state 1; fig. 23A). Two arrangements depart from this pattern: the periotics of Handleyomys, Microryzomys, Neacomys, Oligoryzomys, Oryzomys balneator, and O. hammondi (plus Nyctomys among outgroups) advance to the carotid canal, usually contributing to the wall of the canal (state 0; fig. 23B); and the ectotympanics of Holochilus and Nesoryzomys (plus Wiedomys among outgroups) are enlarged, blocking the periotic from ventral view (state 2; fig. 23C). My analyzed samples of Oecomys (Oe. bicolor, Oe. concolor, Oe. trinitatis) and of Oryzomys rostratus included specimens with the periotic reaching or not reaching the internal carotid canal (scored as polymorphic, 01). This character was treated as ordered (0 ↔ 1 ↔ 2). My scoring for Lundomys differed from that assigned by Carleton and Olson (1999: char. 12): the periotic bone is visible in ventral view in all Lundomys specimens that I analyzed (fig. 23D). No information is currently available about this character for Amphinectomys (coded “?”).

Character 42

Posterior suspensory process of squamosal present and connected to the tegmen tympani (0); or posterior suspensory process absent, tegmen tympani not touching or barely in contact with squamosal (1). All oryzomyines lack the posterior suspensory process of squamosal, a putative synapomorphy for the tribe (Voss and Carleton, 1993; Steppan, 1995). Reithrodon is the only other sigmodontine without the posterior suspensory process (Steppan, 1995). No information is currently available about this character for Amphinectomys (coded “?”).

Character 43

Mastoid completely ossified, or with a diminutive pit in the dorsal contact with the exoccipital border (0); mastoid with conspicuous fenestra (1). The mastoids of most oryzomyines have a conspicuous dorsolateral fenestra (fig. 22B). The size of the fenestra varies from medium-sized to large, but some specimens have a small fenestra that is always enclosed by the bone, not reaching the exoccipital contact area (fig. 22A). In contrast, the mastoids of Holochilus chacarius, Melanomys, Nectomys, Nesoryzomys, Oecomys catherinae, Oryzomys hammondi, O. lamia, O. macconnelli, O. russatus, Scolomys, Sigmodontomys, and Zygodontomys are totally ossified and lack the fenestra (fig. 22C), or have a small opening, resembling a foramen aperture, at the region of contact between the mastoid and the exoccipital (fig. 22D). My analyzed samples of seven taxa (Neacomys spinosus, O. bicolor, O. trinitatis, Oryzomys albigularis,O. subflavus, O. talamancae, and O. xanthaeolus) included specimens with and without the fenestra (scored as polymorphic). Most outgroups also have the fenestra, except Nyctomys. My scoring for several taxa differed from that assigned by Patton and Hafner (1983: char. 9). No information is currently available about this character for Amphinectomys (coded “?”).

Character 44

Mental foramen opens laterally, at body of mandible (0); or mental foramen opens dorsally, at the diastema (1). In most oryzomyines, the mental foramen is situated laterally at the body of the mandible, usually close to the anterior end of the masseteric ridges and below the diastema (restricted here to the dorsal surface of the mandible between the first lower molar and the incisor; fig. 24C). In contrast, in Oecomys catherinae, Oligoryzomys, Oryzomys balneator, O. subflavus, and Zygodontomys the foramen is situated dorsally, at the diastema anterior to the m1 (fig. 24D). Oryzomys chapmani is polymorphic for this character. No information is currently available about this character for Amphinectomys (coded “?”).

Character 45

Capsular process of lower incisor alveolus absent (0); or projection present but reduced as a slight rounded elevation (1); or projection present, well developed as a conspicuous swelling with acute projection (2). The root of the lower incisor in several sigmodontines is contained in a bony capsule on the lateral surface of the mandible (e.g., Voss, 1991: fig. 13). This capsular process exhibits a marked intraspecific variation (Steppan, 1995), which is correlated with age (older specimens have larger processes than do younger conspecifics). Differences between species are also continuous, and establishing discrete states is subtle. Extremes exist between process absent (fig. 24B) and well-developed tubercles with an acute projection (fig. 24C). Between these opposites, some taxa exhibit a subtle elevation (fig. 24A, D), which was coded as an intermediate state. Although variation was observed in the position of the tubercle, intermediate projections are usually situated ventral to the coronoid process, whereas well-developed projections are more posteriorly located. As a final criterion for state recognition, the projection is always present in the juveniles of taxa with well-developed processes as adults (state 2), whereas taxa with reduced processes as adults have no trace of a process as juveniles (state 1). Most oryzomyines have a well-developed capsular process. Handleyomys, Lundomys, Melanomys, Neacomys musseri, Nectomys squamipes, Oecomys catherinae, Oe. concolor, Oe. mamorae, Oryzomys albigularis, O. alfaroi, O. levipes, O. rostratus, O. palustris, O. talamancae, O. xanthaeolus, Scolomys, and Zygodontomys brevicauda have small capsular processes, whereas specimens of Oryzomys hammondi, O. macconnelli, O. megacephalus, O. polius, and O. yunganus have none. My coding of Handleyomys differs from Voss et al. (2002), who reported the projection as absent. Sigmodontomys aphrastus is the only polymorphic taxon (coded as 01). Outgroups are variable for this character, but the two non-sigmodontines Nyctomys and Peromyscus have the intermediate condition. This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 46

Superior and inferior masseteric ridges converge anteriorly as an open chevron (0); or anterior portion of ridges conjoined as single crest (1). The upper and lower masseteric ridges in most oryzomyines converge anteriorly as two separate crests, joining only at their anteriormost point (Voss and Carleton, 1993; see fig. 24A). In some specimens, the ridges run closely parallel for some extent before contacting, but they are always discernible. Conversely, in Holochilus, Melanomys, Nectomys, Nesoryzomys, Oryzomys polius, O. xanthaeolus, and Zygodontomys, the anterior portion of the ridges is made of a single crest because the two ridges fuse to form a single masseteric crest below m1 (fig. 24C). Sigmodontomys alfari is polymorphic for this character. Outgroups uniformly have the open chevron pattern. No information is currently available about this character for Amphinectomys (coded “?”).

Character 47

Anterior edge of masseteric crests below m1 (0); or edge anterior to m1, extending to diastema (1). Although variable among analyzed specimens, most oryzomyines have the anterior edge of the masseteric ridges below the first lower molar, usually at the procingulum (fig. 24A, B), but in specimens of Holochilus, Nesoryzomys, Oligoryzomys, and Zygodontomys, the masseteric ridges extend anterior to m1 and approach the diastema or the body of the mandible ventral to the diastema (fig. 24C, D).

Character 48

Entoglossal process of basihyal present as small knob, basihyal arched, and thyrohyal long, greater than or equal to the length of the basihyal (0); or entoglossal process absent, basihyal straight, and thyrohyal short, less than length of basihyal (1). All sigmodontines examined to date have the same general hyoid conformation, lacking the entoglossal process and with a straight basihyal and a short thyrohyal (Carleton, 1980; Steppan, 1995; Pacheco, 2003). Oryzomyines analyzed here also follow this pattern and contrast with the hyoid morphology observed in most neotomines and nyctomyines (including Peromyscus and Nyctomys), which possess an entoglossal process, a long thyrohyal, and an arched basihyal. This character serves as a diagnostic feature for sigmodontines (Voss, 1993; Voss and Carleton, 1993). Information on this character is missing for 27 species because hyoid bones were unavailable (coded “?” in table 5).

Character 49

Labial accessory root of M1 absent (0); or present (1). Molar teeth of sigmodontines are multirooted, and taxonomic difference in the number of roots has been used extensively in sigmodontine phylogenetic analysis (Carleton, 1980; Carleton and Musser, 1989; Voss, 1991; Voss and Carleton, 1993; Steppan, 1995; Carleton and Olson, 1999). Most oryzomyines have three roots at anterior, posterior, and lingual positions on the first upper molar (M1). A fourth accessory root at the labial position is present in a variety of taxa. An accessory rootlet at central position may also be present in combination or not with the labial one (Voss and Carleton, 1993). The condition of the central accessory root is polymorphic for several taxa and may be caused by droplets of enamel left in the area of furcation of the roots during molar growth (Sicher and Bhaskar, 1972). Therefore, I follow Voss and Carleton (1993) in coding only for the condition of the accessory labial root, which does not show intraspecific variation. Among oryzomyines, the accessory labial root of M1 is present in Holochilus, Melanomys, Neacomys spinosus, Nectomys, Nesoryzomys, Oecomys, Oryzomys alfaroi, O. angouya, O. chapmani, O. couesi, O. rostratus, O. palustris, O. subflavus, O. xanthaeolus, Pseudoryzomys, and Sigmodontomys alfari. It is absent in all other examined oryzomyines. Labial accessory roots are also present in the outgroups Thomasomys and Wiedomys. Some information for this and the next two characters was taken from the literature (in addition to the above-mentioned studies, see Hershkovitz, 1944; Voss, 1993). No information is currently available about this character for Amphinectomys (coded “?” in table 5).

Character 50

Labial and lingual accessory roots of m1 absent (m1 with two roots total) (0); or labial accessory root present (three roots total) (1); or labial and lingual roots present (four roots total) (2). On the first lower molar (m1), the number of roots varies from two to four, due to the presence or absence of accessory roots at labial and lingual positions. Among analyzed specimens, the lingual accessory root is present only in examples with the labial root, so I coded these features as a single ordered character (0 ↔ 1 ↔ 2). Among oryzomyines, the labial and lingual accessory roots are absent in Microryzomys, Neacomys, Oecomys bicolor, Oe. catherinae, Oe. concolor, Oe. mamorae, Oligoryzomys, Oryzomys hammondi, O. megacephalus, O. polius, O. talamancae, O. yunganus, Scolomys, and Zygodontomys; the labial accessory root is present in Oecomys trinitatis, Oryzomys albigularis, O. angouya, O. balneator, O. lamia, O. levipes, O. macconnelli, and O. russatus; and both labial and lingual accessory roots are present in Holochilus, Lundomys, Melanomys, Nectomys, Nesoryzomys, Oryzomys alfaroi, O. chapmani, O. couesi, O. rostratus, O. subflavus, O. xanthaeolus, Pseudoryzomys, and Sigmodontomys. The condition of the m1 accessory roots is variable in specimens of Handleyomys and Oryzomys palustris (coded as 12). No information is currently available about this character for Amphinectomys (coded “?”).

Character 51

Second lower molar with two roots (0); or three roots (1). The second lower molar (m2) of most sigmodontines is anchored by two roots, at anterior and posterior positions. By contrast, the anterior root is replaced by two smaller roots (at anterolabial and anterolingual positions) in Handleyomys, Holochilus, Oryzomys alfaroi, O. chapmani, O. rostratus, O. xanthaeolus, Pseudoryzomys, Sigmodontomys alfari, and Zygodontomys. Lundomys and Oryzomys palustris are polymorphic for this character. The condition of Amphinectomys, Nesoryzomys swarthi, Oryzomys couesi, O. hammondi, O. polius, and Sigmodontomys aphrastus could not be assessed (coded “?”).

Character 52

Incisors opisthodont (0); or orthodont (1). The degree of upper incisor procumbency is defined by the position of the cutting edge of the incisor relative to the vertical-incisive plane (Hershkovitz, 1962; Steppan, 1995). Despite the gradient of incisor curvature observed among taxa, a distinct interval is observed among orthodont and opisthodont categories. Most oryzomyines, and all outgroups, have opisthodont teeth with the cutting edge terminating posteriorly to the vertical-incisive plane. Amphinectomys, Handleyomys, Melanomys, Oryzomys hammondi, Scolomys, and Sigmodontomys aphrastus have orthodont incisors with the cutting edge perpendicular to the vertical-incisive plane. No oryzomyine has proodont or hyper-opisthodont incisors (see Steppan, 1995, for definition of these categories).

Character 53

Enamel band of upper incisors smoothly rounded, or flattened but without labial bevel (0); or band flattened medially, with distinct labial bevel (1). The putatively derived condition (state 1) is found only in species of Holochilus, where medial and lateral facets create a beveled enamel surface (Voss and Carleton, 1993: fig. 12). The remaining taxa all have incisors with smoothly rounded or flattened enamel surfaces. No information is currently available about this character for Amphinectomys (coded “?”).

Character 54

Molars bunodont and brachydont (0); or molars planar and hypsodont (1). The molars of Holochilus are hypsodont, with the cusps situated at the same occlusal level as the central elements of the molar teeth, forming a planar surface. Remaining oryzomyines have bunodont and brachyodont molars, with the lateral apices of the cusps situated higher then the central teeth elements.

Character 55

Labial flexi not patent, closed off by labial cingula (0); or labial flexi patent, cingula absent (1). The molars of all outgroups and most oryzomyines have labial cingula that close off the lateral apertures of all the labial flexi. In Holochilus and Lundomys, however, the labial flexi are patent (open), because the labial cingula are absent. No information is currently available about this character for Amphinectomys (coded “?”).

Character 56

Maxillary toothrows parallel (0); or anteriorly convergent (1). Although the direction of the toothrows appears to vary continuously among analyzed taxa, an unambiguous gap was detected between the more-or-less parallel toothrows seen in most oryzomyines and all outgroups, and the anteriorly convergent toothrow observed in Holochilus and Lundomys. No information is currently available about this character for Amphinectomys (coded “?”).

Character 57

Flexi of M1 and M2 do not interpenetrate (0); or flexi meet at midline, enamel overlaps (1); or flexi interpenetrate (2). The molars of most oryzomyines are nonlophodont, with opposite labial and lingual flexi conspicuously far from the end of the opposite flexi (e.g., fig. 25P). Several oryzomyines, however, display a certain degree of lophodonty, which varied from minimal lophodonty, where only the enamel border of the flexi reaches the end of the opposite flexi (e.g., fig. 25I), to incipient lophodonty, where flexi interpenetrate beyond the end of the opposite flexi (e.g., fig. 25H). Despite the apparent continuous nature of such variation, distinctions between the three states is unambiguous; the labial flexi (paraflexus and metaflexus) of nonlophodont taxa (state 0) are sharply curved (90°) before the molar midline (e.g., fig. 25P), whereas the labial flexi of minimally lophodont taxa (state 1) are gently curved (e.g., fig. 25I). Incipiently lophodont taxa have transversely oriented paraflexi and metaflexi (e.g., fig. 25H). See the matrix in table 5 for character-state distributions among taxa. This character was treated as ordered (0 ↔ 1 ↔ 2). No information is currently available about this character for Amphinectomys (coded “?”).

Character 58

Anterocone of M1 divided into labial and lingual conules by anteromedian flexus (0); or anterocone partially divided into labial and lingual conules by internal fold of procingulum, anteromedian flexus absent (1); or anterocone undivided, anteromedian flexus and internal fold of procingulum absent (2). The anteromedian flexus (fig. 25O, amf) is observed in all outgroup taxa and in examples of Lundomys, Microryzomys (fig. 25P), Neacomys minutus, N. musseri, Nesoryzomys narboroughi, Oecomys concolor, Oligoryzomys (fig. 25O), Oryzomys albigularis (fig. 25F), O. angouya (fig. 25A), O. balneator, O. levipes, O. polius, and O. xanthaeolus among oryzomyines. Oryzomys subflavus, O. palustris (fig. 25B), and Pseudoryzomys (fig. 25K) are the only taxa with the internal fold, which is sometimes fused with the anteroflexus. Carleton and Olson (1999) scored Zygodontomys brevicauda as having this last state, but I could not detect the presence of the internal fold in any exemplar of this taxon (e.g., fig. 25M). The remaining oryzomyines also have undivided anterocones (e.g., fig. 25L). Although the degree of excavation of the anteromedian flexus varies among taxa, I could not identify a clear distinction between weakly or deeply excavated states (e.g., fig. 25I vs. fig. 25O). This character was scored in young individuals because modifications of molar topography due to occlusal wear, such as the disappearance of the anteromedian flexus or the presence on an internal fold in the procingulum derived from the anteroflexus, can lead to erroneous scoring. This character was treated as ordered (0 ↔ 1 ↔ 2). Information about this and the next 19 dental characters was not available for Amphinectomys and Nesoryzomys swarthi (coded “?” in table 5).

Character 59

Anteroloph on M1 well-developed and discrete, reaching the labial cingulum, anteroflexus present (0); or anteroloph present but small, not reaching the labial cingulum, anteroflexus absent (1); or anteroloph fused with anterocone labially, anteroflexus present as small fossette (2); or anteroloph and anteroflexus absent (3). The anteroloph and anteroflexus are present and well developed among most oryzomyines (e.g., in Oryzomys albigularis, fig. 25F), with the following exceptions: Lundomys has a small anteroloph that does not reach the labial cingulum (state 1; fig. 25I); in Melanomys, Nectomys, Oryzomys xanthaeolus, Scolomys, and Sigmodontomys, the anteroloph is fused with the anterocone labially (state 2; fig. 25N); and in Holochilus, Pseudoryzomys, and Zygodontomys, the anteroloph is absent (state 3; fig. 25J). The anteroloph is present and well developed in all outgroups. Young specimens are necessary for scoring this character because modifications of molar topography due to occlusal wear can lead to erroneous scoring. Hershkovitz (1962) postulated that taxonomic variation in the development of the anteroloph is correlated with that of the mesoloph, and Carleton (1980) found a Spearman rank correlation of 94% between the condition of the anteroloph and mesoloph in a matrix that included a wide variety of New World muroids. Nevertheless, these structures show several cases of contrasting conditions, and thus they have previously been treated as separate characters in several analyses (e.g., Voss and Carleton, 1993; Steppan, 1995; Carleton and Olson, 1999). This character was treated as unordered.

Character 60

Protostyle on M1 absent (0); or present (1). The protostyle is absent in all analyzed taxa with the exception of Oryzomys angouya (fig. 25A), which has a small crest connected to the protocone.

Character 61

Paracone of M1 connected to protocone by enamel bridge situated at posteriormost end of protocone (0); or paracone connected to protocone by enamel bridge situated at anterior portion of protocone (1); or protocone and paracone forming single dentine basin without enamel connection (2). The paracone is connected to the posterior terminus of the protocone by an enamel bridge in several oryzomyines and in all outgroups (e.g., fig. 25C). This enamel connection also contacts the median mure in those taxa with the protocone-median mure connection (see character 63). Conversely, in several taxa the paracone and protocone are linked by an enamel bridge connected to the middle or to the anterior part of the protocone; these taxa exhibit an independent enamel connection to the median mure (e.g., fig. 25D). State 2 refers to Zygodontomys, in which the paracone and protocone are connected by a wide dentine surface rather than by an enamel bridge (fig. 25M). Although taxa displaying this latter pattern may have separated intercuspal dentine basins in unworn teeth (cf. Voss, 1988), they do not display a definitive connecting bridge, but rather the dentine basins are slightly obliterated by the penetration of the paraflexus. This character was analyzed in individuals with a moderate level of occlusal wear, as enamel connections are not apparent in unworn teeth. See the matrix in table 5 for character-state distributions among the remaining taxa, of which only Oryzomys rostratus and O. subflavus were scored as polymorphic (i.e., 01). This character was treated as unordered.

Character 62

Mesolophs on M1 and M2 well-developed, extending from the median mure to the labial cingulum, fused with mesostyle (0); or mesolophs small, not extending to labial cingulum and not fused with the mesostyle (1); or mesolophs on M1 and M2 absent (2). Most oryzomyines have well-developed mesolophs (fig. 25B, ml). Exceptions include Pseudoryzomys (fig. 25K), Lundomys (fig. 25I), and Holochilus brasiliensis (fig. 25J), which have small mesolophs (state 1); and Holochilus chacarius and Zygodontomys (fig. 25M), which lack mesolophs (state 2). The mesoloph is well developed in all outgroups. This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 63

Median mure connected to protocone on M1 (0); or median mure not connected to protocone (1). The median mure is an enamel crest connecting the anterior (protocone and paracone) and posterior (hypocone and metacone) pairs of molar cusps (fig. 25M, mm). In most oryzomyines and in all outgroups, the median mure is connected to the posterior end of the protocone, whereas the median mure is connected to the paracone in Holochilus (fig. 25J).

Character 64

Protoflexus of M2 present (0); or absent (1). Most oryzomyines have a protoflexus on M2 (fig. 25C, pf), usually as a shallow indentation that does not attain the degree of excavation of other major folds. In contrast, the protoflexus is completely absent and the anterolingual border of M2 forms a smooth and rounded surface continuous with the protocone in Holochilus (fig. 25J), Nectomys, Nesoryzomys narboroughi, Oryzomys hammondi (fig. 25E), O. xanthaeolus, Scolomys (fig. 25N), Sigmodontomys (fig. 25L), and Zygodontomys (fig. 25M). Among outgroups, only Wiedomys specimens lack the protoflexus. The protoflexus can disappear with moderate wear, so younger specimens were used to score this character.

Character 65

Paracone on M2 without accessory loph (0); or accessory loph present posterior to paracone (1). An accessory loph is present in specimens of Oecomys and Nyctomys due to the presence of an additional fossette anterior to the mesoflexus (fig. 25G, acc). This loph is situated between the paracone and the mesoloph on M2 (and M3). All other taxa lack this accessory loph; the labial fossette of some species (see character 66) sometimes creates the impression of an accessory loph (e.g., Oryzomys angouya, fig. 25A), but it never reaches the labial cingulum.

Character 66

Mesoflexus present as single internal labial fossette on M2 (0); or mesoflexus divided into labial and medial fossetti (1). The internal fossetti observed on M2 between the mesoloph and the paracone are derived from the engulfment of the mesoflexus caused by wear of the occlusal surface. The presence of a single labial fossette, sometimes extending into the central portion of the tooth (e.g., fig. 25F), is the widespread condition among pentalophodont oryzomyines. The only taxa with an additional central fossette (fig. 25D, cf) are Handleyomys and several species of Oryzomys: O. alfaroi, O. angouya, O. chapmani, O. couesi, O. macconnelli, O. rostratus, O. palustris, O. russatus, O. subflavus, and O. yunganus. The present character is not applicable for taxa that lack a mesoflexus, such as Holochilus chacarius and Zygodontomys, and for taxa without an engulfed mesoflexus, such as Peromyscus, Lundomys, Holochilus brasiliensis, and Pseudoryzomys (coded “–” in table 5).

Character 67

Mesoloph on M3 present and well developed (0); or absent or vestigial (1). The mesoloph on M3 is usually present in taxa with mesolophs on M1 and M2, but some taxa do not follow this pattern. Lundomys and Pseudoryzomys have small mesolophs on M1 and M2 but not on M3 (Voss and Carleton,1993; Carleton and Olson, 1999); Holochilus chacarius does not have mesolophs on M1 and M2 but has one on M3 (fig. 25J); and Scolomys has mesolophs on M1 and M2 but lacks one on M3 (fig. 25O). All outgroups and remaining oryzomyines, with the exception of Zygodontomys, have a mesoloph on M3.

Character 68

Posteroloph on M3 present (0); or absent (1). The posteroloph is one of the first structures to lose its identity on the molar occlusal surface, joining the metacone in early stages of wear. Nevertheless, the posteroloph still can be identified on moderately worn third upper molars (M3) by the presence of an internal fossette derived from the posteroflexus. This fossette separates the metacone from the posterior rim of tooth, and thus creates an additional loph, the posteroloph. Several oryzomyine taxa have a posteroloph (fig. 25F, pl), including Holochilus, Lundomys, Melanomys, Nectomys, Nesoryzomys narboroughi, Oecomys, Oligoryzomys, Pseudoryzomys, Sigmodontomys, some species of Oryzomys (O. albigularis, O. angouya, O. couesi, O. hammondi, O. levipes, O. palustris, O. polius, O. subflavus, O. xanthaeolus), and Zygodontomys brevicauda. In several other oryzomyines, however, the metacone is the posteriormost structure on M3 (e.g., fig. 25P). Such taxa include Handleyomys, Microryzomys, Neacomys, Scolomys, most species of Oryzomys, and Zygodontomys cherriei. Among non-oryzomyine sigmodontines and outgroups, only Peromyscus specimens lack a posteroloph on M3.

Character 69

Hypoflexus on M3 present, remaining excavated until later wear stages (0); or hypoflexus absent or diminutive, disappearing with little occlusal wear (1). Most oryzomyines have a distinct hypoflexus on M3 (fig. 25G, hf) that is conspicuous in moderately worn dentition, but usually disappears in older specimens with heavily worn teeth. Conversely, the M3 hypoflexus of several other oryzomyines is absent or diminutive even in younger specimens, never reaching the degree of excavation that this structure exhibits on M2, and it disappears in the earliest stages of molar wear (e.g., fig. 25H). My analyzed sample of Oryzomys subflavus included young specimens with and without the hypoflexus (scored as polymorphic, 01). See the matrix in table 5 for character-state distributions among remaining taxa.

Character 70

Anteromedian flexid and anteromedian fossettid absent on first lower molar (0); or anteromedian flexid absent but anteromedian fossettid present (1); or anteromedian flexid present and anteromedian fossettid absent (2). Most analyzed taxa have an undivided m1 anteroconid with an internal enamel pit, the anteroconid internal fold or anteromedian fossettid (state 1; fig. 26G, aif). Departing from this pattern, specimens of Thomasomys, Wiedomys, Microryzomys, and Oryzomys balneator have a deep anteromedian flexid (fig. 26P, amf) dividing the anteroconid into distinct anterolabial and anterolingual conulids (state 2). The putatively plesiomorphic condition (anteroconid without anteromedian flexid or anteromedian fossettid) was observed only in specimens of Peromyscus. The character description is based on the assumption that the anteromedian fossettid is actually an engulfed anteromedian flexid, and is thus homologous to the latter structure (Steppan, 1995; Carleton and Olson, 1999). This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 71

Anterolabial cingulum on m1 absent (0); or long anterolabial cingulum present (1). A distinctive anterolabial cingulum is observed in all oryzomyines (e.g., fig. 26O, alc) except Oryzomys lamia and Pseudoryzomys (fig. 26K). The anterolabial cingulum is usually fused with the anterior labial surface of the protoconid, leaving the protoflexid as an internal fossettid. Among non-oryzomyines, the anterolabial cingulum was observed in specimens of all taxa except Nyctomys.

Character 72

Ectolophid and ectostylid on m1 absent (0); or present (1). Most oryzomyines lack an ectolophid and an ectostylid on the first and second lower molars (e.g., fig. 26J). Several oryzomyine species, however, have distinctive ectolophids and/or ectostylids. The ectolophid may be developed and reach the ectostylid (fig. 26G, el), or it can be smaller, not reaching the ectostylid, or even merged to the mesoconid. I could not sort these configurations into different states because they are observed within samples of the various species displaying the ectostylid and ectolophid. The putative apomorphic state is observed in Nyctomys, Delomys, Thomasomys, Handleyomys, Oecomys, and several species of Oryzomys (O. albigularis, O. alfaroi, O. angouya, O. hammondi, O. levipes, O. macconnelli, O. megacephalus, O. talamancae, and O. yunganus).

Character 73

Mesolophids present and well developed on m1 and m2 (0); or mesolophids present in unworn dentition but small, not extending to lingual cingulum (1); or mesolophids completely absent (2). Almost all analyzed taxa have the same condition for the mesolophid on m1/m2 and for the mesoloph on M1/M2 (character 62; see the matrix in table 5 for character-state distributions among taxa). Nonetheless, specimens of Pseudoryzomys simplex and Holochilus brasiliensis have small mesolophs on their upper molars but do not have mesolophids on their lower molars (e.g., fig. 26K), and I therefore included the two features as independent characters (following Voss and Carleton, 1993). In adult individuals with advanced molar wear, the mesolophid fuses with the entoconid, with the consequent loss of the entoflexid. This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 74

Anterolabial cingulum present on m2 (0); or absent (1). Almost all analyzed taxa have an anterolabial cingulum present on the second lower molar (fig. 26C, ac), but the structure is absent in specimens of Sigmodontomys alfari. The character definition is modified from the description of Steppan (1995: char. 22P; described as procingulum on m2). In his description, Steppan (1995) defined three states: absence; cingulum appears as groove, wearing away with age; and cingulum well developed. I could not find a suitable discrete interval for the last two conditions, which I joined in a single state.

Character 75

Anterolophid absent or weakly expressed on m2 and m3 (0); or anterolophid and companion metaflexid distinct (1). A distinct anterolophid on m2 and m3 (fig. 26B, al) was observed only in Pseudoryzomys and three species of Oryzomys (O. couesi, O. palustris, and O. polius). The remaining taxa lack distinct anterolophids on both m2 and m3 (e.g., fig. 26D).

Character 76

Anterolabial margin of m3 with shelflike cingulum, separated from protoconid by protoflexid (0); or anterolabial margin of m3 smoothly rounded, without cingulum, protoflexid absent (1). Most analyzed taxa have a distinct anterolabial cingulum on m3 (fig. 26A, alc), but specimens of Neacomys musseri, Nectomys squamipes, Oryzomys hammondi, and Sigmodontomys lack the cingulum (e.g., fig. 26E). The distribution of states of the anterolabial cingulum of m3 is not identical to that of the serially homologous structure on m2, as several taxa with the cingulum present on m2 do not possess it on m3. I scored this character only in young specimens because the protoflexid disappears and the anterolabial cingulum joins the protoconid even in lightly worn teeth. I could not define discrete states related to the development of the cingulum (broad flange or narrow ridge) as did Carleton and Musser (1989: char. 16). Information about this character is not available for Amphinectomys, Handleyomys, and Nesoryzomys swarthi (coded “?” in table 5).

Character 77

Posteroflexid on m3 present, well developed (0); or posteroflexid present as a small groove, obvious only in juveniles, obliterated with wear (1); or posteroflexid absent (2). In most oryzomyines, the posteroflexid is present as a conspicuous internal fossettid on the m3, even in most old exemplars with advanced wear on teeth (fig. 26F, pf). In contrast, Pseudoryzomys displays a minute posteroflexid only in young individuals (fig. 26K), whereas specimens of Holochilus, Lundomys, and Zygodontomys lack the posteroflexid altogether, even in juveniles with unworn teeth (e.g., fig. 26J). The posteroflexid was observed in all outgroup taxa except Peromyscus. This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 78

13 ribs present (0); or 12 ribs (1). All oryzomyine species have a modal count of 12 ribs (see also Voss, 1993; Steppan, 1995; Voss et al., 2002; Pacheco, 2003). Although intraspecific variation in rib counts is widespread (Steppan, 1995, table 5), most outgroup taxa display 13 ribs, but Wiedomys has 12. The definition of this character follows Steppan (1995: char. 79P; see also Voss, 1993; Voss et al., 2002), who hypothesized that the presence of 12 ribs is a putative synapomorphy for oryzomyines. Carleton (1980: char. 36), Steppan (1995: char. 26S), and Pacheco (2003: chars. 96S, 124T) used modified definitions for this character; instead of ribs, these authors counted the number of thoracic vertebrae, also including the number of lumbar vertebrae in the character definition. In this context, taxa with 12 (rib-bearing) thoracic vertebrae have 7 lumbar vertebrae, whereas all taxa with 13 thoracics have 6 lumbars. Character information for this and the next six postcranial osteological characters is missing for several species that are not represented by complete skeletal preparations in my material, but some data were retrieved from Steppan (1995).

Character 79

Tuberculum of first rib articulates with transverse process of first thoracic vertebra only (0); or first rib contacts transverse processes of both the first thoracic and seventh cervical vertebrae (1). All sigmodontine rodents display the dual articulation of the first rib with the first thoracic and seventh cervical vertebrae, a synapomorphy that supports the monophyly of Sigmodontinae sensu stricto (Carleton, 1980; Voss, 1993; Voss and Carleton, 1993). Both of my non-sigmodontine outgroups, Nyctomys and Peromyscus, exhibit the putatively plesiomorphic single articulation. Information for this character is unavailable for 13 taxa not represented by suitable skeletal material in my samples (coded “?”).

Character 80

Anapophyses present on the 17th thoracico-lumbar vertebra (0); or anapophyses absent or vestigial (1). The 17th thoracico-lumbar vertebra (TL17) is the 5th lumbar (L5) in specimens with 12 thoracic and 7 lumbar vertebrae, or the 4th lumbar (L4) in specimens with 13 thoracics and 6 lumbars (see character 78). In both cases, TL17 is always the third-to-last lumbar. The anapophyses (or processus accessorius; Nomina Anatomica Veterinaria, 1994) are spinous processes that “leave the caudal borders of the [vertebral] pedicle and, when well developed, form a notch lateral to the caudal articular process that receives the cranial articular process of the vertebra behind” (Evans, 1993: 175). In sigmodontines, anapophyses were consistently observed on TL11–TL16 (some specimens also have vestigial anapophyses on TL10), but the condition on TL17 is taxonomically variable. Most oryzomyines, both non-sigmodontines outgroups, and Delomys have conspicuous anapophyses on TL17 that approach the size of the anapophyses on TL16 (fig. 27A). In contrast, specimens of Thomasomys, Wiedomys, Melanomys, Microryzomys, Neacomys spinosus, Oecomys bicolor, Oe. mamorae, Oe. trinitatis, Oligoryzomys flavescens, Ol. fulvescens, Ol. nigripes, Oryzomys chapmani, O. couesi, O. megacephalus, O. rostratus, O. palustris, Pseudoryzomys, Scolomys, and Zygodontomys do not have anapophyses on TL17, or they have a vestigial process much smaller than the one on TL16 (fig. 27B). Information for this character is unavailable for 18 taxa not represented by suitable skeletal material in my samples (coded “?”).

Character 81

Hemal arches absent between caudal vertebrae 2 and 3 (0); or hemal arches present, with simple posterior border (1); or present, with spinous posterior border (2). Hemal arches (arcus hemales; Nomina Anatomica Veterinaria, 1994) are discrete ossifications that articulate with the ventral surface of the caudal ends of the second and third caudal vertebrae in some mammals (Evans, 1993; Steppan, 1995).7 Hemal arches were observed between the second and third caudal vertebrae in almost all oryzomyines except Scolomys and Nesoryzomys narboroughi (fig. 28A). The posterior border of the hemal arch has two configurations, with or without a spinous process (fig. 28B, C, respectively). Taxa without the spinous process are Handleyomys, Microryzomys, Neacomys spinosus, Oecomys bicolor, Oe. concolor, Oe. mamorae, Oe. trinitatis, Oryzomys albigularis, O. chapmani, O. levipes, O. macconnelli (fig. 28B), O. megacephalus, O. rostratus, O. russatus, O. talamancae, and O. yunganus. The posterior spinous border is observed in the hemal arches of Holochilus brasiliensis, Lundomys, Melanomys, Nectomys, Nesoryzomys swarthi, Oligoryzomys fulvescens, Ol. nigripes, Oryzomys angouya, O. couesi, O. palustris (fig. 28C), O. subflavus, O. xanthaeolus, Pseudoryzomys, and Sigmodontomys alfari. Both species of Zygodontomys exhibit intraspecific variation for this feature, with all three states observed among analyzed specimens. Among outgroups, hemal arches were observed in Wiedomys (without posterior spinous border), whereas Thomasomys is polymorphic for this character (coded as 01). This character was treated as unordered. Information about the condition of hemal arches is unavailable for Amphinectomys, Holochilus chacarius, Neacomys minutus, N. musseri, Oecomys catherinae, Oligoryzomys flavescens, Ol. fornesi, Ol. stramineus, Oryzomys alfaroi, O. balneator, O. hammondi, O. lamia, O. polius, and Sigmodontomys aphrastus (coded “?”).

Character 82

Entepicondylar foramen of humerus present (0); or absent (1). The entepicondylar foramen perforates the distal end of the humerus above the medial epicondyle of most neotomines (e.g., Peromyscus) and all tylomyines (e.g., Nyctomys). The foramen is absent in all sigmodontines and might be a putative synapomorphy for the subfamily (Carleton, 1980; Voss, 1993; Voss and Carleton, 1993; Steppan, 1995; Pacheco, 2003).8 This character could not be scored for 13 taxa without available humeri or prior literature information (coded “?”).

Character 83

Supratrochlear foramen in humerus absent (0); or present (1). In contrast to the entepicondylar foramen, the supratrochlear foramen is absent in both Peromyscus and Nyctomys but present in most sigmodontines (fig. 29). The supratrochlear foramen perforates the distal portion of the humerus between the medial and lateral epicondyles and is larger than the entepicondylar foramen when both openings are present. The only sigmodontines in the current dataset without the supratrochlear foramen are Thomasomys, Oecomys bicolor, Oe. concolor, and Oryzomys angouya. This character was not scored for 20 taxa without available humeri or prior literature information (coded “?”).

Character 84

Trochlear process of calcaneum at the same level as posterior articular facet, trochlear process broad and shelflike (0); or gap between proximal edge of trochlear process and posterior articular facet, process shorter and less shelflike (1). All sigmodontines in the present dataset, with the exception of Thomasomys, have a gap between the proximal edge of the trochlear process and the posterior articular facet (Carleton, 1980: fig. 16). In Thomasomys and both non-sigmodontine outgroups, the trochlear process is broad and shelflike and reaches the same level as the posterior articular facet. The presence of a gap (state 1) is a putative sigmodontine synapomorphy (Steppan, 1995). This character was not scored for 15 taxa without available humeri or prior literature information (coded “?”).

Character 85

Lateral bacular mounds absent or diminutive (0); or present as large protuberances (1). The tridigitate distal cartilaginous apparatus, characteristic of muroid rodents with a complex penis, is contained by the bacular mounds at the terminal crater of the glans. (Hooper, 1960, 1962; Hooper and Musser, 1964). The lateral mounds containing the lateral bacular digits are strongly developed in the vast majority of sigmodontines analyzed to date (Hooper, 1962; Hooper and Musser, 1964; Carleton, 1980; Patton and Hafner, 1983; Spotorno, 1992; Langguth and Silva Neto, 1993; Malygin and Rosmiarek, 1996). In contrast, Nyctomys has a central bacular mound and vestigial lateral mounds (Hooper and Musser, 1964), and Peromyscus lacks lateral mounds completely (Hooper, 1958). This character was not scored for 18 taxa without preserved male genitalia or prior literature information (coded “?” in table 5).

Character 86

Large bacular cartilaginous apparatus with central digit more robust than lateral digits (0); or reduced cartilaginous apparatus with slim central digit (1). The cartilaginous distal bacular apparatus of most oryzomyines consists of three large digits that approach or exceed half the length of the proximal bone (Hooper and Musser, 1964: table 2). The central digit in these taxa is more robust than the lateral ones (fig. 30A). In contrast, Melanomys, Nesoryzomys, Oryzomys alfaroi, O. chapmani, O. rostratus, O. subflavus, O. xanthaeolus, Pseudoryzomys, and Sigmodontomys alfari have a reduced tridigitate apparatus (approximately less than one-third of the proximal bone length) in which the slender central digit is less robust than the lateral digits (fig. 30B; see also Hooper and Musser, 1964; Patton and Hafner, 1983). Sigmodontine outgroups scored for this character display a robust central digit. The present character is not applicable for taxa without a tridigital apparatus (Nyctomys and Peromyscus; coded “–”). This character was not scored for 18 taxa without preserved male genitalia or prior literature information (coded “?”).

Character 87

Nonspinous tissue of crater rim does not conceal bacular mounds (0); or nonspinous tissue conceals bacular mounds (1). Most of the exterior surface of the oryzomyine penis is densely covered with small spines, but a distinct distal band of softer tissue surrounding the crater rim is nonspinous (Hooper and Musser, 1964). In some oryzomyines (Lundomys, Melanomys, Neacomys, Oligoryzomys flavescens, Ol. fulvescens, Ol. nigripes, Oryzomys alfaroi, O. balneator, O. chapmani, O. couesi, O. rostratus, O. subflavus, O. xanthaeolus, and Sigmodontomys alfari), this band of soft nonspinous tissue is broad enough to conceal the bacular mounds, but in others (Holochilus brasiliensis, Oecomys bicolor, Oe. concolor, Oe. mamorae, Oryzomys albigularis, O. angouya, O. levipes, O. macconnelli, O. megacephalus, O. palustris, O. talamancae, O. yunganus, Scolomys, and Zygodontomys), the nonspinous tissue is less extensive and the bacular mounds are exposed. This character was not scored for 21 taxa lacking preserved male genitalia or prior literature information (coded “?”). Neither non-sigmodontine outgroup could be scored for this character because bacular mounds are not developed (coded “–”).

Character 88

Dorsal papilla of glans penis spineless (0); or spinous (1). The dorsal and lateral surfaces of the dorsal papillae are free of spines (fig. 31A) in most examined oryzomyines, but the dorsal papilla is studded with numerous spines in Oligoryzomys, Oryzomys couesi, and O. palustris (fig. 31B; see also Hooper and Musser, 1964). This character is inapplicable for Peromyscus (in which the dorsal papilla is absent; coded “–”) and was not scored for 17 taxa without preserved male genitalia or prior literature information (coded “?”).

Character 89

Subapical lobule on ventral surface of urethral processes absent (0); or present (1). The urethral processes are simple in most oryzomyines (fig. 32A), but a fleshy subapical lobule (fig. 32B, sl) is present on the ventral surface of each process in Oryzomys couesi, O. palustris, and Holochilus brasiliensis (see also Hooper, 1962; Hooper and Musser, 1964; Langguth and Silva Neto, 1993; Voss, 1988). This character is inapplicable for Peromyscus (in which urethral processes are absent; coded “–”) and was not scored for 18 taxa lacking preserved genitalia or prior literature information (coded “?”).

Character 90

Two pairs of preputial glands present (0); or one pair present (1); or preputial glands absent (2). One pair of preputial glands is the widespread condition among sigmodontines (Arata, 1964; Carleton, 1980; Voss and Linzey, 1981; Patton and Hafner, 1983; Carleton and Musser, 1989), but preputial glands are absent (at least macroscopically) in Oligoryzomys fulvescens and Ol. nigripes (Myers and Carleton, 1981; Voss and Linzey, 1981).9 Among outgroups, Nyctomys possesses two pairs of preputial glands (Carleton, 1980; Steppan, 1995), whereas preputial glands are macroscopically absent in Peromyscus (Linzey and Layne, 1969). Information about this and the following seven characters are missing for 27 taxa for which no information on accessory reproductive glands is available (coded “?”). This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 91

Two pairs of ventral prostate glands present (0); or ventral prostate glands absent (1). The derived condition of this character is found only in Nesoryzomys narboroughi (see Patton and Hafner, 1983) and Nyctomys (see Voss and Linzey, 1981; contra Carleton, 1980). All remaining muroid rodents analyzed to date have at least one pair of ventral prostates, and most remaining sigmodontines have two pairs (“Akodon” bogotensis and Thaptomys nigrita, two taxa not analyzed herein, have a single pair; see Arata, 1964; Carleton, 1980; Voss and Linzey, 1981). Among the five specimens of Nyctomys dissected by Voss and Linzey (1981), four lacked the ventral prostates entirely, while in one specimen a single small pair was present. I consider this a variation within the apomorphic condition.

Character 92

Anterior prostate glands present (0); or absent (1). Like the previous character, the apomorphic absence of the anterior prostate glands is found only in Nesoryzomys narboroughi (see Patton and Hafner, 1983) and Nyctomys (see Voss and Linzey, 1981; contra Carleton, 1980). All remaining taxa with available information about male accessory glands have one pair of anterior prostates (Arata, 1964; Carleton, 1980; Voss and Linzey, 1981).

Character 93

Vesicular glands present, large, shaped like a cane or inverted “J” (0); or vesicular glands present, shaped like small diverticula (1); or vesicular glands absent (2). The vesicular glands are usually the largest of the male glands complement, resembling J-shaped sacs (Voss and Linzey, 1981). In Nesoryzomys narboroughi, however, the vesicular glands are considerably smaller pear-shaped diverticulae (Patton and Hafner, 1983). The vesicular glands are absent in Nyctomys (Voss and Linzey, 1981; contra Carleton, 1980). This character was treated as unordered.

Character 94

Preputial glands extend to or beyond the ventral flexure of the penis (0); or do not extend to the ventral flexure of the penis (1). The preputial glands of all but one oryzomyine exceed the prepuce in length, reaching and sometimes extending cranially (anteriorly) beyond the ventral flexure of the penis (Voss and Linzey, 1981). The sole exception is Neacomys spinosus, in which the preputial glands do not reach the ventral flexure of the penis. Oligoryzomys fulvescens and Ol. nigripes could not be scored for this character because the preputial glands are absent (coded “–”).

Character 95

Dorsal prostates absent (0); or one pair present (1); or two pairs present (2). The presence of a second pair of dorsal prostates is reported by Arata (1964) for Sigmodontomys alfari, the only sigmodontine taxon possessing this condition. All other oryzomyines in the present study have only one pair of dorsal prostate glands. Among outgroups, Thomasomys has one pair and Nyctomys does not have prostate glands. Voss and Linzey (1981) argued that dorsal prostatic tissue encroaches upon the lateral surface of the male reproductive tract, and hypertrophy of this portion could result in the presence of two pairs of prostate glands in Sigmodontomys alfari. This character was treated as ordered (0 ↔ 1 ↔ 2).

Character 96

Ampullary glands forming tufts of tubules that extend cranially from the base of the vas deferens (0); or ampullary glands compact, not elaborate (1). The ampullary glands of most described oryzomyines and outgroups are compact and closely associated with the vas deferens. In contrast, the ampullary gland is composed of a series of tufts of tubules extending from the base of the vas deferens in Oryzomys megacephalus and O. talamancae (see Voss and Linzey, 1981).10

Character 97

Subterminal flexure of vesicular gland rounded and smooth (0); or irregularly lobed and notched (1); or small and finger-shaped (2). The subterminal region of the vesicular gland is smooth and rounded, not folded, in most oryzomyines and in Thomasomys (see Voss and Linzey, 1981). In contrast, the subterminal flexure is rough and irregular in Oryzomys palustris, O. megacephalus, O. alfaroi, and O. talamancae, whereas in O. rostratus it is reduced and constricted (Voss and Linzey, 1981). Patton and Hafner (1983) reported that the vesicular gland of Nesoryzomys narboroughi is reduced in size, pear-shaped, and smooth, but they did not specifically describe the condition of the subterminal flexure; therefore, this character was not scored for this taxon (coded “?”). Nyctomys could not be scored for this character because the vesicular glands are absent (coded “–”). This character was treated as unordered.

Character 98

Gall bladder present (0); or absent (1). The gall bladder is absent in all oryzomyines, a condition considered to be a diagnostic synapomorphy of the tribe (Carleton, 1980; Voss, 1991, 1993; Voss and Carleton, 1993; Steppan, 1995; Voss et al., 2002). The gall bladder is present in all outgroups for which information is available, with the exception of Nyctomys. Information for this character is unavailable for Wiedomys, Amphinectomys, Neacomys minutus, Nesoryzomys swarthi, Oecomys catherinae, Oligoryzomys fornesi, Ol. stramineus, and Oryzomys polius (coded “?”).

Character 99

Gastric glandular epithelium of stomach limited to antrum, not extending beyond incisura angularis (0); or gastric glandular epithelium covers antrum and proximal portion of corpus near esophageal opening (1). Oryzomyines have the basic unilocular-hemiglandular stomach pattern characteristic of sigmodontines, with a shallow incisura angularis that does not split the stomach in two major chambers, and a glandular epithelium that covers the anterior gastric chamber or antrum (Carleton, 1973; Vorontsov, 1979). Carleton (1973) recognized two major configurations of the glandular epithelium among taxa with the unilocular-hemiglandular pattern: (1) a group with the glandular epithelium restricted to the antrum that included most sigmodontines (fig. 33A); and (2) a smaller group with glandular epithelium extending into the corpus (the posterior chamber) that included Holochilus brasiliensis and Nectomys squamipes. Subsequently, the latter condition was also described for Microryzomys minutus (see Carleton and Musser, 1989) and Pseudoryzomys (see Voss and Carleton, 1993). Among the specimens I analyzed, state 1 was observed in Holochilus, Microryzomys, Neacomys musseri, Nectomys, Oryzomys balneator, Pseudoryzomys, and Sigmodontomys aphrastus (fig. 33B). The remaining analyzed oryzomyines, as well as all of my outgroups, have restricted glandular epithelium (state 0). Information on the stomach morphology is not available for Wiedomys, Amphinectomys, Neacomys minutus, Nesoryzomys swarthi, Oecomys catherinae, Oligoryzomys flavescens, O. fornesi, O. stramineus, Oryzomys angouya, O. lamia, O. levipes, O. polius, O. russatus, and Sigmodontomys alfari (coded “?”). This character is inapplicable for Peromyscus (which has a bilocular-discoglandular stomach structure; coded “–”).

Morphology-Only Analyses

A heuristic analysis of the morphological data matrix with polymorphic entries analyzed as composites (CO) resulted in one most parsimonious tree of 522 steps (CI = 0.25, RI = 0.63; table 7) consistent with the assumption of ingroup (oryzomyine) monophyly (fig. 34). After rooting the tree using Nyctomys and Peromyscus, the basal tree structure depicts the sequence (Thomasomys (Delomys (Wiedomys + Oryzomyini))). Oryzomyines are split basally into two major clades, the first with 28 and the second with 21 taxa. The first clade contains the genera Handleyomys, Microryzomys, Neacomys, Oecomys, Oligoryzomys, Scolomys, Zygodontomys, and 10 of the 19 analyzed species of Oryzomys. The second clade is composed of Amphinectomys, Holochilus, Lundomys, Melanomys, Nesoryzomys, Pseudoryzomys, Nectomys, Sigmodontomys, and the remaining nine Oryzomys species. Of the eight genera with multiple representatives, only Oryzomys and Sigmodontomys are not recovered as monophyletic.

The first oryzomyine clade is divided basally into two large subclades. In the first, Scolomys and Zygodontomys form a monophyletic group (labeled A in fig. 34) that is sister to the group containing Microryzomys, Neacomys, Oligoryzomys, and Oryzomys balneator (labeled C). Neacomys and Oligoryzomys are both recovered as monophyletic genera, and the latter is the sister group to a clade formed by Microryzomys and Oryzomys balneator. The second oryzomyine clade (labeled B*) contains Handleyomys, Oecomys, and eight species of Oryzomys. Oecomys is recovered as monophyletic, with O. hammondi as its sister group. Going up the tree, a clade with the three species of the nitidus species group (O. macconnelli, O. russatus and O. lamia) is joined by O. yunganus and then by a clade with O. megacephalus, O. talamancae, O. albigularis, O. levipes, and Handleyomys. The latter genus is the sister group of the albigularis species group (O. albigularis and O. levipes).

Oryzomys polius appears as the sister group to the remaining members of a second large group (labeled D* in fig. 34). The next branch contains O. angouya, and the remaining taxa are split into two clades. The first clade contains O. subflavus as the most basal taxon, followed by (1) the alfaroi group of Oryzomys—O. chapmani, O. alfaroi and O. rostratus—of which the latter two appear as sister species; (2) the palustris group of Oryzomys (O. palustris and O. couesi); and (3) a clade containing Pseudoryzomys, Lundomys, and Holochilus, with the latter two as sister taxa. Another clade contains Nesoryzomys as the most basal taxon, followed by six taxa in the sequence (Oryzomys xanthaeolus (Amphinectomys (Melanomys (Sigmodontomys aphrastus ( S. alfari + Nectomys))))).

A heuristic analysis of the morphological data with polymorphic entries analyzed as transformation series (TS) resulted in 22 most parsimonious trees of 693 steps each (CI = 0.22, RI = 0.62). The consensus tree (fig. 35) is the least resolved among the analyzed datasets, with only 31 of 51 (61%) resolved nodes and major polytomies between and within the major oryzomyine clades. The TS tree differs from the CO tree in several important aspects: (1) clade A is not recovered in the consensus tree, appearing in only in a few fundamental cladograms; (2) Amphinectomys appears as the sister taxon of Handleyomys within clade B*; (3) the alfaroi group of Oryzomys is recovered within clade B*; and (4) Sigmodontomys is recovered as monophyletic. Nevertheless, many relationships are congruent between the two analyses, including: (1) monophyly of Oryzomyini; (2) monophyly of clade C; (3) the three tetralophodont taxa Pseudoryzomys, Holochilus, and Lundomys and the palustris group of Oryzomys form a clade; (4) Oryzomys polius as basal member of clade D*; and (5) Nectomys and Sigmodontomys form a clade. Additionally, clades B* and D* are similar between the two analyses, with only minor differences in taxa content, and the monophyletic genera recovered in the CO analysis are also observed in the TS tree. Note also that all nodes at which the two analyses conflict are weakly supported in both trees.

Irbp-Only Analysis

A heuristic analysis of the IRBP dataset resulted in four equally parsimonious trees of 667 steps (CI = 0.55, RI = 0.71; table 7). The consensus topology (fig. 36) is identical to that previously recovered by Weksler (2003: fig. 5) when pruned of the outgroup taxa not included here. Within oryzomyines, a basal polytomy involving Scolomys, Zygodontomys, and a clade containing all of the remaining ingroup taxa is present in the consensus tree and in all fundamental cladograms. The largest group in this trichotomy is further divided into two clades. The first (labeled B in fig. 36) contains the genera Oecomys, Handleyomys, and 9 of the 16 analyzed species of Oryzomys. The internal structure of clade B in the strict consensus topology is characterized by a five-fold polytomy resulting from two solutions found in the fundamental cladograms, which depict either (Oryzomys talamancae (O. macconnelli (O. lamia + O. russatus))) or (Handleyomys intectus (O. alfaroi + O. rostratus)) as the basal clade relative to the remaining taxa.

The second major group, containing the remaining 11 oryzomyine genera and 7 species of Oryzomys, is divided into two clades. The first (labeled C) contains Microryzomys, Oligoryzomys, Neacomys, and Oryzomys balneator. The second (labeled D) consists of six species of Oryzomys interspersed among eight other genera. Within this group, (1) Oryzomys polius appears as the sister group to remaining species, and (2) three pairs of genera are recovered as sister groups: Holochilus + Pseudoryzomys, Amphinectomys + Nectomys, and Melanomys and Sigmodontomys. The species of Oryzomys included in this clade are not recovered as monophyletic.

Combined Analyses

A heuristic search of the combined data analyzed with CO polymorphic entries resulted in two most parsimonious trees of 1214 steps (CI = 0.38, RI = 0.65; table 7). Oryzomyines are again recovered as monophyletic, and Wiedomys is again its sister group (fig. 37). Oryzomys hammondi appears as the sister group of remaining oryzomyines, followed by the clade containing Scolomys and Zygodontomys (labeled clade A in fig. 37). The basal phylogenetic structure of remaining oryzomyines is similar to the one recovered by the IRBP-only analysis, with three large clades (labeled B, C, and D) containing 17, 10, and 18 species each. Clades C and D appear as sister groups. Within clade B, the clade containing the albigularis species group (O. albigularis and O. levipes) plus O. talamancae is recovered as sister group of the (O. megacephalus + O. yunganus) clade. Going up the tree, this clade is joined by a cluster containing Handleyomys and three species of Oryzomys (O. rostratus, O. chapmani, and O. alfaroi) and then by the monophyletic Oecomys. Oryzomys chapmani is found as the sister species to O. rostratus; the three species of the nitidus species group (O. macconnelli, O. russatus, and O. lamia) form a clade that is the sister group of the remaining species.

Within clade C, Microryzomys and Oryzomys balneator are recovered as sister groups and are joined successively by the monophyletic genera Oligoryzomys and Neacomys. Oryzomys polius appears as the sister group to remaining members in clade D, which is then split into two subclades. The first contains two species of Oryzomys (O. palustris and O. couesi) as the sister group to the clade containing Pseudoryzomys, Lundomys, and Holochilus, with the latter two as sister groups. The phylogenetic sequence of the second subclade starts with Oryzomys angouya, followed by O. subflavus, Nesoryzomys, and O. xanthaeolus. Nested within the subclade, (Amphinectomys + Nectomys) and (Melanomys + Sigmodontomys) are sister groups. Nectomys is recovered as monophyletic, but Sigmodontomys is not: S. aphrastus is recovered as the sister group of Melanomys, and the two are joined to S. alfari.

The polytomy present in the consensus tree (fig. 37) is the result of different arrangements within the genus Oligoryzomys in the fundamental cladograms: O. fulvescens is recovered either as the sister group of the (O. fornesi + O. flavescens) clade or as the sister group of the (O. stramineus + O. nigripes) clade.

Heuristic search of combined datasets with polymorphic entries analyzed as TS resulted in nine most parsimonious trees of 1388 steps (CI = 0.34, RI = 0.64). The consensus tree (fig. 38) resembles the CO combined tree, having six differences: (1) hammondi is within clade B; (2) the basal topology of major oryzomyines clades is unresolved; (3) the internal topology of clade B is unresolved; (4) Microryzomys and O. balneator do not form a monophyletic group; (5) Oligoryzomys is the basal taxon in clade C; and (6) O. angouya is the sister group of all members of clade D except O. polius. The polytomies present in the consensus tree (fig. 38) are the result of (1) different arrangements within the Sigmodontomys-Melanomys clade; (2) different ordering of xanthaeolus and Nesoryzomys; (3) different positioning of O. hammondi within clade B, either as sister group to Oecomys or as the most basal taxon in clade B; and (4) different positioning of clade A, either as basal to all oryzomyines or as sister group to clade B.

A node-by-node description of all groups recovered in the combined analysis with CO coding for polymorphisms (fig. 37) is presented in appendix 3.

Reduced Analysis

Heuristic search of the reduced dataset (i.e., excluding the five oryzomyines without molecular sequences) resulted in eight most parsimonious trees of 1174 steps (CI = 0.40, RI = 0.65; table 7). The recovered trees and the consensus cladogram (fig. 39) are similar to those retrieved by the CO combined analysis, except for three changes: (1) in clade B, Oryzomys albigularis is recovered as the most basal taxon, while the nitidus group is recovered as sister group to O. talamancae (i.e., O. albigularis and the nitidus group swap places); (2) in clade C, O. balneator and Microryzomys are not recovered as sister taxa, and instead are placed in a polytomy with Oligoryzomys and Neacomys; and (3) in clade D, Oryzomys angouya is recovered in a polytomy also containing the two major subclades of clade D.

Size

Of the 31 genera, species groups, or isolated oryzomyine lineages recovered in the analysis, 16 have species with the adult range of head-and-body length (HBL) between 90 and 175 mm (considered medium-sized; table 8). Two additional groups have HBL between 100 and 200 mm (considered large-medium) and two others have HBL between 80 and 145 (small-medium). Five groups are composed of small taxa, with HBL between 60 and 110 mm, while six groups are composed of large taxa, with HBL between 150 and 260 mm (some Nectomys and Holochilus can be as small as 125 mm, but are probably young adults). Thus 71 of 116 oryzomyines (61%) are either medium, small-medium, or large-medium in size, 29 (25%) are small-sized, and 16 (14%) are large-sized.

Inspection of body size distribution among the major oryzomyine lineages (fig. 43) reveals that all members of clade B are medium- or small-medium-sized, all members of clade C are small-sized, and all members of clade D are medium, large-medium, or large in size. O. hammondi is a large rat, while clade A includes both medium and small taxa. Reconstruction of size transformation in the recovered phylogeny (fig. 44) shows that the putative primitive condition for oryzomyines is that of a medium-sized rat. There are two transformations into the small class (Scolomys and clade B) and four transformations into the large class (hammondi, Lundomys + Holochilus, angouya, and Amphinectomys + Nectomys).

Arboreal and Semiaquatic Specializations

Oecomys is the only oryzomyine taxon with obvious specializations for arboreal life. Although species belonging to other oryzomyine clades (such Oligoryzomys and the subflavus group of Oryzomys) are sometimes reported to have some arboreal capacity (Alho, 1982; Fonseca and Redford, 1984; Alho and Villela, 1985; Mares et al., 1986), most individuals of these taxa are collected on the ground (Mares et al., 1989; Bonvicino et al., 2005). In contrast, various studies have shown that Oecomys specimens are found mostly, and sometimes exclusively, in trees, often as high as 15–20 m above the ground (Hershkovitz, 1960; Mares et al., 1989; Patton et al., 2000; Voss et al., 2001). The main adaptations for arboreal life observed in Oecomys include: more robust and compact hindfeet, with lateral broadening of the metatarsus; first and fifth digits more powerful and opposable; and plantar pads modified for grasping (Hershkovitz, 1960). These features are also observed in other arboreal sigmodontine rodents (e.g., Rhipidomys).

In contrast, semiaquatic (amphibious) specializations are observed in seven oryzomyine taxa: Amphinectomys, Nectomys, Lundomys, Holochilus, Sigmodontomys alfari, Pseudoryzomys, and in the palustris group of Oryzomys. The degrees of morphological adaptations of these taxa are proportionate to the amount of time the animals spend in water and to the type of activities performed there (i.e., foraging and nestbuilding; Stein, 1988). Specimens of Holochilus, Lundomys, Amphinectomys, and Nectomys, which are found almost exclusively alongside bodies of freshwater, display well-developed interdigital webbing and natatory fringes, whereas Oryzomys palustris, Pseudoryzomys simplex, and Sigmodontomys alfari, which are less dependent on water bodies, have reduced interdigital webbing and do not display natatory fringes. The same pattern is observed in relation to the size of these rats: Holochilus, Amphinectomys, Nectomys, and Lundomys are the largest living oryzomyines, while Sigmodontomys, O. palustris, and Pseudoryzomys are all medium-sized rodents. This size gradient is in agreement with the hypotheses of larger body size for semiaquatic small mammals (Wolff and Guthrie, 1985).

All semiaquatic oryzomyines are members of clade D, where they are divided into two lineages: (1) the palustris group, Pseudoryzomys, Lundomys, and Holochilus; and (2) Amphinectomys, Nectomys, and Sigmodontomys alfari, together with the terrestrial S. aphrastus and Melanomys caliginosus. Reconstruction of ancestral states of the characters related to the semiaquatic habitus (fig. 45) indicates that such adaptations occurred at least twice within oryzomyines. Natatory fringes appeared in the last common ancestor (LCA) of Lundomys and Holochilus and in the LCA of Amphinectomys and Nectomys. Optimizations of interdigital webbing transformations, however, are ambiguous (fig. 45). In ACCTRAN optimization, interdigital webs evolved twice and were lost twice: small membranes (not extending to first interphalangeal joints) appeared in the LCA of the palustris group + Lundomys + Holochilus + Pseudoryzomys, and in the LCA of Amphinectomys + Nectomys + Melanomys + Sigmodontomys; well-developed webs (extending to or beyond first interphalangeal joints) appeared in the LCA of Lundomys + Holochilus, and in the LCA of Amphinectomys + Nectomys; webs were lost in the lineage leading to Oryzomys couesi and in the LCA of Melanomys + Sigmodontomys aphrastus. In DELTRAN optimization, webbing appeared five times and was not lost in any lineage: small webs appeared in the LCA of Lundomys + Holochilus + Pseudoryzomys, and independently in the lineages leading to O. palustris and Sigmodontomys alfari, whereas large webs appeared in the LCA of Holochilus + Lundomys and in the LCA of Nectomys + Amphinectomys. Note that in ACCTRAN optimization, the sequence of modifications of the two hindfoot specializations follow the same pattern in the two semiaquatic clades, with the appearance of small webs preceding the synchronous development of long webs and natatory fringes.

Other hindfoot characters also seem to be influenced by the transition to a semiaquatic mode of life (fig. 45). Hypothenar pads, interdigital pads, and ungual tufts, for example, are all reduced or lost in semiaquatic taxa. Changes in these characters are synchronous with ACCTRAN-optimized changes in known semiaquatic specializations. Interdigital pads and ungual tufts were reduced, and hypothenar pads were lost in the LCA of the Oryzomys palustris group + Lundomys + Holochilus + Pseudoryzomys, and in the LCA of Amphinectomys + Nectomys + Melanomys + Sigmodontomys. Ungual tufts were lost in the LCA of Lundomys + Holochilus and in the lineage leading to Nectomys apicalis.12 Functional morphological research might shed light on the role (if any) of these features in semiaquatic locomotion.

Habitat

Twenty-one oryzomyine clades, encompassing 61% of all oryzomyine species, are found exclusively in forest environments, especially in ombrofilous forest. Five forest-dwelling clades (Nectomys, Oecomys, angouya, nitidus, and megacephalus) are also distributed in open vegetation biomes, such as the Cerrado and Llanos, but are mostly found in forest patches within open vegetation, such as gallery forests and “cerradão” (Alho et al., 1986; Mares et al., 1986; Nitikman and Mares, 1987; Mares et al., 1989; Bonvicino et al., 1996, 1998; Talamoni and Dias, 1999; Lacher and Alho, 2001). Five other oryzomyine clades representing 10% of oryzomyine species, however, are found only in open-vegetation biomes: Lundomys in the Pampas; Pseudoryzomys in the Cerrado, Caatinga, and Chaco; Zygodontomys in the Llanos and other northern South American and Central American savannas; and the xanthaeolus group of Oryzomys and Nesoryzomys in the dry coastal biomes of Western Peru, Ecuador, and the Galapagos Islands (Patton and Hafner, 1983; Voss, 1991; Voss and Myers, 1991; Voss and Carleton, 1993; Dowler et al., 2000; Guabloche et al., 2002). Finally, species of five clades (28% of oryzomyine species) are found in both forest and nonforest environments: Holochilus, Microryzomys, Oligoryzomys, palustris, and subflavus.

Forest-dwelling taxa occur in all oryzomyine lineages (fig. 46), whereas exclusively open-vegetation dwellers are found only in clades A and D. All members of clade B are exclusively sylvan taxa, while clades A, B, and D contain both types. Reconstruction of this feature in the recovered cladogram (fig. 46) indicates that the ancestral oryzomyine was a forest-dwelling taxon, and that invasions of open-vegetation environments occurred at least four times in oryzomyine evolution.

Three other significant modifications from the generalized oryzomyine bauplan are observed in restricted clades: the volelike appearance of Zygodontomys, Melanomys and Handleyomys species (Allen, 1913; Voss, 1991; Voss et al., 2001); the spiny pelage of Scolomys and Neacomys; and the hypsodonty of Holochilus molars (modified for intake of grass; Hershkovitz, 1955). Members of 11 lineages retained the primitive, unspecialized morphotype: S. aphrastus, O. polius, nitidus group, subflavus group, albigularis group, talamancae group, alfaroi group, chapmani group, melanotis group, megacephalus group, and yunganus group. As expected, all of these taxa, with the exception of S. aphrastus, are currently part of Oryzomys, highlighting, in a phylogenetic framework, the notion of the genus as a wastebasket group of unspecialized, mostly forest-dwelling, rats. As inferred by the reconstruction on the combined tree of size and the ecological adaptations discussed above, evolutionary niche shifts occurred de novo in different clades of these unspecialized oryzomyines. In turn, lineages that went through such evolutionary transitions are now among the most speciose oryzomyine clades assigned to genera: Oligoryzomys (16 spp.), Oecomys (15 spp.), Nectomys (8 spp.), and Neacomys (8 spp.). This indicates the conquest of different ecological roles in South American biomes that opened the possibilities for further diversification of oryzomyines.

Trans-Andean Distribution (Fig. 47)

This category encompasses taxa primarily distributed in lower-montane and lowland habitats west of the Andes, such as trans-Andean lowland rainforests (Voss and Emmons, 1996) and the arid coastal region of Peru and Ecuador (including the Galapagos Islands). Nine oryzomyine taxa are found in such trans-Andean landscapes: Melanomys, Sigmodontomys, Nesoryzomys, and six groups of Oryzomys (palustris, talamancae, melanotis, alfaroi, chapmani, and xanthaeolus). The chapmani and melanotis groups are restricted to forests of Central America and Mexico (Goldman, 1918), whereas Nesoryzomys occurs only in the Galapagos Islands (Dowler et al., 2000), and the xanthaeolus group occurs in both the Galapagos Islands and in the Pacific littoral zone of Peru and Ecuador (Patton and Hafner, 1983). The remaining trans-Andean taxa are found in forests from northwestern South America into Central America, or in the case of the palustris group, in the United States (Allen, 1913; Goldman, 1918; Hall, 1981; Musser and Carleton, 1993; Carleton and Musser, 1995; Voss and Emmons, 1996; Musser et al., 1998; Sanchez-H. et al., 2001).

Andean Distribution (Fig. 47)

This category encompasses taxa found only in Andean habitats such as montane forest and Paramos, usually on both sides of the cordillera and above 1500–2000 m. Six taxa are found exclusively or primarily in the Andes mountains: Microryzomys, Handleyomys, and four groups of Oryzomys (albigularis, balneator, hammondi, and polius). Microryzomys and the albigularis group are widely distributed in the Andes, from central Bolivia to northern Venezuela (Carleton and Musser, 1989; Patton et al., 1990; Musser and Carleton, 1993; Percequillo, 2003). The four remaining taxa have restricted range: Handleyomys is found in cloud forest of the Colombian Occidental and Central Cordilleras (Voss et al., 2002); O. balneator occurs in lower montane forest in both west and east slopes of the Andes in Ecuador and Peru (Musser et al., 1998); O. hammondi is known from a single locality from Ecuadorian western Andes (Thomas, 1913; specimens from UMMZ); and O. polius is known from a few localities from the eastern Peruvian Andes (Osgood, 1913; specimens from AMNH and FMNH).

Cis-Andean Distribution (Fig. 47)

This category encompasses taxa primarily distributed in lowland or lower montane biomes east of the Andes, such as Amazon, Atlantic and coastal Venezuelan rainforests, Llanos, Cerrado, Chaco, Pampas, and Caatinga. This category includes 15 taxa: Nectomys, Amphinectomys, Holochilus, Lundomys, Pseudoryzomys, Neacomys, Oecomys, Scolomys, and five groups of Oryzomys (megacephalus, yunganus, nitidus, angouya, and subflavus). Lundomys, Pseudoryzomys, and the angouya and subflavus groups occur only in southeast South America (central and eastern Brazil, Bolivia, Paraguay, Uruguay, and Argentina) (Voss and Myers, 1991; Musser and Carleton, 1993; Voss and Carleton, 1993; Percequillo, 1998; Langguth and Bonvicino, 2002; Bonvicino, 2003). Holochilus, Nectomys, Oecomys, and the megacephalus and nitidus groups are widely distributed in the Amazon basin, eastern Andean piedmont, and southeast South America (Hershkovitz, 1944, 1955, 1960; Musser et al., 1998; Percequillo, 1998; Weksler et al., 1999; Patton et al., 2000). Neacomys and the yunganus group occur throughout Amazonia (Musser et al., 1998; Patton et al., 2000; Voss et al., 2001), whereas Amphinectomys and Scolomys are found only in western Amazonia (Malygin et al., 1994; Patton and da Silva, 1995; Gómez-Laverde et al., 2004).

As expected in such simplifications, the geographic range of various taxa blurs the exact limits of the three categories. One species of the albigularis group, O. devius, is found in the highlands of Costa Rica and Panama (Carleton and Musser, 1995; Percequillo, 2003). One species of Oecomys (Oe. bicolor) and two species of Neacomys (N. tenuipes and N. pictus) are found in eastern Panama and western Colombia (Hall, 1981; Musser and Carleton, 1993); another species of Oecomys (Oe. trinitatis) reaches Costa Rica. Two species of Nectomys (N. magdalenae and N. grandis) are found in the inter-Andean valleys of the Magdalena and Cauca Rivers (Hershkovitz, 1944; Bonvicino, in prep.). One species of the yunganus group (Oryzomys tatei) is restricted to the eastern Andean piedmont (Musser et al., 1998) and could be considered an Andean taxon. Sigmodontomys and Melanomys occur in a few localities at the coastal Venezuelan forests east of the Andes, and Melanomys also occurs in some localities in the eastern Ecuadorian Andes piedmont (Voss and Emmons, 1996). In each of these cases, however, taxa have restricted distributions outside their main geographic category, and I assume this is a result of secondary, recent range expansion.

In contrast, two taxa could not be effectively categorized into the patterns. Oligoryzomys is distributed from Mexico to Terra del Fuego, being found east, west, and at the Andes mountains (Carleton and Musser, 1989; Patton et al., 1990; Carleton and Musser, 1995). Zygodontomys is widely distributed in open vegetation biomes of northern South America and eastern Central America, occurring east and west of the Andes (Voss, 1991). The species of these two genera are treated individually in the present analysis. Zygodontomys cherriei has trans-Andean distribution, whereas Z. brevicauda is distributed east of the Cordillera de Mérida (Voss, 1991). Among Oligoryzomys species, O. fulvescens is distributed from Mexico to the eastern Amazon basin in Brazil; remaining species are encompassed in the cis-Andean category, being restricted to southeastern South America.

The two major methodological categories currently in use in taxon biogeography (sensu Hovenkamp, 1997) can be divided in “projection-rule biogeography” and “vicariant biogeography” (Seberg, 1988). These approaches are considered separately bellow.

Vicariance Biogeography

No major vicariance scenario has been proposed for the biogeographic history of oryzomyines, or for any other sigmodontine tribe. The most obvious vicariance scenario for oryzomyines would be related to the Andean uplift, with the separation of trans- and cis-Andean lineages (van der Hammen, 1974). Molecular clock estimates for the initial diversification of oryzomyines point to a period between 5 and 9 mybp. This time range is based on the bounds estimated for the diversification of all sigmodontine lineages except sigmodonts and ichthyomyines by Steppan et al. (2004) using four nuclear genes, as well as on estimates for the origin of the oryzomyine lineage by Engel et al. (1998) using 1340 bp of the mitochondrial genome. Smith and Patton (1999: fig. 10) also placed the origin of oryzomyines in this period in their analysis of cytochrome b sequences. This time boundary is also congruent with the time estimated by DNA-DNA hybridization for the divergence of Akodon and Oryzomys (Catzeflis et al., 1993). If these estimates are corroborated in future studies that use better calibration points and denser taxonomic sampling, then oryzomyines could have been affected by the final orogenic surge of the Andes between 3 and 5 mybp (Irving, 1975; Simpson, 1979; Kellogg, 1984; Helmens and van der Hammen, 1994), a scenario proposed for other neotropical groups such as Heliconius butterflies (Brower, 1996), sand flies (Arrivillaga et al., 2002), tropidurine lizards (Harvey and Gutberlet, 2000), and various bird groups (Cracraft and Prum, 1988; Prum, 1988; Brumfield and Capparella, 1996).

The cis- and trans-Andean patterns, however, do not show correspondence to the major oryzomyine lineages (fig. 48). Instead, each major lineage displays all or most of the described biogeographic patterns. Clades B and D contain Andean, cis-Andean, and trans-Andean taxa, whereas clade C has Andean and cis-Andean taxa; clade A has cis-Andean and trans-Andean taxa. Even within each lineage, the distribution categories are not grouped into monophyletic units (fig. 48). In clade A, the two cis-Andean species are paraphyletic. In clade B, two unrelated taxa, Handleyomys and the albigularis group, have Andean distribution; three trans-Andean taxa (O. chapmani, O. alfaroi, and O. rostratus) are grouped, but a fourth taxon, O. talamancae, is not included in this clade; and the cis-Andean taxa form a paraphyletic grade. In clade C, the two Andean taxa (Microryzomys and O. balneator) do form a clade, but the cis-Andean taxa are observed within three clades: one formed by Neacomys and two subclades of Oligoryzomys. Finally, cis- and trans-Andean taxa of clade D are dispersed over several lineages. Only two clades in the present analysis follow a cis–trans-Andean sister-group pattern: the palustris (trans) and tetralophodont (cis) clade, and the Melanomys + Sigmodontomys (trans) and Amphinectomys + Nectomys (cis) clade. A third clade that follows this pattern is recovered with moderate support by the IRBP analysis, containing the talamancae (trans) and nitidus (cis) groups.

It is clear from the above patterns (or lack of thereof) that any analytical methodology within the vicariance framework will have to include several cases of over-Andean dispersal within oryzomyine phylogeny.

Progression Rule Biogeography

The center of origin concept was once ousted from historical biogeography (Croizat et al., 1974; Nelson and Platnick, 1981), but several procedures (Bremer, 1992, Ronquist, 1994, 1997; Hausdorf, 1998) have tried to reinstate it in cladistic framework under the label “ancestral area methodologies” (Ebach, 1999; Crisci, 2001). The basic premise of these methods is the progression rule of Hennig (1966), which Bremer (1994: 255–256) rephrased as “(1) areas positionally more plesiomorphic (present on ‘deep’ branches) in a cladogram of a particular group are more likely parts of the ancestral area for that group than are positionally more apomorphic areas and (2) areas represented on numerous branches of the cladogram are more likely parts of the ancestral area than are areas represented on a few branches.”

This methodological formulation fits well to biogeography of sigmodontines, as scenarios for the group have relied heavily on the center of origin concept, with a strong dispersalist element. Almost all classical sigmodontine biogeographic scenarios agree that oryzomyines, as all sigmodontines, are descendants from proto-sigmodontine ancestors that invaded (i.e., dispersed to) South America from North America in the late Cenozoic (Simpson, 1950; Hershkovitz, 1966b, 1969, 1972; Patterson and Pascual, 1968; Patterson and Pascual, 1972; Savage, 1974; Baskin, 19781986; Marshall, 1979; Reig, 1980; Simpson, 1980; Jacobs and Lindsay, 1984; Reig, 1984; Slaughter and Ubelaker, 1984; Reig, 1986; Czaplewski, 1987; Baskin, 1989). The South American fossil record strongly corroborates this hypothesis, as sigmodontine rodents suddenly appear in the well-known Argentinean fossil sequence at the Pliocene (Montehermosan and Chapadmalalan; Pardiñas et al., 2002).

The major disagreements between these dispersalist scenarios are the time of the dispersion and the place of the initial diversification for the sigmodontine groups (i.e., the place of the ancestral area of the group). Scenarios range from an early arrival (early or middle Miocene, 15–24 mybp) of a primitive stock of muroids in South America by waif dispersal or island hopping across the Bolivar Trough, and further radiation in the South American continent (e.g., Hershkovitz, 1966b; Reig, 1980, 1984), to a relatively recent entrance (Early or Middle Pliocene 2.5–4 mybp) of an already diversified sigmodontine stock through the Panama Isthmus (e.g., Patterson and Pascual, 1968; Patterson and Pascual, 1972; Baskin, 1986; Baskin, 1989; Czaplewski, 1987).

The oryzomyine fossil record is extremely poor and cannot provide, by itself, any evidence for the timing or the place of such initial radiation. The earliest oryzomyines in the South American fossil record are from the Pleistocene (Steppan, 1996; Pardiñas et al., 2002). Steppan (1996) estimated the age of Holochilus primigenus from Bolivia at between 0.7 and 1 mybp. Forms of Holochilus, Lundomys, Nectomys, and Oligoryzomys were retrieved from the Ensenadense (Early-Middle Pleistocene of Argentina; Pardiñas et al., 2002). All these taxa are placed at advanced, or apomorphic, positions in the tree, suggesting that diversification of the tribe must have occurred before the Pleistocene. Oryzomyines (and Sigmodontines in general) are absent from the middle Miocene (13 mybp) Honda group of La Venta, the richest Miocene tropical fossil deposit in South America (Kay and Madden, 1997). Sampling for small vertebrates at this site has been exemplary, as indicated by the recovery of examples of small mammals groups, such as echimyid rodents (Walton, 1997) and didelphid marsupials (Goin, 1997), that are often captured alongside oryzomyines in present-day tropical forest biomes (Voss and Emmons, 1996). Thus, based on this strong negative evidence, the upper bound for oryzomyine absence in South America is about 13 mybp.

The North American record also does not provide clues for timing and place of oryzomyine diversification. Previous suggestions that Oryzomys is present in the Pliocene or even Miocene in North America (Jacobs and Lindsay, 1984; Baskin, 1986, 1989) are unfounded. Oryzomys? pliocaenicus from the Miocene (Hemphilian) of Kansas (Hibbard, 1939) is “generically indeterminate, but may represent Bensonomys” (Baskin, 1986: 295; see also Hershkovitz, 1966b: 737). ?Oryzomys from the Miocene (Hemphillian) of Oregon (Shotwell, 1970) is more closely related to “Peromyscus” pliocenicus, putatively an ancestral to the fossil neotomine †Repomys (see May, 1981; Baskin, 1986). “Oryzomys” from the Early Pliocene (Blancan) of New Mexico (May inJacobs and Lindsay, 1984; Repenning and May, 1986) is “close dentally to †Jacobsomys and †Symmetrodontomys and might belong to the genus †Jacobsomys” (Czaplewski, 1987: 194). †Bensonomys and †Symmetrodontomys were placed by McKenna and Bell (1997, following Musser, in lit.) in the Peromyscini tribe, whereas †Jacobsomys was tentatively assigned to sigmodontines sensu lato (i.e., sensu Carleton and Musser, 1984) as Sigmodontinae incertae sedis. The earliest confirmed oryzomyines present in the North American fossil record are referred to Oryzomys fossilis (= Oryzomys palustris), from the Pleistocene of Florida, Georgia, Kansas, and Texas (Hibbard and Taylor, 1960; Dalquest, 1962; Hibbard, 1963; Webb, 1974; Kurten and Anderson, 1980; Webb and Wilkins, 1984; Hulbert and Pratt, 1998). All fossil localities of O. palustris with a more precise dating are from middle (Rancholabrean; 0.3 mybp) and late (Sangamonian; 0.13 mybp) Pleistocene deposits (Webb, 1974; Webb and Wilkins, 1984).

Fossils from other sigmodontine tribes, in turn, are present in both North and South American Pliocene. Fossils of Phyllotini and Akodontini are present in South America in the Montehermosan (4–5 mybp) and Chapadmalalan (3.5–4 mybp), respectively (Pardiñas et al., 2002), whereas fossils of †Prosigmodon and Sigmodon are present in North America in the Hemphillian (6.8 mybp) and Blancan (3.3 mybp), respectively (Martin, 1979; Czaplewski, 1987; Korth, 1994). Thus, fossil data alone do not provide much evidence for the ancestral area of oryzomyines.

Application of the progression rule to the current phylogeny suggests a South American ancestral area for oryzomyines. The most basal taxon for each lineage is cis-Andean or, in case of clade D, eastern Andean (fig. 48): Scolomys, nitidus group, Neacomys, and Oryzomys polius for clades A, B, C, and D, respectively. Central American (trans-Andean) taxa are always recovered deeply nested within the major lineages.

Given the estimated time for initial diversification of oryzomyines based on molecular studies, the oryzomyine ancestor must have arrived in South America prior to the formation of the Panamanian land bridge at 3.5–4.0 mybp (Coates et al., 1992; Ibaraki, 1997) by over-water dispersal (but see Coates et al., 2004, for new evidence pointing to a collision of the Central American arc with South America at 7.1 mybp). The capability of oryzomyines to undertake long-distance water dispersal is well known (Carleton and Olson, 1999). Oryzomyines are known from volcanic islands without former subaerial connections to the South American plate, such as the Galapagos Islands (Nesoryzomys and Oryzomys bauri) and Fernando de Noronha (†Noronhomys). These islands are situated at 1000 km and 310 km, respectively, from South America, a distance far exceeding the one among the stepping-stone connections that existed between the North and South American plates since the Miocene (Donnelly, 1992).

Thus, there is no phylogenetic indication that the immediate precursor of oryzomyines was in Central America as previously proposed (Patterson and Pascual, 1968, 1972; Baskin, 1978, 1986; Marshall, 1979; Simpson, 1980; Jacobs and Lindsay, 1984; Czaplewski, 1987; Baskin, 1989; Engel et al., 1998). Such a hypothesis would involve several ad hoc events of dispersal and extinction. The principal argument used for a North American diversification of oryzomyines was the supposed presence of members of the tribe (among other sigmodontines) in the North American Tertiary fossil record, that is, before the formation of the Panamanian land bridge. As discussed above, no undisputed fossil oryzomyine is known from the Tertiary of North America.

Current oryzomyine diversity in Central and North America is more likely a product of independent colonizations made by the different clades within oryzomyines that are currently found in that area (Zygodontomys, alfaroi-melanotis-chapmani, talamancae, Oligoryzomys, palustris, Melanomys, and Sigmodontomys). This scenario is similar to the one proposed by Hershkovitz (1966b), who suggested nonsynchronous dispersion of various oryzomyine lineages from northern South America in a timeframe ranging from before the completion of the Panamanian bridge to recent times. The palustris group was included by Hershkovitz in his “Stratum III: Old South American Migrants in North American”, which “returned over water routes to Middle America [during the Pliocene] and differentiated significantly [in situ]” (94Hershkovitz, 1966: 733). Melanomys, Sigmodontomys, talamancae, alfaroi, Zygodontomys, and Oligoryzomys were considered members of “Stratum IV: Late South American Migrants in Middle America”, which “spread over Panamanian land-bridge into Middle America [during Pleistocene]; [with] low grade subspeciation” (94Hershkovitz, 1966: 737). Oryzomys melanotis and Sigmodontomys aphrastus were of doubtful position. 94Hershkovitz (1966) also included in the recent migrants group some taxa that were considered here as with borderline distribution in Central America, such as albigularis, Oecomys, and Neacomys.

A slightly different scenario is the concomitant range expansion of all the independent oryzomyine lineages into Central America after the completion of the Panamanian land bridge, without over-water dispersal events in the Pliocene. Such invasion would be second only to the Didelphidae dispersal into Central America as part of the “Great American Interchange” in terms of number of genera, and would be the most successful in terms of species diversity. Much of the oryzomyine radiation is restricted to tropical North America, with only one taxon extending into the Nearctic region, a pattern also observed in other groups of South American invaders such as Didelphidae, Echimyidae, and Xenarthra (Webb and Wilkins, 1984). Some of the invading oryzomyine groups have experienced larger in situ diversification, such as the alfaroi-melanotis-chapmani clade, the palustris group, and Oligoryzomys (Hershkovitz, 1966b; Hall, 1981; Carleton and Musser and 1995), whereas others appear to be in early stages of dispersion into Central America, especially those cis-Andean taxa with minimal distribution in Central America, such as Neacomys and Oecomys.

A more precise location of the oryzomyine ancestral area in South America requires delimitation of smaller units for analysis than the three general categories provided here. Two areas are likely candidates: the region of premontane forests of the northern Andes and the western Amazon lowland forests. All basal taxa within each lineage—Neacomys, polius, hammondi, the nitidus group, and Scolomys—are distributed in the western Amazon and/or submontane Andes. The few biogeographical analyses done with oryzomyine genera or species groups also placed western Amazon/eastern Andes taxa as basal to their own lineages (Costa, 2003). In contrast, taxa restricted to other South American landscapes, such as southeastern South America (Lundomys, Pseudoryzomys, subflavus and angouya groups), are situated farther from the oryzomyine root in the present phylogeny and in lower level analyses (Patton et al., 2001; Costa, 2003).