Oreoryzomys, WEKSLER & PERCEQUILLO & VOSS, 2006

WEKSLER, MARCELO, PERCEQUILLO, ALEXANDRE REIS & VOSS, ROBERT S., 2006, Ten New Genera of Oryzomyine Rodents (Cricetidae: Sigmodontinae), American Museum Novitates 3537 (1), pp. 1-29 : 21-23

Oreoryzomys , new genus

TYPE SPECIES: Oryzomys balneator Thomas, 1900 .

CONTENTS: balneator Thomas, 1900 (including hesperus Anthony, 1924).

DISTRIBUTION: In humid montane (‘‘cloud’’) forest on the Andean slopes of southern Ecuador and northern Peru.

MORPHOLOGICAL DIAGNOSIS: Dorsal pelage dark olive-brown; ventral pelage abruptly paler (superficially whitish or yellowish), but ventral hairs mostly gray-based (a few specimens have irregular gular and/or pectoral patches of selfwhitish fur). Pinnae small, not reaching eye when laid forward. Mystacial vibrissae extending posteriorly beyond caudal margins of pinnae when laid back against cheeks; superciliary vibrissae shorter, not extending to caudal margins of pinnae when laid back. Pes with conspicuous tufts of long ungual hairs at bases of claws on dII–dV; plantar surface covered with distinct squamae distal to thenar pad; hypothenar pad present and distinct; claw of dI extends to middle of phalange 1 of dII; claw of dV extends to first interphalangeal joint of dIV. Tail unicolored (all-dark) or indistinctly bicolored basally, much longer than combined length of head and body.

Skull with short, narrow rostrum flanked by shallow zygomatic notches; interorbital region hourglass shaped, with rounded supraorbital margins; braincase rounded and globose, without temporal, lambdoidal, or nuchal crests. Posterior margin of zygomatic plate usually anterior to M1 alveolus. Jugal absent (maxillary and squamosal zygomatic processes in contact). Nasals extending posteriorly beyond lacrimals; lacrimals small, equally sutured to maxillary and frontal bones. Frontosquamosal suture usually colinear with frontoparietal suture. Parietals with or without small lateral expansions. Incisive foramina short, sometimes extending posteriorly to but not between M1 alveoli, and widest posteriorly (with anteriorly convergent lateral margins). Posterolateral palatal pits large and recessed in shallow fossae; mesopterygoid fossa extending anteriorly between maxillae but not between molar rows; bony roof of mesopterygoid fossa usually perforated by narrow sphenopalatine vacuities (the type of balneator has a completely ossified mesopterygoid roof). Alisphenoid strut absent (buccinator–masticatory foramen and accessory foramen ovale confluent). Stapedial foramen, squamosal–alisphenoid groove and sphenofrontal foramen present (5 carotid circulatory pattern 1 of Voss, 1988). Postglenoid foramen large and rounded, subsquamosal fenestra large and patent. Periotic broadly exposed posteromedially between ectotympanic and basioccipital, extending anteriorly to carotid canal; mastoid perforated by conspicuous posterodorsal fenestra. Capsular process of lower incisor alveolus strongly developed below base of coronoid process; superior and inferior masseteric ridges converge anteriorly as open chevron below m1.

Labial and lingual flexi of M1 and M2 not interpenetrating. First upper molar (M1) with anterocone divided into labial and lingual conules by anteromedian flexus; anteroloph well developed and fused with anterostyle on labial cingulum, separated from anterocone by persistent anteroflexus; protostyle absent; paracone connected by enamel bridge to anterior moiety of protocone. Second upper molar (M2) protoflexus present; mesoflexus usually present as single internal fossete. Third upper molar (M3) without a posteroloph, but with persistent hypoflexus. Labial accessory root of M1 absent.

Lower first molar (m1) anteroconid without a distinct anteromdian flexid (an indistinct flexid visible in some newly erupted dentitions is presumably obliterated with light wear); anterolabial cingulum present on all lower molars; anterolophid distinct on unworn m1 but absent on m2 and m3; ectolophid absent on all lower molars; mesolophid present and distinct on m1, reduced or absent on m2; m2 hypoflexid short. Accessory roots absent on m1; m2 and m3 each with one large anterior root and one large posterior root.

Postcranial skeletal characters unknown.

Stomach with extension of glandular epithelium into corpus. Distal bacular cartilage of glans penis large and trifid (with a robust central digit); nonspinous tissue on crater rim concealing bacular mounds; dorsal papilla nonspinous; urethral processes without subapical lobules.

COMPARISONS: ‘‘ Oryzomys ’’ balneator was consistently recovered as a member of clade C in the phylogenetic analyses of Weksler (2003, 2006). Although a sister-group relationship between Oreoryzomys and Microryzomys was moderately well supported in some analyses, Oreoryzomys was recovered as the sister taxon of Neacomys (albeit with only trivial support) in other analytic permutations. In effect, its relationships within clade C remain to be resolved convincingly, and comparisons with all three member genera (including Oligoryzomys ) seem appropriate.

As noted by Musser and Carleton (2005), Oreoryzomys and Microryzomys exhibit noteworthy similarities, but they also differ in many characters. Among the most useful morphological features for distinguishing these taxa, the pelage of Oreoryzomys is distinctly countershaded (the pelage is not countershaded in Microryzomys ); the claw of pedal digit V extends only to the first interphalangeal joint of dIV (the claw of dV extends well beyond the first interphalangeal joint of dIV in Microryzomys ); the tail is unicolored or indistinctly bicolored basally (the tail is more or less distinctly bicolored in Microryzomys ); the premaxillae do not extend as far posteriorly as the nasals do (the premaxillae and nasals extend posteriorly to about the same extent in Microryzomys ); the incisive foramina do not extend posteriorly between the M1 alveoli (as they usually do in Microryzomys ); the foramen magnum is more caudally oriented (versus more ventrally oriented in Microryzomys ); and the anteroconid of m1 is undivided (the anteroconid of m1 is deeply divided into anterolabial and anterolingual conulids by a persistent anteromedian flexid in Microryzomys ).

Oreoryzomys differs from Neacomys by its soft fur ( Neacomys has spiny fur); much longer tail (the tail of Neacomys seldom exceeds the combined length of head and body); shallower zygomatic notch (the zygomatic notch is moderately deep in Neacomys ); hourglass-shaped interorbital region with rounded supraorbital margins (the interorbital region is anteriorly convergent with beaded supraorbital margins in Neacomys ); absence of the jugal (the jugal is small but present in Neacomys ); and shorter palate (the mesopterygoid fossa extends anteriorly between the maxillary bones in Oreoryzomys but not in Neacomys ). We observed additional character differences between Oreoryzomys and some species of Neacomys , but none that appear to represent generically consistent distinctions.

Oreoryzomys differs from Oligoryzomys by its unicolored or indistinctly bicolored tail (the tail is distinctly bicolored in Oligoryzomys ); premaxillae that do not extend as far posteriorly as the nasals do (the premaxillae and nasals extend posteriorly to about the same level in Oligoryzomys ); shallower zygomatic notch (the zygomatic notch is moderately deep in Oligoryzomys ); shorter palate (the mesopterygoid fossa does not extend anteriorly between the maxillae in Oligoryzomys ); more deeply excavated parapterygoid fossae (the flat parapterygoid fossae of Oligoryzomys are almost level with the palate); smaller sphenopalatine vacuities (these openings are very large in Oligoryzomys ); complete stapedial circulation (the dorsal ramus of the stapedial artery is absent in Oligoryzomys ); more posterior position of the masseteric crest (which extends anterior to m 1 in Oligoryzomys ); absence of spines on the dorsal papilla of the gland penis (the dorsal papilla is spinous in examined species of Oligoryzomys ); and extension of glandular epithelium into the gastric corpus (the gastric corpus is entirely lined by cornified epithelium in examined species of Oligoryzomys ).

REMARKS: The available morphological character data for Oreoryzomys is incomplete because postcranial skeletons and male accessory reproductive gland dissections are currently unavailable. Although new morphological information from these (and other) systems might help resolve the relationships of this genus within clade C, the trenchant differences it exhibits with other member taxa suggests that such resolution will not affect its status as a distinct clade. Oreoryzomys is currently unrepresented by sequence data from any gene other than IRBP, hence the lack of prior information concerning phylogenetic relationships in published molecular studies of oryzomyines (e.g., Myers et al., 1995; Bonvincino and Moreira, 2001).

ETYMOLOGY: From oros (Greek for mountain), in reference to the montane distribution of this distinctive taxon.