Eremoryzomys, 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 : 10-11

Eremoryzomys , new genus

TYPE SPECIES: Oryzomys polius Osgood, 1913 .

CONTENTS: polius Osgood, 1913 .

DISTRIBUTION: Known only from a few localities in the upper Río Marañón valley of northern Peru.

MORPHOLOGICAL DIAGNOSIS: Dorsal pelage coarsely grizzled-grayish (but brownish- or yellowish-gray in some old and possibly soiled specimens); ventral pelage paler (superficially whitish), but ventral hairs always gray-based. Pinnae small, not reaching eye when laid forward. Mystacial and superciliary vibrissae not extending posteriorly beyond pinnae when laid back. Hind foot with conspicuous tufts of long ungual hairs at bases of claws on dI–dV; plantar surface densely covered with distinct squamae distal to thenar pad; hypothenar pad large and distinct; claw of dI extending almost to first interphalangeal joint of dII; claw of dV extending just beyond first interphalangeal joint of dIV. Tail longer than combined length of head and body; distinctly bicolored (dark above, pale below).

Skull with long, stout rostrum flanked by moderately deep zygomatic notches; interorbital region anteriorly convergent, with beaded supraorbital margins; braincase rounded, with more or less distinct temporal crests; lambdoidal and nuchal crests developed in older adults. Posterior margin of zygomatic plate dorsal to M1 alveolus; jugal present and large (the maxillary and squamosal zygomatic processes widely separated, not overlapping in lateral view). Nasals short, not extending posteriorly beyond lacrimals; lacrimals equally sutured to maxillary and frontal bones. Frontosquamosal suture usually colinear with frontoparietal suture. Parietals with broad lateral expansions. Incisive foramina very long, usually extending posteriorly between M1 anterocones or protocones; with subparallel lateral margins. Posterolateral palatal pits large, complex, and recessed in deep fossae; mesopterygoid fossa penetrating anteriorly to or slightly between molar rows; bony roof of mesopterygoid fossa perforated by large sphenopalatine vacuities. Alisphenoid strut usually present (buccinator-masticatory foramen and accessory foramen ovale separate), but strut unilaterally absent on some skulls. Stapedial foramen and posterior opening of alisphenoid canal small; squamosal– alisphenoid groove and sphenofrontal foramen absent; secondary anastomosis of internal carotid crosses dorsal surface of pterygoid plate (5 carotid circulatory pattern 3 of Voss, 1988). Postglenoid foramen large and round- ed; subsquamosal fenestra large and patent. Periotic exposed posteromedially between ectotympanic and basioccipital but not extending anteriorly to carotid canal; mastoid perforated by small or large posterodorsal fenestra. Capsular process of lower incisor alveolus indistinct or absent. Superior and inferior masseteric ridges usually conjoined anteriorly as single crest below m1.

Labial and lingual flexi of M1 and M2 not interpenetrating. First upper molar (M1) anterocone not divided into labial and lingual conules (but a small internal fossette obviously derived from the anteromedian flexus is present); anteroloph usually well developed and fused with anterostyle on labial cingulum, separated from anterocone by persistent anteroflexus; protostyle absent; paracone connected by enamel bridge to middle or to posterior moiety of protocone. Second upper molar (M2) protoflexus present but shallow; mesoflexus present as one or more internal fossettes (both conditions occurring on opposite sides of some specimens: e.g., AMNH 64054). Third upper molar (M3) with posteroloph and diminutive hypoflexus (the latter tending to disappear with moderate to heavy wear). Labial accessory root of M1 absent.

First lower molar (m1) anteroconid without an anteromedian flexid; anterolabial cingulum and anterolophid present on all lower molars; ectolophid absent on m1 and m2; mesolophid variably developed on m1 and m2, large and distinct in some specimens but much reduced or absent in others; m2 hypoflexid short. Accessory roots absent on m1; m2 and m3 each with one large anterior root and one large posterior root.

COMPARISONS: ‘‘ Oryzomys ’’ polius was consistently recovered as the most basal lineage of clade D and not as the sister group of any other terminal taxon in the phylogenetic analyses of Weksler (2003, 2006). In his original description of ‘‘ O. ’’ polius, Osgood (1913) contrasted it with ‘‘ O. ’’ xanthaeolus , a geographically adjacent species, but he emphasized the lack of any close resemblance between them. Indeed, the genera to which we now refer these species are strikingly divergent in several characters.

Among other contrasts, Eremoryzomys differs from Aegialomys by its much grayer dorsal pelage (the dorsal fur is distinctly yellowish or brownish in Aegialomys ); larger jugal (the jugal of Aegialomys is much smaller); longer incisive foramina (these openings never extend posteriorly between the M1 protocones in Aegialomys ); shorter palate (the mesopterygoid fossa never extends anteriorly to the molar rows in Aegialomys ); alisphenoid strut separating the buccinator–masticatory and accessory oval foramina (the alisphenoid strut is invariably absent and the foramina are confluent in Aegialomys ); absence of a distinct capsular process of the lower incisor alveolus (the capsular process is well developed in Aegialomys ); absence of accessory roots on M1/m1 (accessory roots are normally present on these teeth in Aegialomys ); and presence of the anterolophid on m2 and m3 (the anterolophid is absent on these teeth in Aegialomys ).

Comparisons with other new genera belonging to clade D, none of which appear to be closely related to Eremoryzomys , are summarized in table 2.

ETYMOLOGY: From eremia (Greek for a lonely place), in reference to the isolated distribution of this monotypic genus.