Aegialomys, 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 : 5-8

Aegialomys , new genus

TYPE SPECIES: Oryzomys xanthaeolus Thomas, 1894 .

CONTENTS: galapagoensis Waterhouse, 1839 (including bauri J.A. Allen, 1892 ) and xanthaeolus Thomas, 1894 (including baroni J.A. Allen, 1897, and ica Osgood, 1944).

DISTRIBUTION: In the lowland dry forests of western Ecuador (including the Galapagos Islands) and western Peru, but also at higher elevations (to about 2500 m) in the upper Marañón valley of northern Peru.

MORPHOLOGICAL DIAGNOSIS: Dorsal pelage coarsely grizzled yellowish- or grayish-brown; ventral pelage abruptly paler (superficially whitish or pale yellow), 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 ungual hairs at bases of claws on dI–dV; plantar surface densely covered with distinct squamae distal to thenar pad; hypothenar pad present and large; claw of dI extending beyond middle of phalange 1 (almost to first interphalangeal joint) of dII; claw of dV extending just beyond first interphalangeal joint of dIV. Tail about as long as head and body in A. galapagoensis but distinctly longer than head and body in A. xanthaeolus ; weakly to distinctly bicolored (dark above, pale below).

Skull with stout rostrum flanked by deep zygomatic notches; interorbital region anteriorly convergent with strongly beaded supraorbital margins; braincase oblong, usually with well-developed temporal crests; lambdoidal and nuchal crests often well developed in older adults. Posterior margin of zygomatic plate dorsal to M1 alveolus in some examined specimens, anterior to M1 alveolus in others. Jugal present but small (the maxillary and squamosal zygomatic processes broadly overlapping in lateral view but not in contact). Nasals extending posteriorly behind lacrimals in A. galapagoensis but shorter (extending to but usually not behind lacrimals) in A. xanthaeolus ; lacrimals usually with longer maxillary than frontal sutures. Frontosquamosal suture usually colinear with frontoparietal suture. Parietals with broad lateral expansions. Incisive foramina long, typically extending posteriorly to or between M1 alveoli; almost parallel-sided (in A. galapagoensis ) or widest at midlength and tapering symmetrically anteriorly and posteriorly (in A. xanthaeolus ). Posterolateral palatal pits large, complex, and recessed in deep fossae; mesopterygoid fossa penetrating anteriorly between maxillae in A. galapagoensis but often not in A. xanthaeolus ; bony roof of mesopterygoid fossa perforated by very large sphenopalatine vacuities. Alisphenoid strut absent (buccinator-masticatory foramen and accessory foramen ovale confluent). 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 rounded; subsquamosal fenestra small but distinct in most forms, but vestigial or absent in an unnamed species from coastal Ecuador. Periotic exposed posteromedially between ectotympanic and basioccipital, but usually not extending anteriorly to carotid canal; mastoid unfenestrated or with a small but distinct posterodorsal fenestra (in specimens from coastal Ecuador). Capsular process of lower incisor alveolus well developed in most fully adult specimens; superior and inferior masseteric ridges conjoined anteriorly as single crest below m1.

Labial and lingual flexi of M1 and M2 not interpenetrating. First upper molar (M1) anterocone divided into anterolabial and anterolingual conules by distinct anteromedian flexus in some forms (e.g., A. galapagoensis and an undescribed species from coastal Ecuador), undivided in others (e.g., A. xanthaeolus , which, however, has a small internal fossette that seems to represent a vestigial anteromedian flexus); anteroloph well developed and fused with anterostyle on labial cingulum, separated from anterocone by persistent anteroflexus in some species (e.g., A. xanthaeolus ) but fused with anterocone (anteroflexus reduced or absent) in others; protostyle absent; paracone usually connected by enamel bridge to posterior moiety of protocone. Second upper molar (M2) protoflexus present; mesoflexus present as single internal fossette. Third upper molar (M3) with posteroloph and diminutive hypoflexus (the latter tending to disappear with moderate to heavy wear). Accessory labial root of M1 often present.

First lower molar (m1) anteroconid usually without an anteromedian flexid; anterolabial cingulum present on all lower molars; anterolophid present on m1 but absent on m2 and m3; ectolophid absent on m1 and m2; mesolophid distinct on unworn m1 but reduced on m2; m2 hypoflexid short. Accessory lingual and labial roots of m1 present; m2 and m3 each with two small anterior roots and one large posterior root.

Fifth lumbar (17th thoracicolumbar) vertebra with well-developed anapophysis. Hemal arch between second and third caudal vertebrae with posterior spinous process. Supratrochlear foramen of humerus present.

Stomach without extension of glandular epithelium into corpus. One pair of preputial glands present. Distal bacular cartilage of glans penis small and trifid (with a short and slender central digit); nonspinous tissue on rim of terminal crater does not conceal bacular mounds; dorsal papilla spineless; urethral processes without subapical lobules.

COMPARISONS: Aegialomys was represented by ‘‘ Oryzomys ’’ xanthaeolus 4 in the phylogenetic analyses of Weksler (2003, 2006), who consistently recovered it as a member of clade D. Within clade D, ‘‘ O. ’’ xanthaeolus usually appeared as the sister taxon of a group composed of Amphinectomys , Melanomys , Nectomys , and Sigmodontomys (as in fig. 1 ). Phenetically, however, Aegialomys more closely resembles Oryzomys sensu stricto (the ‘‘ palustris group’’ of authors), Cerradomys (the ‘‘ subflavus group’’), and Eremoryzomys (the ‘‘ polius group’’). Comparisons with Oryzomys sensu stricto and Cerradomys are provided here, and comparisons with Eremoryzomys are included in the account for that genus (below). Table 2 summarizes key morphological comparisons among all of the new taxa belonging to clade D.

Aegialomys differs from Oryzomys in numerous traits, among which the most noteworthy are its large, distinct hypothenar pad on the hind foot (the hypothenar pad is absent or vestigial in Oryzomys ); conspicuous tufts of long ungual hairs at the bases of the claws on pedal digits II–V (the ungual hairs are sparse and short in Oryzomys ); M1 anteromedian flexus present or vestigial (the anteromedian flexus is unambiguously absent in Oryzomys ); M1 paracone usually attached by an enamel bridge to the posterior moiety of the protocone (the attachment is usually to the anterior moiety in Oryzomys ); M2 mesoflexus forming a single internal fossette (the M2 mesoflexus usually forms two internal fossettes in Oryzomys ); lack of distinct anterolophids on

4 Subsequent study indicates that this terminal taxon was a composite based on material of the unnamed Ecuadorean species mentioned in the preceding diagnosis together with Aegialomys xanthaeolus sensu stricto. Taxonomic differences therefore account for some of the character variation scored as polymorphisms of A. xanthaeolus in Weksler’s (2006) analyses.

TABLE 2 Selected Morphological Comparisons Among New Genera from Clade D

m2 and m3 (anterolophids are usually distinct on unworn m2 and m 3 in Oryzomys ); and mesolophids that tend to disappear as distinct structures with only moderate wear (mesolophids are persistent as distinct structures in Oryzomys ). In addition, A. xanthaeolus has a well-developed anapophysis on the fifth lumbar vertebra that is absent in O. couesi and O. palustris ; a tridigitate bacular cartilage with a short and slender central digit (the central digit is robust in O. couesi and O. palustris ); a spineless dorsal papilla (the dorsal papilla is provided with spines in O. couesi and O. palustris ); and urethral processes that lack subapical lobules (subapical lobules are present on the urethral processes of O. couesi and O. palustris ).

Although Aegialomys xanthaeolus and Cerradomys subflavus differ in numerous morphological characters and were never recovered as sister taxa in Weksler’s (2003, 2006) phylogenetic analyses, only a few traits distinguish their respective genera as recognized herein. This difficulty arises from substantial character variation among species within each genus: for example, as documented in Langguth and Bonvicino’s recent (2002) descriptions of new species of Cerradomys , and by our remarks about character variation in Aegialomys (above). In fact, Aegialomys and Cerradomys do not appear to differ consistently in any integumental or cranial feature that we have been able to identify. Several dental and genitalic characters, however, suggest that these are distinct taxa that merit formal recognition. Because they are so few, each character merits particular attention.

In Aegialomys galapagoensis , the unworn anterocone of M1 is divided into subequal anterolabial and anterolingual conules by an anteromedian flexus, but in A. xanthaeolus (from coastal Peru) the anterocone is undivided and the anteromedian flexus is present only as an internal fossette whose faint connection to a shallow median sulcus in the anterior face of the anterocone is transient and can only be seen on minimally worn teeth (e.g., AMNH 10111, 42398). By contrast, the anterocone of M 1 in Cerradomys is never divided into labial and lingual conules by an anteromedian flexus, and the internal fossette of the procingulum that is visible in some unworn dentitions (e.g., AMNH 134566, illustrated by Musser et al., 1998: fig. 144) is clearly derived from the anteroflexus, a labial enamel infolding.

The mesoflexus of M2 is present as a single internal fossette in Aegialomys . Although occasional rare variants are to be expected in such traits, this morphology appears to be exhibited consistently by examined specimens of A. galapagoensis , A. xanthaeolus (including baroni), and the unnamed form from coastal Ecuador. The mesoflexus of Cerradomys , however, is usually represented by two internal fossettes, of which one is labial and other is nearer the midline of the tooth (as illustrated for ‘‘ Oryzomys ’’ subflavus by Musser et al., 1998: fig. 144).

The male genitalia of Aegialomys galapagoensis (‘‘ Oryzomys bauri ’’) and A. xanthaeolus (‘‘ O. xantheolus ’’) were described and illustrated by Patton and Hafner (1983). In both species, the distal bacular cartilage is unambiguously trifid, with a slender but distinct central digit. By contrast, the glans penis of Cerradomys scotti , C. subflavus , and an undescribed congener from northeastern Brazil have a bifid distal bacular cartilage because the middle digit is vestigial or absent.

REMARKS: Although ‘‘ Oryzomys ’’ galapagoensis and ‘‘ O. ’’ xanthaeolus have long been recognized as closely related species (e.g., by Thomas, 1894; Gardner and Patton, 1976; Patton and Hafner, 1983), no published phylogenetic analysis of biochemical or morphological data is currently available to support the monophyly of Aegialomys as constituted herein. The presence of at least one undescribed species among the material we examined, together with questions that have been raised elsewhere concerning the taxonomic status of ica (by Musser and Carleton, 2005: 1156) and our own reservations about baroni, suggest that a revision of this group is needed to identify the terminal taxa that should be represented in future phylogenetic analyses.

ETYMOLOGY: From aegialos (Greek for the seashore), in reference to the predominantly coastal distribution of these species in western South America.

Figures

Fig. 1. Phylogenetic relationships of oryzomyines based on a heuristic parsimony analysis of sequence data from the Interphotoreceptor Retinoid Binding Protein (IRBP, 1266 bp from exon 1) and 99 morphological characters (after Weksler, 2006: fig. 37). Numbers above and below branches represent jackknife support (.50%) and decay indices (.1), respectively. Vertical bars on the right-hand side of the figure indicate taxon membership in clades A–D. See Weksler (2006: 14–17) for methodological details.