Hylaeamys, 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 : 14-16

Hylaeamys , new genus

TYPE SPECIES: Mus megacephalus Fischer, 1814 .

CONTENTS: acritus Emmons and Patton, 2005 ; laticeps Lund, 1840 (including saltator Winge, 1888, and seuanezi Weksler et al., 1999 ); megacephalus Fischer, 1814 (including capito Olfers, 1818 , cephalotes Desmarest, 1819, velutinus J.A. Allen and Chapman, 1893 , goeldii Thomas, 1897, and modestus, J.A. Allen, 1899 ); oniscus Thomas, 1904 ; perenensis J.A. Allen, 1901 ; tatei Musser et al., 1998 ; and yunganus Thomas, 1902 .

DISTRIBUTION: In moist (evergreen and semi-evergreen) forests of cis-Andean tropical and subtropical lowlands and foothills (to about 1500 m above sea level) from Venezuela and the Guianas southward throughout Amazonia and the Atlantic rainforest to Paraguay and northern Argentina. Numerous records from drier landscapes (e.g., in the Chaco and Cerrado) are probably all from gallery forests or relictual moist forest fragments formerly continuous with either Amazonian or coastal Brazilian rain forests.

MORPHOLOGICAL DIAGNOSIS: Dorsal pelage finely grizzled-brownish, typically drab grayish-brown in young adults but often tawny or buffy in mature specimens; ventral pelage usually abruptly paler (superficially whitish), but ventral hairs always gray-based. Pinnae large, extending to eye when laid forward. Mystacial and superciliary vibrissae not extending posteriorly beyond pinnae when laid back. Pes with tufts of long ungual hairs at bases of claws on dII–dV (also on dI of H. acritus ); plantar surface smooth or sparsely covered with indistinct squamae distal to thenar pad; hypothenar pad distinct in most species but often very small in H. megacephalus and sometimes absent in H. yunganus ; claw of dI extending just beyond base of phalange 1 of dII; claw of dV extending almost to first interphalangeal joint of dIV. Tail usually about as long as or slightly shorter than combined length of head and body; unicolored (all dark) or weakly bicolored (dark above, pale below) near base.

Skull with moderately long, tapering rostrum flanked by deep zygomatic notches; interorbital region usually convergent anteriorly with weakly beaded supraorbital margins (some specimens of H. laticeps and H. megacephalus have almost hourglass-shaped interorbits with squared dorsolateral margins); braincase oblong, usually with distinct temporal crests; lambdoidal and nuchal crests developed in older adults. Posterior margin of zygomatic plate usually dorsal to M1 alveolus; jugal present but small (maxillary and squamosal zygomatic processes overlapping in lateral view but not in contact). Nasals short, not extending posteriorly beyond lacrimal; lacrimals equally sutured to maxillary and frontal bones in some species (e.g., O. yunganus ) or sutured primarily to maxilla in others (e.g., O. megacephalus ). Parietals with small lateral expansions. Incisive foramina short, not extending posteriorly between M1 alveoli, and broader posteriorly than anteriorly. Posterolateral palatal pits variable in size, number, and morphology, but usually large and recessed in shallow fossae; mesopterygoid fossa usually not extending anteriorly between maxillae; roof of mesopterygoid fossa completely ossified or perforated by small (slit-like) sphenopalatine vacuities. Alisphenoid strut absent (buccinator–masticatory foramen and accessory oval foramen confluent). Stapedial foramen and posterior opening of alisphenoid canal large, squamosal–alisphenoid groove and sphenofrontal foramen absent (5 carotid circulatory pattern 2 of Voss, 1988). Postglenoid foramen large and rounded; subsquamosal fenestra large and patent. Periotic exposed posteromedially between ectotympanic and basioccipital but usually not extending anteriorly to carotid canal; mastoid perforated by conspicuous posterodorsal fenestra. Distinct capsular process of lower incisor alveolus absent; superior and inferior masseteric ridges converge anteriorly as open chevron below m1.

Labial and lingual flexi of M1 and M2 not (or shallowly) interpenetrating except in H. tatei (which has more deeply interpenetrating flexi). First upper molar (M1) anterocone not divided into labial and lingual conules (anteromedian flexus absent); anteroloph well developed and fused with anterostyle on labial cingulum, separated from anterocone by anteroflexus in minimally worn dentitions; protostyle absent; paracone connected by enamel bridge to posterior moiety of protocone. Second upper molar (M2) protoflexus present or absent; mesoflexus present as single internal fossette in some species (e.g., H. megacephalus ) or as two fossettes (e.g., in H. yunganus ). Third upper molar (M3) without posteroloph; hypoflexus deep and persistent in some species, shallow and transitory in other. Labial accessory root of M1 absent.

First lower molar (m1) anteroconid without an anteromedian flexid; anterolabial cingulum present on all lower molars; anterolophid usually distinct on m1 but absent on m2 and m3; ectolophid often present on m1 and m2; mesolophid present and distinct on m1 and m2; m2 hypoflexid usually short in some species (e.g., H. yunganus ) but usually long in others (e.g., H. megacephalus ). Accessory roots absent on m1; m2 and m3 each with one large anterior root and one large posterior root.

Fifth lumbar (17th thoracicolumbar) vertebra usually without anapophysis. Hemal arch between second and third caudal vertebrae without 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 large and trifid (with robust central digit); shelf of nonspinous tissue on crater rim does not conceal bacular mounds; dorsal papilla spineless; urethral processes without subapical lobules.

COMPARISONS: Weksler’s (2003, 2006) phylogenetic analyses of morphological and molecular characters consistently recovered Hylaeamys (represented by ‘‘ Oryzomys ’’ megacephalus , and ‘‘ O. ’’ yunganus ) as part of clade B along with Euryoryzomys , Transandinomys , Nephelomys , Oecomys , and Handleyomys . Phenetically, Hylaeamys is most similar to two other genera that contain species formerly referred to the so-called ‘‘ capito complex’’ of Oryzomys sensu lato ( Musser et al., 1998), namely Euryoryzomys and Transandinomys . Because comparisons with Euryoryzomys have already been provided in the account for that genus (above), only comparisons with Transandinomys are discussed here.

Hylaeamys is similar to Transandinomys in size and in most qualitative external features, but its superciliary vibrissae are shorter, not extending posteriorly behind the pinnae. Although T. talamancae has much shorter superciliary vibrissae than T. boliviae (see Musser et al., 1998: fig. 53), these tactile hairs extend posteriorly behind the pinnae in both species of Transandinomys and are diagnostically longer than they are in Hylaeamys . Otherwise, the two genera are difficult (if not impossible) to distinguish per se based on integumental comparisons.

In cranial features, the two genera are most readily distinguished by their alternative patterns of carotid circulation. Whereas Hylaeamys possesses only the infraorbital branch of the stapedial artery, Transandinomys also has an intact supraorbital branch. The presence of the latter vessel is indicated by a translucent groove across the internal surfaces of the squamosal and alisphenoid bones and by the presence of a sphenofrontal foramen. Both of these osteological markers are constant features of examined skulls of Transandinomys , but they are just as consistently absent in Hylaeamys (see Musser et al., 1998: fig. 151).

Hylaeamys and Transandinomys do not appear to differ consistently in any other character that we have been able to score in all member species. However, the potential diagnostic value of an anapophysis on the fifth lumbar (17th thoracicolumbar) vertebra, a process that is usually absent in H. megacephalus but present in T. talamancae , merits evaluation as postcranial skeletal material becomes available for other congeneric taxa.

REMARKS: The monophyly of Hylaeamys is not supported by analyses of morphological character data ( Weksler, 2006: figs. 34, 35) or mtDNA sequences (Bonvincino and Moreira, 2001). Instead, compelling evidence for generic monophyly comes primarily from nuclear sequences ( Weksler, 2003). Because the latter are only available from two species ( H. megacephalus and H. yunganus ), our concept of Hylaeamys is primarily based on the absence of the supraorbital branch of the stapedial artery. This trait was optimized as an unambiguous synapomorphy of H. megacephalus + H. yunganus in Weksler’s combined analyses of morphological and IRBP characters; within clade B, it is uniquely shared by the species that we refer to Hylaeamys .

ETYMOLOGY: For the hylaea, Humboldt’s name for the the rainforested lowlands of cis- Andean South America, the principal habitat of species belonging to this clade.