Leiuromys Emmons & Fabre 2018, new genus
Creators: name: Emmons, Louise H.
name: Fabre, Pierre-henri
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Rodentia
Echimyidae
Leiuromys

description: Leiuromys, new genus   
 TYPE SPECIES: Echimys occasius Thomas, 1921: 450.   
CONTENTS: Only the type species.  
 ETYMOLOGY: From the Greek leios (“smooth, bald”) and uros (“tail”), thus, “bald-tailed mouse,” in reference to the slick, hairless appearance of the tail, which is more naked than that of any other arboreal echimyine except lowland species of Dactylomys.   
DIAGNOSIS: Small, arboreally adapted brown rats, heavily spined dorsally with abundant wide, sharp, spines on midback from neck to rump, spines intermixed with fine wavy rust-red underhairs (fig. 3; tables 3–6). Tail hairs short, fine, and inconspicuous, so that the tail looks bare and even shiny. Auditory tympanic bullae small, flattened in lateral aspect. Inferior jugal process reduced to nearly absent, but jugal fossa deep and sharply pointed anteriorly. Masticatory and buccinator foramina joined in a single aperture. Incisors bone white; upper incisor root does not extend to within the maxillary base of the zygoma. Lower premolars tetralophodont. Third upper molars trilophodont. The hypoflexids on m1–m3 fall nearly opposite the middle of the mesolophids, such that the teeth viewed from the lingual side are nearly symmetrical, rounded M-shapes. Upper lingual and lower labial hypoflexi short and wide, about a quarter of the width of tooth, but labial meso- and metaflexi long, such that when worn to a mure in M1 and M2, the mure is on the far lingual edge of the tooth. All mandibular cheekteeth have curved posterior borders.  
 DESCRIPTION: Upperparts with abundant, wide, strong spines on midback from neck to rump, spines intermixed with fine, wavy, rust-red underhairs that show through aristiforms, giving back a rusty hue (fig. 3). Spine tips sharp, narrowing abruptly, with short microscopic hairlike processes at tips (fig. 4A). Spines tipped with pale buff sparsely speckle the sides of the hindquarters and tail base. Dorsal spines pale gray-brown at base, with a dusky band, a narrow, subterminal pale whitish or buffy band, and dusky, buff, or black-brown tips (fig. 4A). Upperparts uniform light ochraceous brown, between Cinnamon and Sayal Brown (Smithe, 1975), lined with dusky, or blackish Dusky Brown (the holotype and AMNH 71897). The spines on the middorsum from neck to rump are the darkest and nearly unbanded (figs. 3, 4A). Head and muzzle brown above, without any red color. Cheeks below the ears, forelimbs, and hindlimbs tinged grayish. Feet pale beige to rusty above; silvery ungual tufts reach to the ends of the claws. Ventral pelage of soft aristiforms, colored strong to pale buff, grading gradually from the sides (fig. 3); inner thighs slightly darker pinkish buff; the chest and lower abdomen and inguinal regions can have self-white patches, including the tail base, or are crossed by a white band between the elbows. Vibrissae black, moderately robust and the longest reaches to the shoulder. Ears short, pigmented, and nearly naked, lined with a few wispy, dusky hairs. Tail slightly longer than head and body length, brown pigmented, with a naked and somewhat shiny appearance. Dorsal hair ends abruptly close to the body at the tail base; distal tail hairs inconspicuous and closely adpressed. Scales on the tail base evenly hexagonal, wider than long, tapered at the lateral ends. Dorsally each scale has one robust, scalelike dark hair about two and a half scale rows long, and one shorter, fine, hair (fig. 5B). Ventrally, tail hairs somewhat longer (three scale rows), adpressed, and silvery. An adult female (FMNH 84259) has two used lateral pairs of mammae spaced about evenly between the limbs and 1.5 cm into the dorsal pelage field, and possibly an unused inguinal pair. As far as can be seen on dried specimens, the feet have small digital pads collared with rings of tubercles as in Toromys species (fig. 6A), and with numerous plantar tubercles on the bottom and sides of the feet around the pads. Cranium lightly built for the subfamily, inferior jugal process much reduced or essentially absent (figs. 7D, 8, 9D, 10D). Mastoid processes are short, reach to the middle of the auditory meatus, and are adpressed to the cranium; the space above the auditory meatus is nearly as wide as the meatus. Parietal ridges not raised posterior to the postglenoid fossa. Masticator and buccinator foramina joined in a narrow, horizontal slit or a large oval foramen (MCZ 37964) (fig. 8B). Posterior opening of the alisphenoid canal thin walled and opens in a tube beneath the strut (pterygoid plate) between the pterygoid canal and the foramen ovale (fig. 8A); it sometimes lacks the dorsal wall and appears absent. There are no sphenopalatine vacuities. Auditory bullae not inflated, nearly flat in lateral profile, and markedly smaller than those of most other echimyines (figs. 7D, 8B); meatus rim pointed outward. Postglenoid foramen slitlike and enclosed below by a crest of petrosal. The ridges and grooves posterior to the end of the incisive foramina terminate anterior to the premolars, with shallow or absent extensions onto the palate between the toothrows (fig. 8A, B). Viewed dorsally, the interparietal is an evenwidth, narrow strip bowed smoothly posteriorly at the parietal suture (fig. 9D). Coronoid process of the mandible slender and recurved, above a shallow sigmoid notch (fig. 7D). Masseteric crest strongly developed and curved upward anteriorly for about half the width of the mandible; spine of the condyloid ridge on the interior of the mandible does not extend much more than halfway up the condyloid process, except on a specimen from southern Peru. Maxillary toothrows short, 7.3–10.75 mm (table 6); incisors bone white; roots of the upper incisors lie outside the maxillary base of the zygoma. Lower premolar quadralophodont, comprised of a curved anteroloph and a straight metolophid that join with wear into a D-shape around a central fossettid, which joins with wear by a central mure to the lingually opening V-shaped posterior lophid pair (fig. 11B). Hypoflexids on m1 to m3 fall nearly opposite the middle of the mesolophids, such that the teeth viewed from the lingual side are nearly symmetrical, rounded M-shapes. Upper lingual and lower labial reentrant folds (hypoflexi) short and wide, about 1/4 of the width of tooth, when worn they form subcircular fossettes; labial reentrant folds are long, and of equal length (meso- and metaflexi), such that when worn to a mure in M1 and M2, the mure is on the far lingual edge of the tooth (fig. 11B). Mandibular cheekteeth all with curved posterior borders. M3 trilophodont in all specimens examined, with the third loph much reduced (fig. 11C). Worn lower molars nearly circular, somewhat resembling those of Isothrix (fig. 11C).   
 COMPARISONS WITH OTHER GENERA: Leiuromys is most easily confused externally with the distantly related taxa Mesomys species and Phyllomys pattoni. Species of Mesomys all have conspicuously hairy tails with brown, outward-curling hairs and a slight-to-prominent pencillike tapering at tip (Patton et al., 2000). In Mesomys the buff or whitish ventral pelage field is sharply demarcated from the spiny dorsal pelage. The molar occlusal pattern has flexi/flexids compressed into narrow parallel lines that do not open on sides of the teeth. Phyllomys pattoni have rusty-tipped spines and rusty sides, and an occlusal pattern of separate laminae (Emmons et al., 2002). Pattonomys species are larger, with grizzled gray heads, sides, and feet, white lower cheeks and throat, a fuzzy tail, and a distinct molar occlusal pattern, including straight posterior edges of the lower cheekteeth (fig. 12E–G). The spines of Pattonomys species taper more sharply at the tips (fig. 4B). Makalata species have tails slightly but distinctly hairy, with banded tail hairs, and short, rectangular tail scales in even rings (fig. 5D); a uniformly ochraceous- or reddish-agouti pelage of dusky and yellowish; multibanded, spinous aristiform hairs with short, pale, hairlike tips that are not as wide or sharp as in Leiuromys (fig. 4C); rostra that are reddish anteriorly to above and behind the eyes (fig. 13C), and rusty pelage around the tail base. Phyllomys species and Echimys chrysurus have one inguinal and three lateral pairs of mammae, whereas the other genera have two lateral pairs, and if present, inguinal mammae seem obsolete (unused in parous females). All other Echimyini usually have a quadralophodont M3, but P. semivillosus can lack the fourth loph. In Makalata the lower premolars are pentalophodont (illustrated in Emmons, 2005). The lower premolar hypoconid is a rounded point in genera of the Echimys clade, while it is flat in Pattonomys and Toromys; in the latter often slightly indented (Iack-Ximenez et al., 2005: fig. 11).
description: Published as part of Emmons, Louise H. & Fabre, Pierre-henri, 2018, A Review of the Pattonomys / Toromys Clade (Rodentia: Echimyidae), with Descriptions of a New Toromys Species and a New Genus, pp. 1-52 in American Museum Novitates 2018 (3894) on pages 13-24, DOI: 10.1206/3894.1, http://zenodo.org/record/5369712