Nactus acutus Kraus, 2005, sp. nov.

Nactus acutus sp. nov.

Figs. 5 B, 6

Holotype: BPBM 20755 (field tag FK 10213), non­reproductive male, collected by F. Kraus on ridge on S side of Mt. Rossel, Rossel Island, 11.35552 S, 154.22459 E, 720 m elevation, Milne Bay Province, Papua New Guinea, 8 May 2004.

Diagnosis: A medium (36.6 mm) species of Nactus lacking dorsal tubercles and enlarged postnasals; and having granular, largely tricarinate dorsal and ventral scales; lamellate, unexpanded subdigital lamellae; 16 lamellae under the 4th toe; claws lying within four scales; granular postmentals; elongate first infralabials; a single row of scales between the orbit and supralabials; 5 supralabials to center of eye and 7 to rictus; 7 infralabials; keeled scales on the tail; elongate, slit­like ear opening; relatively narrow head (HW/HL = 0.64); long, narrow snout (EN/SV = 0.087, IN/SV = 0.033, EN/IN = 2.7); small eye (EE/SV = 0.055) and interorbital distance (ID/SV =0.063); relatively narrow tail (TW/TL = 0.10); four sternal/xiphisternal ribs; and uniformly dark gray dorsal color.

Comparison with other species: Nactus acutus may be distinguished from N. sphaerodactylodes in its larger size; 16 (vs. 10–13) lamellae under 4th toe; relatively narrow head, snout, and tail; relatively small eye and interorbital distance; carinate (vs. smooth) scales on tail; absence of enlarged postnasals; and narrow ear opening. From all other members of the genus except N. coindemirensis, N. acutus differs in 1) lacking rows of enlarged tubercles on the body, 2) having the claws lie within four (vs. two or three) scales, and 3) having elongate infralabials such that the mental extends no further posteriorly than the center of the first infralabials (vs. to or past suture with 2nd infralabial) and the suture between the first and second infralabials lies behind the center of the second supralabial (vs. at beginning of second supralabial). From N. coindemirensis, N. acutus differs in its elongate first infralabial as well as in having granular (vs. imbricate) ventral scales, carinate (vs. smooth) dorsal and ventral scales, lamellate (vs. granular) distal subdigital scales, unexpanded (vs. expanded) proximal subdigital lamellae, and 7 (vs. 10–11) infralabials. From the closely related N. vankampeni, N. acutus further differs in having tricarinate (vs. multicarinate) dorsal and ventral keeling, one (vs. two) scale row between orbit and supralabials, and four (vs. five) sternal/xiphisternal ribs.

Description of holotype: A non­reproductive male with right lateral incision; part of liver removed. Animal of medium (SV = 36.6 mm, TrL = 15.5) size. Head relatively long (HL/SV = 0.28) and narrow (HW/HL = 0.64), somewhat depressed (HH/HL = 0.37), not very distinct from neck (Fig. 6A). Loreal and interorbital region slightly inflated; canthus rostralis absent. Snout relatively long (SN/HL = 0.43), much longer than eye diameter (SN/EY = 2.2), depressed (Fig. 6 B). Eye relatively large (EY/HL = 0.20); pupil vertical, with entire margins; supraciliaries small, conical, not well differentiated from adjacent granules. Ear opening a small, elongate, oblique slit (Ear/HL = 0.069); distance between ear and eye 1.5 times eye diameter. Rostral less than half (0.70 mm) as high as wide (1.55 mm), highest just medial to nares and shallowly depressed between these points; length (1.15 mm) greater than height; partially divided dorsally by shallow rostral groove. Two enlarged supranasals separated by a single granular internasal. Rostral in contact with first supralabials, two supranasals, and single internasal. External nares oval, long dimension running antero­posteriorly; each bordered by rostral, supranasal, first supralabial, and nasal. First supralabial has an extension that reaches dorsally to behind the posteriormost limit of the naris. Nasal bordered posteriorly by two (right) or three (left) small granular scales; enlarged postnasals absent. One row of small scales separates orbit from supralabials. Mental subtriangular, wider (1.9 mm) than long (1.5 mm). No enlarged postmentals (Fig. 6 C); mental bordered posteriorly by eight tiny granules. Infralabials bordered by tiny granules, these somewhat larger along middle of series than posteriorly or anteriorly. Supralabials to mid­orbital position five on each side, to start of granules six on each side. Infralabials seven. Scale rows between center of orbits ~42.

Body rather slender (TrL/SV = 0.42), lacking ventrolateral folds (Fig. 5 B). Dorsal scales on head, body, and limbs tiny, conical, juxtaposed granules; each wider than long and with three sharp keels; granules on venter same. Exceptions are the top of snout and anterior of chin, which have unicarinate scales. Organs on posterior end of each scale 3–6. Enlarged tubercles absent on head, body, and limbs.

Precloacal pores absent (Fig. 6 D). Granules in precloacal and femoral regions equal to or slightly enlarged relative to those on abdomen; granules posterior to this conspicuously smaller. No precloacal groove. Scales on palms and soles rounded, smooth.

Fore­ and hindlimbs relatively short, stout (FA/SV = 0.11, CS/SV = 0.16). Digits short, only modestly reflected at basal interphalangeal joints; all with robust, slightly recurved claws. Subdigital lamellae narrow, smooth, rounded, and slightly expanded distally. Lamellae on manus 7­11­12­11­9 on right, 7­11­13­12­10 on left; on pes 9­11­13­16­15 on right, 8­11­14­16­15 on left. Relative lengths of digits on manus I <V <II <III <IV; on pes I <II <V <III <IV. No interdigital webbing present on manus or pes.

Tail relatively short (TL/SV = 0.87), with moderately wide base (TW/TL = 0.10), tapering to a point, slightly constricted at base. Cloacal sacs small, external orifices small, approximately 3 scales in width, situated near lateral margins of vent. Scales of tail large, flat, tricarinate, imbricate; enlarged postcloacal spurs absent.

Color in preservative: Dorsal granules medium or dark brown, with a few scattered granules of lighter brown; keels usually darker than remainder of scale. Interstitial skin darker brown. Overall appearance to the naked eye is of a uniformly dark brown color. Rostral and labials chocolate brown with narrow unpigmented strips along margins. Granules of venter much like those on dorsum except that scattered granules and (more commonly) clusters of 3–4 granules are unpigmented, imparting to the venter a dark gray appearance speckled with light gray spots. Unpigmented scales on chin and throat typically in larger clusters, giving the impression of larger light gray spots on a dark ground. Tail black, with some scattered unpigmented scales on ventral side.

Color in life: “Charcoal gray with lighter gray flecks ventrally. Iris brown; tongue pink.” No trace of a dorsal pattern was evident in life (Fig. 5 B).

Osteology: The following is based on X­rays of the holotype. Frontal with a narrow supraorbital waist that is one­fourth as wide as its posterior margin and half as wide as the parietals; anterior margin strongly tridentate. Parietals paired, longer than wide. Premaxillae with 13 teeth loci. Maxillae extend posterior to approximately mid­orbit, each with approximately 30–35 teeth loci. Quadrate long, narrow, slightly curved. Cranial vault filled with endolymphatic space as far forward as to include the posterior parietals; endolymphatic sacs extend from just behind quadrate posteriorly to reach first ribs; ducts to endolymphatic space of cranial vault obvious on right side. Phalangeal formula 2­3­4­5­ 3 for manus, 2­3­4­5­4 for pes. Presacral vertebrae 26, three of which are cervical vertebrae lacking ribs; two sacral vertebrae; anteriormost eight caudal vertebrae with lateral processes, those on the first five caudal vertebrae thick and obliquely oriented, remainder thin and transverse.

Ecology: The sole specimen was collected in a sticky trap placed in a partially protected hollow leading under a large rock. The type locality is a wind­blown ridge in cloud forest (5–10 m canopy) near the top of Rossel Island. Understory here is dense, with many gingers and tree ferns; progress through the vegetation is very difficult. Soil is muddy on the slopes but with pockets of humus, especially on the ridgeline; the area is highly unstable and prone to landslides. The site remained moist during our stay but almost all moisture was due to fog drip from the clouds blowing over the ridge. This location and the dominant vegetation of the site were described by Brass (1959: 57).

Etymology: The name is a masculine Latin adjective meaning “sharp” and is in reference to the long, pointed snout of the species (Fig. 5 B).

Range: Known only from the type locality on the eastern end of Rossel Island (Fig. 7). Rossel Island rises only another 80 m in elevation beyond the type locality and the large majority of reptiles and amphibians collected by us at the type locality also occurred widely at lower elevations. Despite this, considerable effort on our part failed to disclose another specimen. It may be that Nactus acutus is ecologically associated with large boulders. We found much of the forest on Rossel to be heavily strewn with boulder fields apparently resulting from earlier landslides, among which effective collecting of reptiles and amphibians was virtually impossible. Hence, it is possible that this species will prove to have a wider range on the island but may be difficult to locate for ecological reasons. Alternatively, but seemingly less likely given the elevational tolerance of its congeners, N. acutus may be restricted to the cloud forest of Rossel Island.