Adult female ZMKU R 00947 from Pa La-U Waterfall, Kaeng Krachan National Park, Huai Sat Yai Subdistrict, Hua Hin District, Prachuap Khiri Khan Province, Thailand (12.53685°N, 99.45972°E, 368 m a.s.l.), collected by Attapol Rujirawan, Siriporn Yodthong, Korkhwan Termprayoon, Natee Ampai, and Piyawan Puanprapai on 15 September 2017.

Adult female ZMKU R 00946 bearing the same data as the holotype.

Cyrtodactylusrivularis sp. nov. can be separated from all other species of the brevipalmatus group by the combination of having 12 or 13 supralabials, 9–11 infralabials, 33 or 34 paravertebral tubercles, 18–20 rows of longitudinally arranged tubercles, 34–37 transverse rows of ventrals, 160–166 longitudinal rows of ventrals, nine expanded subdigital lamellae on the fourth toe, 12 or 13 unexpanded subdigital lamellae on the fourth toe, 21 or 22 total subdigital lamellae on the fourth toe; eight expanded subdigital lamellae on the fourth finger, 10–12 unexpanded subdigital lamellae on the fourth finger, 18–20 total subdigital lamellae on the fourth finger; 14–16 total enlarged femoral scales; 15 enlarged precloacals; enlarged femorals and enlarged precloacals not continuous, and lacking pores; proximal femorals less than one-half the size of the distal femorals; small tubercles on forelimbs and flanks; large dorsolateral caudal tubercles and wide ventrolateral caudal fringe; ventrolateral caudal fringe composed generally homogeneous scales; tail square in cross-section; single enlarged unpaired medial subcaudals not posteromedially furrowed; maximum SVL 73.9 mm; three or four dark transverse body bands (Tables 4, 5).

(Fig. 7). Adult female SVL 73.9 mm; head moderate in length (HL/SVL 0.27), width (HW/HL 0.73), depth (HD/HL 0.40), distinct from neck, triangular in dorsal profile; lores concave slightly anteriorly, weakly inflated posteriorly; prefrontal region concave; canthus rostralis rounded; snout elongate (ES/HL 0.41), rounded in dorsal profile; eye large (ED/HL 0.29); ear opening horizontally elongate, small; eye to ear distance greater than diameter of eye; rostral rectangular, divided dorsally by a deep furrow, bordered posteriorly by large left and right supranasals and one slightly smaller azygous internasal, bordered laterally by first supralabials; external nares bordered anteriorly by rostral, dorsally by large supranasal, posteriorly by two smaller postnasals, bordered ventrally by first supralabial; 13R/12L rectangular supralabials, first two largest, then tapering abruptly below eye; 11R/10L infralabials tapering smoothly to just below eye and then more rapidly beyond posterior margin of eye; scales of rostrum and lores flat to domed, larger than granular scales on top of head and occiput; scales of occiput intermixed with distinct, small tubercles; superciliaries subrectangular, largest anteriorly; mental triangular, bordered laterally by first infralabials, posteriorly by large left and right elongate postmentals contacting medially for approximately 40% of their length posterior to mental; one row of two (R) and four (L) slightly enlarged sublabials extending posteriorly to third(L) and second(R) infralabial, subsequent sublabials much smaller; gular and throat scales small, granular, grading posteriorly into slightly larger, flatter, smooth, imbricate, pectoral and ventral scales.

Body relatively short (AG/SVL 0.47) with well-defined ventrolateral folds; dorsal scales small, granular, interspersed with larger conical, semi-regularly arranged, weakly keeled tubercles; tubercles extend from occipital region onto base of tail and slightly beyond as paravertebral rows; tubercles of nape and occiput small; approximately 20 longitudinal rows of tubercles at midbody; approximately 34 paravertebral tubercles; tubercles on flanks nearly same size as those on dorsum; 34 longitudinal rows of flat, imbricate, ventral scales much larger than dorsal scales; 160 transverse rows of ventral scales; no pore-bearing, precloacal scales; 15 enlarged precloacal scales; no deep precloacal groove or depression; and three rows of post-precloacal scales on midline.

Forelimbs moderate in stature, relatively short (ForL/SVL 0.13); granular scales of forearm slightly larger than those on body, interspersed with tubercles; palmar scales rounded, slightly raised; digits well-developed, relatively short, inflected at basal interphalangeal joints; digits narrower distal to inflections; subdigital lamellae wide, transversely expanded proximal to joint inflections, narrower transverse lamellae distal to joint inflections; claws well-developed, claw base sheathed by a dorsal and ventral scale; 8R/8L expanded and 11R/11L unexpanded lamellae beneath the fourth finger; hind limbs larger and thicker than forelimbs, moderate in length (TibL/SVL 0.15), covered dorsally by granular scales interspersed with moderately sized, conical tubercles dorsally and posteriorly and anteriorly by flat, slightly larger, subimbricate scales; ventral scales of thigh flat, subimbricate, larger than dorsals; subtibial scales flat, imbricate; no pore-bearing femoral scales; 8R/8L enlarged femoral scales; enlarged femoral scales not contiguous with enlarged precloacal scales, terminating distally at knee; proximal femoral scales smaller than distal femoral scales, the former forming an abrupt union with much smaller, rounded, ventral scales of posteroventral margin of thigh; plantar scales flat; digits relatively long, well-developed, inflected at basal interphalangeal joints; 9R/9L wide, transversely expanded subdigital lamellae on fourth toe proximal to joint inflection that extend onto sole, and 13R/13L unexpanded lamellae beneath the fourth toe; and claws well-developed, sheathed by a dorsal and ventral scale at base.

Posterior one-half of tail regenerated, tail long 91.5 mm (TL/SVL 1.24), 4.8 mm in width at base, tapering to a point; nearly square in cross-section; dorsal scales flat, square bearing large tubercles forming a discontinuous dorsolateral longitudinal row; slightly larger, posteriorly directed, semi-spinose tubercles forming large distinct ventrolateral caudal fringe; scales of ventrolateral fringe generally homogeneous; single medial subcaudals enlarged but not paired; subcaudal scales, larger than dorsal caudal scales; base of tail bearing hemipenial swellings; 2R/2L conical postcloacal tubercles at base of hemipenial swellings; and postcloacal scales flat, imbricate.

(Fig. 8). Ground color of the head, body, limbs, and tail pale brown; faint, diffuse mottling on rostrum; lores darkly colored; wide, distinct, pale-colored post-orbital stripe; nuchal band faint, bearing two posterior projections; three very faint, wide irregularly shaped body bands edged in slightly darker brown between limb insertions; band interspaces bearing irregularly shaped, faint, dark-colored markings; dark-colored speckling on limbs and digits; digits bearing pale-colored bands; four wide faint dark-colored caudal bands separated by three pale-colored bands on original portion of tail; all caudal bands encircle tail; all ventral surfaces beige, generally immaculate; iris orangish to coppery in color.

(Fig. 8). The paratype (ZMKU R 00946) closely approximates the holotype in overall coloration and pattern except that it is more boldly marked. It has four dark-colored body bands as opposed to three and a complete original tail (TL 89.0mm, TW 4.1 mm) bearing eight dark-colored and seven pale-colored bands. The pale-colored postorbital stripe is slightly thinner and less distinct. Meristic and morphometric differences are listed in Table 5.

Cyrtodactylusrivularis sp. nov. is currently known from the type locality at Pa La-U Waterfall, Kaeng Krachan National Park, Huai Sat Yai Subdistrict, Hua Hin District Prachuap Khiri Khan Province, Thailand.

The specific epithet rivularis is derived from the Latin rivus, meaning stream, brook, or creek refers to rocky brook or stream habitat of the new species.

Cyrtodactylusrivularis sp. nov. is the sister species to C.rukhadeva (Fig. 2) from which it differs by an uncorrected pairwise sequence divergence of 3.61% (Table 2). It differs from C.rukhadeva and C.cf.rukhadeva by having large versus small dorsolateral caudal tubercles and a wide versus narrow ventrolateral fringe. Although no statistical mean differences were recovered between Cyrtodactylusrivularis sp. nov. and C.rukhadeva which we attribute to the small sample sizes of both species (N = 2), they do respectively differ discretely (at this point) in their ranges of SL (12 or 13 vs. 9–11), PVT (33–34 vs. 27–30), VSM (160–166 vs. 152–154), and TL4T (21–22 vs. 18–20) and the morphometric characters of HumL, ForL, FemL, TibL, HD, ED, and IO (Table 5). Discrete differences among Cyrtodactylusrivularis sp. nov. and all other species and populations are presented in Tables 4, 5.

The holotype and paratype were collected at night (1900–2055 h) on granite boulders by a rocky stream dry evergreen forest at 368 m in elevation (Fig. 9) with a temperature of 25.3 °C and relative humidity of 86.9%. The new species was found to co-occur with two other species of gekkonid lizards, Cyrtodactylusoldhami (Theobald, 1876) and Gehyramutilata (Wiegmann, 1834).

Cyrtodactylusrivularis sp. nov. and C.cf.rukhadeva occur on separate branches of the phylogeny, thus neither is embedded within one another’s branch indicating there is no evidence of gene flow between them. Both species are reported to occur within the boundaries of Kaeng Krachan National Park (Grismer et al. 2022). Kaeng Krachan National Park is the largest national park in Thailand, encompassing 2,914.7 km² across five districts through the provinces of Phetchaburi and Prachuap Khiri Khan and extending approximately 300 km in latitude. As such, it harbors a wide range of habitats from lowland forests near sea level to cloud forests at its highest peak of 1,513 m in elevation. The seven specimens of C.cf.rukhadeva (THNHM 01807, 24622, 24838, 03251–54) were obtained by various collectors between 11 March 1991 and 25 October 2015 with no specific collection data other than Phetchaburi Province. The locality reported by Grismer et al. (2021c) was an estimate obtained from Ulber (1993) from the central portion of the park in Phetchaburi Province who referred to a specimen he examined (THNHM 24838) as C.brevipalmatus. Cyrtodactylusrivularis sp. nov. however, is known only from the Park’s southernmost limit in Prachuap Khiri Khan Province. Additional collecting and DNA sequencing will be required to establish the approximate distribution limits of each population.