An adult male, QCAZ 14814 (Figs 2, 4, 5), field number SC55292, collected on 29 July 2016 at 1630 h by Rubén Jarrín at Yasuní National Park in a lagoon close to Yasuní River, 6 km S Rumiyacu River mouth (0.967°S, 75.875°W, 190 m), Orellana Province, Ecuador.

ECUADOR: Orellana: Oasis community, 500 m from Napo River, 0.460°S, 76.135°W, 220 m, QCAZ 10740; Puerto Yuturi, 0.557°S, 76.008°W, 230 m, QCAZ 3609; Yasuní National Park, near PUCE (Pontificia Universidad Católica del Ecuador) scientific station, 0.677°S, 76.401°W, 273 m, QCAZ 5901; Yasuní National Park, near PUCE scientific station, 0.674°S, 76.398°W, 264 m, QCAZ 17519; Yasuní National Park, near PUCE scientific station, north trail of 50 ha dynamic forest plot, 0.681°S, 76.403°W, 285 m, QCAZ 10229; Puerto Francisco de Orellana (El Coca), 0.467°S, 76.983°W, QCAZ 185; Río Bigal Biological Reserve, 0.537°S, 77.425°W, 955 m, QCAZ 17646; Taracoa, 0.490°S, 76.773°W, 251 m, QCAZ 1118. Pastaza: Jatun Yaku community, 5 km SE Montalvo, 2.084°S, 76.921°W, 286 m, QCAZ 4839; K10 camp, Villano camp, Bloque 10-Agip Oil, 1.475°S, 77.534°W, 468 m, QCAZ 11832. Sucumbíos: La Selva Amazon Ecolodge, 0.498°S, 76.374°W, 229 m, QCAZ 2926; Cuyabeno Wildlife Reserve, PUCE scientific station, 0.002°S, 76.176°W, QCAZ 2623; Cuyabeno Wildlife Reserve, bridge on Cuyabeno River, 0.031°S, 76.321°W, QCAZ 7138.

Mabuya yasuniensis sp. nov. can be distinguished from other congeners by the combination of the following character states: (1) prefrontals paired; (2) frontoparietals paired; (3) supraciliaries four, second longest; (4) supraoculars four, second largest; (5) supralabials seven, fifth longest and below eye; (6) nuchals in one pair; (7) dorsals smooth; (8) scales around midbody 26–30; (9) dorsals 47–52; (10) ventrals 32–40; (11) caudals smaller and similar in shape to dorsals; (12) lamellae under fourth finger 12–16; (13) lamellae under fourth toe 16–19; (14) palms and soles dark; (15) dark dorsolateral stripes absent; (16) pale dorsolateral stripes absent.

Among species of Mabuya known to occur in Ecuador—M. altamazonica and M nigropunctata—, M. yasuniensis is most similar to M. altamazonica in having (1) seven supralabials, with the fifth being the largest and located under the eye (eight supralabials, with the sixth being the largest and located under the eye in M. nigropunctata), and (2) parietals in contact posterior to interparietal (no contact in M. nigropunctata). However, M. yasuniensis can be readily distinguished by having four supraciliaries, of which the second is longer than the others (usually five subequal supraciliaries in both M. altamazonica and M. nigropunctata). The new species is closely related to M. bistriata Spix, 1825 (Fig. 1), which also has four supraciliaries, the second the longest. It differs from M. bistriata in (1) lacking a pair of thin, pale dorsolateral stripes from neck to midbody (Fig. 2), and having (2) dark palms and soles (Fig. 4), darker than venter (palms and soles cream or tan, similar in color to venter in M. bistriata), and (3) 47–52 transverse rows of dorsal scales (53–59 in M. bistriata; Table 2).

Adult male (QCAZ 14814) (Figs 2, 4, 5); SVL 72.98 mm; TL 126.72 mm; head length 14.39 mm, head width 11.29 mm, head depth 8.45 mm; rostral wider than high (2.86 × 1.31 mm), in contact with first supralabials, nasals and supranasals; paired, posterolaterally oriented supranasals in median contact, with posterolateral end contacting anteriormost loreal; frontonasal with rounded anterior border and pointed posterior border, wider than long (3.76 × 2.50 mm), in contact with anterior loreal laterally; paired prefrontals roughly rectangular, anterolaterally oriented, in contact medially, and contacting frontonasal anteriorly, both anterior and posterior loreals laterally, first supraciliaries and first and second supraoculars posterolaterally, and frontal posteromedially; frontal rhomboid, longer than wide (3.93 × 2.59 mm), wider anteriorly, in contact with prefrontals anteriorly, second supraoculars laterally, and frontoparietals posteriorly; supraoculars four, the first the smallest, the second the largest; first supraocular in contact with prefrontal anteriorly, first and second supraciliaries laterally, and second supraocular medially; second supraocular in contact with prefrontal anteromedially, frontal medially, frontoparietal and third supraocular posteriorly, and second supraciliary laterally; third supraocular in contact with frontoparietal laterally, fourth supraocular posteriorly, second and third supraciliaries laterally and second supraocular anteriorly; fourth supraocular in contact with frontoparietal medially, parietal and one pretemporal posteriorly, and third and fourth supraciliaries laterally; supraciliaries four, the second more than twice the length of others, which are similar in length; frontoparietals paired, longer than wide, in broad contact at midline, in contact with frontal anteriorly, all supraoculars except the first laterally, parietal and interparietal posteriorly; interparietal rhomboid, longer than wide (3.03 × 2.54 mm); parietal eye distinct on posterior end of interparietal; parietals more than twice as long as interparietal, wider than long, in contact with each other posterior to interparietal and overlapping the upper anterior temporal scales; each parietal in contact with frontoparietal and fourth supraocular anteriorly, two pretemporals anterolaterally, two temporals (overlapped by parietal) posterolaterally, nuchal posteromedially, and interparietal medially; nasal subrectangular; nostril approximately half the size of nasal, located posteriorly; postnasal small, in contact with supranasal dorsally, anterior loreal and first and second supralabials ventrally, and nasal anteriorly; two subrectangular loreals posterior to nasal, subequal in size, the posteriormost slightly higher; anterior loreal in contact with second supralabial, posterior loreal in contact with second and third supralabials; presubocular single, in contact with fourth and fifth supralabials; preocular single, between presubocular and posterior loreal, in contact with third and fourth supralabials; lower eyelid with large transparent disk, one elongated scale across its dorsal edge (an additional small, anterior scale on right side); supralabials seven, the fifth ventral to eye and more than twice as long as the others; infralabials seven on right side and six on left side; temporals imbricate, smooth, cycloid, not distinctly delimited from scales on the nape or sides of the neck; pretemporals two; primary temporal single, two secondary temporals in contact with three tertiary tempo­rals; ear opening relatively small (maximum diameter = 1.04 mm), round, with two scales overlapping anterior margin; auricular lobules absent; mental about three times as wide as long (3.21 × 1.05 mm), with straight posterior margin; postmental wider than long, in contact with first and second infralabials laterally and one pair of chin shields posteriorly; three pairs of chin shields, anterior one in contact medially, posterior ones separated by three longitudinally arranged gular scales; anterior pair of chin shields in contact with second infralabial and first sublabial laterally, posterior pairs separated from infralabials by sublabials; gulars similar in size and shape to ventrals, separated from supralabials by sublabials; palms and soles covered with small tubercles, subequal in size, both regions delimited by a row of larger and flatter scales; subdigital lamellae smooth, single, 14 under fourth finger and 18 under fourth toe on both sides; finger and toes clawed; relative length of the toes in the following order: I < II < V < III < IV; conspicuous posthumeral mite pocket with slit-like opening; all scales, except head shields and scales on sole and digits, cycloid, smooth and imbricate; longitudinal scale rows around midbody 26, transverse rows of dorsal scales 49, transverse rows of ventral scales 36; preanals four, larger than adjacent ventral scales; median subcaudal series of scales twice as wide as long on posterior half of tail.

Dorsal background olive-bronze; ventral side of head, body, and limbs immaculate bluish-grey; ventral aspect of tail with dense brown pigmentation; dorsum with series of small (~1/3 of a dorsal scale) dark brown spots more or less longitudinally arranged from scapular region onto tail; lateral and dorsal sides of limbs spotted with small, fused dark dots; palms and soles dark brown; preanals pale cream; dark lateral and ventrolateral stripes present; dark lateral stripe wider, about three scales wide at midbody, extending from nostrils through loreals, upper half of supralabials, around eyes, temporals, upper half of ear openings, neck, above arms, and along flanks up to insertion of hindlimbs; dark ventrolateral stripe diffuse, extending from corner of mouth, along ventral aspect of ear opening, above forelimbs, up to insertion of hindlimbs; dark lateral and ventrolateral stripes separated from each other by ill-defined pale lateral stripe, same color as venter, about one scale wide at midbody, with brown pigmentation (denser on posterior half).

The following description is based on two partially everted hemipenes of two male paratypes of Mabuya yasuniensis (Fig. 6); character numbers in parentheses correspond to those listed by Sánchez-Martínez et al. (2020). Organ tubular, bilobed, not capitate, with both external and internal membranous layers; sulcus spermaticus diagonally oriented, bifurcating at base of lobes, with crotch before branches deep and exposed; proximal half of sulcus spermaticus overlapped by large thick fold (1); two small horizontal ridges below the base of each lobe (2) on sulcate side; obtuse projection (lower fold, 3) proximal to each pair of small horizontal ridges; broad, medial bell-shaped fold (4) below lobes on asulcate side, its upper tip (5) extending on surface between lobes; two large protuberances (6) proximal to bell-shaped fold; lateral fold (7) connecting lower fold (3) of sulcate side with large protuberance (6) of asulcate side.

Intraspecific morphological variation in M. yasuniensis is presented in Table 2. One of the most distinctive characters of M. yasuniensis is having four supraciliaries, of which the second is elongated; however, of 14 examined specimens, one had only three supraciliaries, possibly as a result of scale fusion. Contact of parietals behind interparietal, a diagnostic character for M. altamazonica, was observed in 79% of the specimens. The dorsal color pattern of M. yasuniensis varies from having a few longitudinally aligned dark brown spots (usually restricted to scale tips) to a densely spotted back, whereas the dark ventrolateral stripe can be ill-defined as a few longitudinally arranged dark spots on flank scales (Fig. 7).

The specific epithet yasuniensis derives from the words “Yasuní” (Spanish) and “-ensis” (Latin adjectival suffix denoting place or locality). It refers to Yasuní National Park, where M. yasuniensis was discovered. With 9,820 km², Yasuní is the largest protected area in continental Ecuador and one of the most biodiverse places on Earth (Bass et al. 2010). In 1989 Yasuní was declared a UNESCO Biosphere Reserve.

Mabuya yasuniensis is known from the Amazonian lowlands of Ecuador—Orellana, Pastaza and Sucumbíos provinces, at elevations between 200–955 m (Fig. 3). Some localities lie within protected areas, including Yasuní National Park, Cuyabeno Wildlife Reserve and Río Bigal Biological Reserve, suggesting that some populations of M. yasuniensis are well protected. Specimens were found active mostly on the ground between 1030 h and 1630 h in primary and secondary forests. The holotype was found within moss 1 m above water on a tree standing within a lagoon. Mabuya yasuniensis occurs in sympatry with both M. altamazonica and M. nigropunctata.