SAMA R71741 (SJR 3689), adult male, collected adjacent to Utai Village, West Sepik (Sandaun) Province, Papua New Guinea (3.3875°S, 141.5853°E; 210 m a.s.l.) on 29/05/2004 by C. Dahl.

ZMB 91632 (SJR3694), adult male, same details as holotype but collected on 31/05/2004; SAMA R71742 (SJR3872), adult female, collected at Yapsiei Village, West Sepik (Sandaun) Province, Papua New Guinea (4.6284°S, 141.0962°E; 180 m a.s.l.) on 25/06/2004 by C. Dahl; SAMA R71743 (SJR3887), ZMB 91633 (SJR3888), PNGNM (SJR3914), same details as for SAMA R71742, but SAMA R71743 and ZMB 91633 (SJR3888) collected on 27/06/2004 and PNGNM (SJR3914) collected on 01/07/2004.

A species of Xenorhina characterized by the unique combination of: medium size (SUL of two males 29.2–29.9 mm; of four females 29.9–33.0 mm); vomeropalatines each with one short triangular odontoid spike; legs moderately short (TL/SUL 0.40–0.44); all fingers and toe 1 without, and toes 2–5 with, expanded terminal discs; eye-naris distance greater than internarial distance (END/IND 1.10–1.21); tympanum about same size as eye (TyD/ED 0.95–1.16); dorsal surfaces in life different tones of brown with small blackish spots; ventral surfaces light ivory heavily spotted with reddish-brown blotches or reticula; advertisement calls uttered in series containing 7–10 loud hooting calls = notes each lasting 141–165 ms and produced at a rate of 2.19–2.35 calls/s.

Adult male with vocal slits, calling when collected. Measurements are summarized in Table 1, a dorsolateral view in life is shown in Fig. 1a and ventral surfaces in life in Fig. 1b. Head broader than long (HL/HW 0.81); snout acuminate from above and below, distinctly protruding in profile; vomeropalatines each with one short, triangular and acuminate odontoid spike; loreal region oblique, no canthus rostralis; nostrils near tip of snout, directed dorsolaterally, visible from above but not from below; eye-naris distance greater than internarial distance (END/IND 1.21); tympanum visible in life and preservative, its diameter slightly less than that of eye (TyD/ED 0.95); supratympanic fold weakly expressed, extending from behind eye to behind tympanum; fingers moderately short, not webbed; tips of all fingers with barely detectable circum-marginal grooves, tips not wider than penultimate phalanges; subarticular and metacarpal tubercles barely visible; relative lengths of fingers 3>4>2>1 (Fig. 1c); shank short (TL/SUL 0.42); all toe tips with circum-marginal grooves and, with exception of toe 1, tips wider than penultimate phalanges; toes not webbed, most subarticular tubercles and oval inner metatarsal tubercle moderately well defined; relative lengths of toes 4>3>5>2>1 (Fig. 1d). Body laterally with numerous irregularly shaped and irregularly arranged tubercles; dorsally posterior of head with four regularly spaced longitudinal tubercle rows (two paravertebral and two dorsolateral); dorsal surfaces of head, limbs, and all ventral surfaces without tubercles; tip of snout smooth.

In life dorsal surfaces of head, body and extremities mostly ochre-brown (RAL 8001); posterior back with extended daffodil yellow (RAL 1007) flecks; tubercles on dorsum mainly black-brown (RAL 8022) with light ivory (RAL 1015) apices; a black-brown stripe runs along supratympanic ridge. Ventral surfaces mostly light ivory (RAL 1015) with beige-grey (RAL 7006) reticulation and diffuse orange-brown (RAL 8023) spots on extremities and both sides of abdomen; throat orange-brown with black-brown spots. Iris predominantly blackish.

In preservative ground colour of dorsal surfaces of head, back and extremities beige (RAL 1001) with some inconspicuous brown-beige (RAL 1011) spots. Supratympanic ridge and cutaneous tubercles partly (especially on their bases) black-brown; rear of thighs predominantly fawn-brown (RAL 8007). Basic colour of ventral surfaces light ivory; flecks on chest, abdomen, and extremities beige-brown; throat light ivory with mahogany-brown (RAL 8016) flecking. Narrow light ivory middorsal line from between eyes to cloaca continues on to rear of hind limbs and is vaguely detectable on abdomen.

Measurements and proportions of most paratypes show limited variation (Table 1). Two males measure 29.2 and 29.9 mm and four females 29.9–33.0 mm SUL, indicating that sexual size dimorphism is slight. Ground colour of the dorsal surfaces in preservative of SAMA R71742 and ZMB 91633 is beige, as in the holotype, but in the remaining types is light mahogany brown. All paratypes except ZMB 91632 have a narrow light ivory middorsal line; none exhibits lumbar spots. Most specimens have variable numbers of small blackish dorsal and/or dorsolateral spots closely associated with cutaneous tubercles. Tip of snout is light grey and smooth in all specimens. Colouration of ventral surfaces in all specimens is light ivory covered with beige brown (e.g. PNGNM [SJR 3914]) to mahogany brown (e.g. ZMB 91632) spots or reticula (Fig. 2). Size and extent of pigment spots on throat varies from almost unspotted (PNGNM [SJR 3914]) to extensively covered with small (SAMA R71742) or large ZMB 91632 spots.

Eight call series from the holotype recorded at an air temperature of 26.0°C were analysed. Data are presented as range and Mean±SD. Call series contain 7–10 calls (= notes) (mean 9.1±1.13; n=8) lasting 3.2–4.5 s (mean 3.98±0.48 s; n=8) at a repetition rate of 2.19–2.35 calls/s (mean 2.29±0.06; n=8). Call (= note) duration is 141–165 ms (mean 151.6±4.36 ms; n=73), with call intervals of 262–498 ms (mean 322.5±46.7 ms; n=65). Calls are unpulsed and the first call in most series is slightly longer than subsequent ones. Call intervals tend to become longer during the course of each series, and the last interval is clearly greater than preceding ones. Unlike many other Xenorhina species, neither volume (Fig. 3, upper) nor pitch of calls (Fig. 3, lower) increase during the series. Calls start abruptly at maximum amplitude, and amplitude decreases rapidly at the very beginning and then gradually until end of call (Fig. 3, upper). Frequency decreases slightly at the very end of each call (Fig. 3, lower), where this pattern is most evident in the upper harmonics). All calls have four well-defined harmonics, with maximum energy at 0.85 kHz, 1.70 kHz, 2.55 kHz and 3.40 kHz. The first harmonic with peak at 0.85 kHz is clearly the dominant one (Fig. 4).

Xenorhina ventrimaculata sp. nov. is known with certainty from two locations approximately 150 km apart in the lowlands of the Sepik River basin in northwestern Papua New Guinea (Fig. 5). An additional male specimen (SAMA R71744) from Wamangu Village, about 300 km northeast of the type locality (Fig. 5), closely resembles this species in its ventral colouration but is substantially larger than the two male types (39.5 mm vs. 29.2 and 29.9 mm SUL) and further differs in a number of body ratios. We refrain from including it in the type series and consider the taxonomic status of the Wamangu population to be uncertain pending the collection of additional information.

The habitat at the type locality at Utai is secondary lowland forest, where both the holotype and paratype ZMB 91632 were calling from beneath leaf litter after rain at night. At Yapsiei this species was found in primary lowland forest, where all specimens were found between 1.5–3.0 cm beneath the soil surface when attempting to locate calling males. The local name for this species at Utai is Mopepe.

The specific epithet is an adjective compound of two Latin words. Venter is a substantive and means belly or underside of the body and maculata is a feminine adjective meaning flecked or spotted. The specific epithet refers to the conspicuously spotted ventral surfaces of most specimens of the new species.

Xenorhina includes a group of species with one or more distinct odontoid spikes on each vomeropalatine bone (formerly allocated to the genus Xenobatrachus) and another group lacking spikes on the vomeropalatines. Xenorhina ventrimaculata sp. nov. belongs to the former group and we compare it here only with other Xenorhina species of a similar size (25–38 mm SUL) that have a single odontoid spike on the vomeropalatines. Note that the terms call and note are used synonymously.

Xenorhina fuscigula (Blum & Menzies, 1989) has shorter legs than Xenorhina ventrimaculata sp. nov. (TL/SUL < 0.33 vs. > 0.40), a smaller internarial distance (IND/SUL < 0.64 vs. > 0.64), a smaller eye-naris distance (END/SUL 0.064–0.074 vs. 0.070–0.081), a shorter fourth toe (T4L/SUL 0.34–0.41 vs. 0.40–0.44) and its call is a single note (vs. 7–10 notes in Xenorhina ventrimaculata sp. nov.).

Xenorhina huon (Blum & Menzies, 1989) has shorter legs (TL/SUL < 0.38 vs. > 0.40), a ventral colour pattern varying from dark pigmentation near-absent to near-complete dense covering (see fig. 71 in Zweifel 1972 [as Xenobatrachus rostratus (Méhely, 1898)]) and no mid-dorsal line (vs. mid-dorsal line present in almost all specimens of Xenorhina ventrimaculata sp. nov.).

Xenorhina lacrimosa Günther & Richards, 2021 is larger than Xenorhina ventrimaculata sp. nov. (SUL of five adult males 34.5–41.0 mm vs. 29.2–29.9 mm in two male Xenorhina ventrimaculata sp. nov. and 34.3 mm in one female X. lacrimosa vs. 29.9–33.0 mm in four female Xenorhina ventrimaculata sp. nov.). Xenorhina lacrimosa also has wider discs on fourth toes (T4D/SUL 0.036–0.043 vs. 0.029–0.037), wider discs on first toes (T1D/SUL 0.023–0.027 vs. 0.017–0.022), a greater END/IND ratio (1.18–1.48 vs. 1.10–1.21); and a call repetition rate of 0.20–0.27 calls/s vs. 2.2–2.4 calls/s in Xenorhina ventrimaculata sp. nov.

Xenorhina mehelyi (Boulenger, 1898) has longer legs (TL/SUL > 0.44 vs. < 0.44), larger eyes (ED/SUL 0.067–0.079 vs. 0.056–0.070) and advertisement calls containing about 17 calls (vs. 7–10 calls in Xenorhina ventrimaculata sp. nov.) with call intervals of 1500 ms on average (Blum and Menzies 1988) (vs. 323 ms on average) and a mean repetition rate of 0.6 calls/s in X. mehelyi (vs. 2.3 calls/s in Xenorhina ventrimaculata sp. nov.).

Xenorhina schiefenhoeveli (Blum & Menzies, 1989) has shorter legs (TL/SUL < 0.40 vs. > 0.40), larger eyes (ED/SUL 0.071–0.081 vs. 0.056–0.070), and call series containing more than 100 (vs. 7–10) shorter calls (about 100 ms on average vs. about 150 ms on average).

Xenorhina subcrocea (Menzies & Tyler, 1977) has longer legs (TL/SUL > 0.44 vs. < 0.44), a smaller internarial distance (IND/SVL 0.059 vs. 0.064–0.067), a higher END/IND ratio (1.26–1.33 vs. 1.10–1.23), larger eyes (ED/SUL 0.071–0.072 vs. 0.056–0.070), shorter calls (64–69 ms vs. 141–165 ms) with shorter call intervals (154–285 ms vs. 262–498 ms) and a lower dominant frequency (0.40 vs. 0.85 kHz).

Xenorhina tumulus (Blum & Menzies, 1989) is slightly smaller than Xenorhina ventrimaculata sp. nov. (SVL of three males 26.0–28.7 mm vs. 29.2–29.9 mm SUL in two males), has short, round palatine spikes (vs. triangular spikes with pointed tips), ventrum pinkish mottled with brown (vs. light ivory mottled with reddish-brown), rear of thighs very dark (vs. not dark), and call series with up to 17 (vs. 7–10), shorter (100 ms vs. > 140 ms) calls.

Xenorhina wiegankorum Günther & Richards, 2021 is larger than Xenorhina ventrimaculata sp. nov. (SUL of five adult males 32.0–35.7 mm vs. 29.2–29.9 mm in two male Xenorhina ventrimaculata sp. nov.); has longer shanks (TL/SUL 0.44–0.47 vs. 0.40–0.44); longer tarsi (TaL/SUL 0.29–0.31 vs. 0.26–0.29); longer fourth toes (T4L/SUL 0.45–0.47 vs. 0.40–0.44); wider discs on first toes (T1D/SUL 0.023–0.030 vs. 0.017–0.022); wider discs on third fingers (F3D/SUL 0.020–0.028 vs. 0.014–0.022); wider discs on first fingers (F1D/SUL 0.020–0.025 vs. 0.014–0.019); longer call series (13.8–18.1 s vs. 3.2–4.5 s) with shorter calls (60–104 ms vs. 141–165 ms) having a lower dominant frequency (0.55 kHz vs. 0.85 kHz), and produced at a slower repetition rate (1.71–2.15 vs. 2.19–2.35).

Xenorhina woxvoldi Günther & Richards, 2021 has shorter legs than Xenorhina ventrimaculata sp. nov. (TL/SUL 0.36 vs. 0.40–0.44); wider discs on fourth toes (T4D/SUL 0.038–0.040 vs. 0.029–0.037); wider discs on third fingers (F3D/SUL 0.024–0.027 vs. 0.014–0.022); wider discs on first finger (F1D/SUL 0.020–0.021 vs. 0.014–0.019); a shorter distance between eye and naris (END/SUL 0.056–0.060 vs. 0.070–0.079), a lower END/IND ratio (0.80–0.90 vs. 1.10–1.21); and shorter calls = notes (37–84 ms vs. 141–165 ms) produced at a higher repetition rate (4.0–4.5 calls/s vs. 2.19–2.35 calls/s).

Xenorhina zweifeli (Kraus & Allison, 2002) is larger than Xenorhina ventrimaculata sp. nov. (SVL of 10 specimens 33.2–38.0 mm vs. SUL of six specimens 29.2–33.0 mm), has a smaller internarial distance (IND/SVL 0.052–0.063 vs. 0.064–0.067), ventral surfaces sparsely flecked (vs. intensively spotted), and advertisement calls consisting of a single note repeated at irregular intervals with lengths of 207–380 ms (vs. 7–10 calls repeated in rapid succession with lengths of 141–165 ms).