Oligosoma carinacauda, Bell & Patterson, 2024

Bell, Trent & Patterson, Geoff B., 2024, The Okuru skink (Oligosoma carinacauda sp. nov) of South Westland, New Zealand-simply elusive or extinct?, Zootaxa 5536 (1), pp. 123-138 : 126-132

Oligosoma carinacauda sp. nov.

Chresonyms

Oligosoma aff. inconspicuum “Okuru ”

Hitchmough et al. 2010, Hitchmough et al. 2013, Lettink 2013, Chapple et al. 2016, Chapple & Hitchmough 2016, Hitchmough et al. 2016a. Hitchmough et al. 2016b.

Oligosoma “Okuru”

Van Winkel et al. 2018, Hitchmough et al. 2021a, Purdie 2022.

Holotype: NMNZ RE005417 , Mussel Point , Okuru , South Westland, New Zealand (-43.917777 168.878611) ( Figs. 2 , 3 ). Collected by Peter Philpott on 17 June 2000.

Etymology. From the Latin nouns carina (‘keel’) and cauda (‘tail’) in reference to the important diagnostic trait of a keeled tail. The established vernacular name “Okuru skink” refers to the location from which the holotype was collected.

Diagnosis. A medium-sized Oligosoma (SVL 65 mm) with the unique combination of a keeled tail and 3 supraocular scales on each side. A keeled tail is shared only with O. stenotis (Patterson & Daugherty) (from Stewart Island) and with some O. inconspicuum (i.e. populations from North Fiordland and South Westland, but not those from Southland and Otago in which the tail is smooth), but both of these species consistently express four supraocular scales.

Comparison to other species. Table 1 provides tabular comparison of O. carinacauda sp. nov, with similar skink species from the South Island of New Zealand. Excepting the keeled tail, Oligosoma carinacauda sp. nov. is most similar in general scale and colour features to other South Island skinks, O. repens , O. toka , O. tekakahu Chapple, Bell, Chapple, Miller, Daugherty & Patterson , O. aureocola , O. burganae and O. polychroma . The species is not known to co-exist with any other species, but occurs close to the range of O. inconspicuum (i.e. Cascade Plateau and Barn Bay) and O. taumakae Chapple & Patterson (i.e. Open Bay Islands and Barn Stacks).

The new species can be separated from these similar species:

● O. inconspicuum : distinguished from O. carinacauda sp. nov. by the absence of a keeled tail in most individuals, SVL/HW less than O. carinacauda sp. nov. in all individuals, four supraoculars in all individuals versus three supraoculars; occasional projecting scales in ear opening versus no projecting scales; a darker, more notched and flecked patterning that almost always involves some degree of flecking on the belly versus light brown ground colour with no flecks on ventral surface; HL/HW usually <1.63 versus HL/HW = 1.63 in O. carinacauda sp. nov.

● O. repens : no tail keeling; MS> 29; VS <82; HL/HW <1.63; occasional projecting scales in ear opening versus tail keeling; MS = 29; VS = 82; HL/HW <= 1.63 in O. carinacauda sp. nov.

● O. toka : no tail keeling; MS> 29; HL/HW <1.63; SVL/HW <12.04; ground colour usually dark brown, ventral surface often flecked versus tail keeling; MS = 29; HL/HW = 1.63; SVL/HW = 12.04; ground colour light brown, no flecks on ventral surface in O. carinacauda sp. nov.

● O. tekakahu : no tail keeling; MS> 29; VS <82; lamellae <18; HL/HW <1.63; SVL/HW <12.04; projecting scales in ear opening; versus tail keeling; MS = 29; VS = 82; lamellae 18/19; HL/HW = 1.63; SVL/HW = 12.04; no projecting scales in ear opening in O. carinacauda sp. nov.

● O. burganae : no tail keeling; MS> 29; HL/HW <1.63; SVL/HW <12.04; TL/SVL = 1.00 versus tail keeling; MS = 29; HL/HW = 1.63; SVL/HW = 12.04; TL/SVL = 1.25 in O. carinacauda sp. nov.

● O. aureocola : no tail keeling; MS> 29; lamellae> 19; HL/HW <1.63; projecting scales in ear opening versus tail keeling; MS = 29; lamellae 18/19; HL/HW = 1.63; no projecting scales in ear opening in O. carinacauda sp. nov.

● O. polychroma : no tail keeling; usually projecting scales in ear opening; supraoculars three or four (usual) versus tail keeling; no projecting scales in ear opening; three supraoculars in O. carinacauda sp. nov.

● O. taumakae has a black iris and is a darker shade of brown with a more flecked, less bold pattern of stripes. In addition, this species does not have a keeled tail.

Description of holotype. Adult female ( Figs. 3a–c ). Body oval in cross-section; limbs well-developed, pentadactyl; snout blunt. Lower eyelid with a transparent palpebral disc, bordered on sides and below by small, oblong granules. Nostril centred just below middle of nasal, pointing up and back, not touching bottom edge of nasal. Nasals widely separated. Supranasals absent. Rostral broader than deep. Frontonasal wider than deep, touching rostral, anterior loreals, prefrontals and frontal. Prefrontals separated, contact with frontonasal, frontal, 1 st supraocular, 1 st supraciliary and anterior and posterior loreals. Frontal longer than broad, shorter than frontoparietal and interparietal together, in contact with 2 anteriormost supraoculars. Supraoculars 3, the second is the largest. Frontoparietals distinct, slightly larger than interparietal. One pair of parietals, meeting behind interparietal and each bordered posteriorly by nuchals and 1 temporal, also in contact with interparietal, frontoparietal, third supraocular and 1 postocular. Three pairs of nuchals, twice the width of adjacent dorsals. Loreals 2, anterior one the smaller, in contact with first supralabial, posterior loreal, prefrontal, frontonasal, nasal, and 2 nd supralabial; posterior loreal in contact with second supralabial, first subocular, lower preocular, prefrontal and anterior loreal. Supralabials 7, the 6 th largest and 5 th below pupil of eye. Infralabials 6(l)/7(r), 5 th and 6 th (l)/4 th and 5 th /(r) largest. Suboculars 6, 3 rd and 4 th separated by fifth supralabial. Upper ciliaries 6, lower ciliaries 8, supraciliaries 5; nuchals 3 pairs; 1 primary temporal, 2 secondary temporals, 2 tertiary temporals; 1 scale row between tertiary temporals and ear opening. Mental broader but shallower than rostral. Postmental similar size to mental. Chinshields 3 pairs. Dorsal scales with 3–5 weak striations, lateral and ventral scales smooth; ventral scales largest. Subdigital lamellae smooth; palmer scales domed. Top of front feet with series of three enlarged scales in line with toes. Ear opening 1mm at widest point, lacking projecting lobules. Forelimbs shorter than hindlimbs. Digits moderately long, cylindrical; third front digit longer than the fourth; 4 th (longest) toe of hind foot with 19(l)/18(r) subdigital lamellae. Tail keeling occurs on intact tail, on which the normal longitudinal scale striations (i.e. small ridge/groove structures present on most dorsal body scales, typically numbering 3–6 across the entire surface of the scale) are restricted to approximately 2–3 striae in the middle region of each scale, elevated slightly, aligned with the preceding and proceeding scales to produce a longitudinal ridge. These ridges (i.e. the ‘keels’) are most pronounced dorsally, tapering in prominence laterally and are absent ventrally. Measurements (in mm): SVL 65; TL 81 (final 12 regenerated); HLL 21.2, FL 16.0; midbody scale rows 29; ventral scale rows 82. HL 8.8; HW 5.4; AG 35.3; SF 23; SE 11; EF 12; HTL 6.0; diameter of eye 1.6; diameter of ear 1.0. Body ratios: HLL 0.3 x SVL; AG/SF 1.5; SE/EF 0.9; HL/HW 1.6.

Colouration. Iris pale grey, becoming blackish behind pupil; top of head, back and tail almost uniform pale chestnut-brown except row of widely spaced fragments of a mid-dorsal stripe, each fragment consisting of 1–2 black streaks with an accompanying minute cream paravertebral streak, the row initiating after the forelimb and terminating just after the hind limb. Dorsum otherwise unblemished. A relatively broad and smooth cream-white dorso-lateral stripe extends from above eye along the body and part way onto the tail, lined below by a continuous thin black line and above by a greatly fragmented thin black line from behind the forelimb. An unblemished broad upper-lateral band (3.5 scales wide), distinctly darker than the dorsal surface, extends from the loreal scales along the body and onto the tail.A pale subocular stripe, followed after a small gap by a thin white and largely smooth midlateral stripe that begins on the cheeks, passes through and below (not above) the ear, above the forelimb insertion without interruption to the hind limb insertion. The lower-lateral surfaces are pale brown with some indistinct whitish streaks, merging into ventral colours. Throat pale grey with a mid-grey speck in centre of most scales; belly and underside of limbs and tail and palmer scales pale yellow with a bronze tint and unblemished. Underside of toes black. Upper surfaces of limbs and toes the same shade of brown as the back, largely unblemished except for the fragmented rudiments of a black-edged pale forelimb stripe (little more than a few widely spaced flecks).

Phyletic affinities. The morphology of the holotype strongly indicates placement in the Oligosoma grande (Gray) species-group (comprised of O. burganae , O. grande , O. inconspicuum , O. maccanni , O. notosaurus , O. repens , O. stenotis , O. tekakahu and O. toka , see Jewell 2019). In 2023, an additional species in this group was described, O. aureocola , which is closely related to O. repens ( Knox et al. 2023) . Tail keeling, in particular, is a trait unique to this group, being found on O. stenotis and in some populations of O. inconspicuum , and no other known New Zealand skink species. The ventral colour pattern and supraocular scales, too, bear a particularly close resemblance to O. burganae , O. repens , O. aureocola , and O. toka . In the absence of phylogenetic data, however, the precise relationships of O. carinacauda sp. nov. within this group remain uncertain.

Species recognition. Since there is only one known individual, it could be argued that Oligosoma carinacauda sp. nov. is not a cryptic species but an atypical member of a known species. However, the combination of two characters unusual for the New Zealand Scincidae (three supraocular scales and keeled tail) makes this unlikely. O. carinacauda sp. nov. can be separated from most congeners by its keeled tail, and from the two species which share this feature by a lower count of supraocular scales (few Oligosoma species have both three and four supraoculars). It further differs from all known congeners in colour pattern—for example the pale palmer scales contrast with the black palmer scales of O. stenotis , and the dull bronzy-yellow belly contrasts with the brighter yellow belly of O. repens , O. toka and O. aureocola —and in other taxonomic characters (see Table 1 ). O. carinacauda sp. nov. is also geographically isolated from most other species, and occurs on the opposite side of the Southern Alps from most other members of the O. grande species-group (excepting only some O. inconspicuum ). As such, we feel that it is most likely that it is a separate new species.

Distribution. Known only from the type locality at Mussel Point, Okuru, South Westland.

Environment. The type location is within the 51.04 Haast Ecological District (ED). The Haast ED is characterised by arcuate dunes, granite outcrops, swampy alluvial plains, river flats, pakihi wetlands, podocarp and beech forests. Today, the environment in the Haast ED has been modified by land clearing and drainage for agriculture, the arrival of predatory mammals (particularly Mustela , Rattus , and Mus spp. ) and establishment of exotic plants, such as marram ( Ammophila arenaria ) and gorse ( Ulex europaeus ), which have destabilised the dunes and led to increasingly severe erosion. The climate of the Haast ED is mild with high rainfall (3000–4000 mm with little seasonal variation) ( McEwen 1987). LENZ Level IV data indicate the land environment at the type locality is mild, with an annual average temperature of 10.9 °C, a mean minimum temperature of 1.2 °C during the coldest month of the austral winter (July), average annual solar radiation of 13 MJ/m 2 /day and winter solar radiation of 3.9 MJ/m 2 /day ( Leathwick et al. 2002).

Habitat. The holotype was collected from what was described by A.H. Whitaker as “ Sphagnum, coastal wetland” located on the alluvial Haast plain. The original lowland wetland habitat in which the holotype was discovered has since been cleared and drained for agriculture. Today, the type locality is now short-cropped pastoral land bordered by flax ( Phormium tenax )-gorse/marram grass-dominated sand dunes ( Figs. 4 , 5a ). Within 500 m of the type locality, a range of potential habitat types exist: coastal granite outcrops, pakihi/shrub wetland hollows ( Fig. 5b ), forest-covered arcuate dunelands, lowland podocarp forests, tidal wetlands along river and stream margins and even a coastal island (0.15 hectares). The type location is currently about 15 m inland from the high tide mark.

Oligosoma carinacauda sp. nov. probably inhabited the alluvial plains characteristic of the 40-km Haast coastline, extending inland towards the mountain foothills. These plains consist of a series of coastal forest, pakihi, and wetland dunelands formed from alluvial deposits over the Holocene, between the Arawhata River northwards to the Tauperikaka Point. These pakihi and wetland duneland habitats provide an open, sunny mosaic suitable for lizards, particularly low-stature wirerush ( Empodisma minus )- or scrub-dominated wetlands. Sponge Swamp (north of Arawhata River), Mussel Point, and Hapuku Estuary on the alluvial plains have been surveyed for O. carinacauda sp. nov. (T. Bell and T. Jewell, unpub. data, Lettink 2013). Human-modified environments on the alluvial plains often provide artificial refugia, such as roofing iron, timber and rocks; long exotic grasslands on road verges, and garden or farmland debris, which lizards often colonise after clearance or modification of their original natural habitat. So far, no lizards or their sign have been found in either natural or anthropogenic-derived habitats on the alluvial plains during species-specific surveys for O. carinacauda sp. nov. across multiple sites using multiple survey methodologies (visual-encounter surveys, funnel trapping, artificial refuges, hand-searching and even a lizard detector dog between 2009 to present day; T. Bell, unpub. data, Lettink 2013, T. Jewell, S. Purdie, S. Mockett, pers. comm). A public campaign including a newspaper article in a regional newspaper calling for lizard sightings did not result in lizard reports, and in interviews with locals, none could recall seeing any skinks in the Okuru district ( Lettink 2013).

Inland to these arcuate series of wetland/dunelands on the alluvial plains are the lowland podocarp and mixed silver beech-podocarp forests widespread throughout Westland. The inner forest interior, however, is likely too cold, shaded and damp to support terrestrial lizards (note: no other South Island Oligosoma skinks are currently known to inhabit closed-canopy forest). Other potential habitat types are open river-flat grasslands and shrublands; although severe flooding at periodic intervals would make it difficult for lizard populations to establish along rivers. Such sites adjacent to the lower Arawhata River, Okuru River and Tuning Fork Stream have been surveyed, using funnel traps, artificial refuges, and visual encounter and hand searches, during ideal conditions for lizard activity but these were also unsuccessful (T. Jewell, pers. comm., Lettink 2013).

The alluvial plain habitats along South Westland still contain much terrain that remains under-surveyed, and O. carinacauda sp. nov. could potentially persist there in very low numbers, or perhaps the species could be rediscovered inland in alpine herbfields of the adjacent Haast range, or the montane fernlands and shrublands of the Cascade Plateau. There have been some lizard surveys in these locations, but no skinks resembling O. carinacauda sp. nov. have yet been recorded.

Natural history. Likely to be diurnal and terrestrial. Nothing is known about the species’ natural history. Several scars on its chest and tail indicate relatively recent mating and/or aggressive interaction with other skinks prior to its discovery, which does suggest that a population existed within dispersal distance of the type locality.

Conservation. The species has been consistently classed as ‘Data Deficient’ through four consecutive iterations of the NZTCS list ( Hitchmough et al. 2010, 2013, 2016b, 2021a). The latest threat classification includes the qualifier ‘PE’, i.e. ‘Possible Extinction’). The Data Deficient threat category reflects the lack of knowledge about the species. Since the sole individual was found in 2000, there have been several independent specific survey efforts for this skink species undertaken by expert herpetologists (T. Bell, T. Jewell, M. Lettink, S. Purdie, S. Mockett, unpub. data)—all of which have failed to discover populations. In addition, there have been generic lizard surveys along the South Westland coastline, undertaken by the Department of Conservation. All surveys have been undertaken during the day at appropriate times of the year (at the peak of the Austral summer: January–March) both at the type locality and within a 40-km radius of this locality. The surveyed habitats have included supralittoral duneland, wetlands, river flats, grasslands, forest edges as well as human-modified environments—all plausible habitats for the species—to no avail. We must ask the pressing question: is this species simply elusive or is it now extinct? It will be difficult to prove the extinction of a species, particularly if it has a cryptic lifestyle, lives in an unexpected habitat, or has potential to persist in small, isolated populations. The IUCN Red List defines extinction as “…when there is no reasonable doubt that the last individual has died. A taxon is presumed Extinct when exhaustive surveys in known and/or expected habitat, at appropriate times (diurnal, seasonal, annual), throughout its historic range have failed to record an individual. Surveys should be over a time frame appropriate to the taxon’s life cycle and life form.” ( IUCN 2012). Similarly, the NZTCS defines extinction as when “There is no reasonable doubt, after repeated surveys in known or expected habitats at appropriate times (diurnal, seasonal and annual) and throughout the taxon’s historic range, that the last individual has died.” ( Townsend et al. 2007). New Zealand lizards are considered to be relatively long-lived, however a 20+ year lifespan would be remarkable for such a small-sized skink of ~ 65 mm SVL (e.g. Barwick 1959, Cree & Hare 2016). Twenty-four years, likely representing 2–3 generations for a small-sized New Zealand skink species ( Cree & Hare 2016), have now elapsed since the discovery of the only known individual.

Small, cryptic lizards are however notoriously difficult to detect in New Zealand, particularly in highly complex habitat and/or when at unnaturally low abundances as a result of introduced predatory mammals. That said, recent lizard surveys elsewhere throughout lowland West Coast have been consistently successful for at least eight ‘Threatened’ and ‘At Risk’ skink species: Taumaka skink ( O. taumakae ), Kapitia skink ( O. salmo Melzer, Hitchmough, Bell, Chapple & Patterson ), Newman’s speckled skink ( O. newmani Melzer, Hitchmough, Bell, Chapple & Patterson ), cobble skink ( O. aff. infrapunctatum “cobble”), Hokitika skink ( O. aff. infrapunctatum “Hokitika ”), Ōkārito skink (O. “Ōkārito”), Alborn skink ( O. albornense Melzer, Hitchmough, Bell, Chapple & Patterson ), and glossy brown skink ( O. zelandicum (Gray)) (Department of Conservation, unpub. data). There has not been difficulty in finding animals and populations of these species over a similar timeframe; and it would have been expected to be relatively the same for O. carinacauda sp. nov. Due to the species’ unique combination of non-cryptic morphological characteristics (three supraocular scales and a keeled tail) and cognizance of the species in the New Zealand herpetological community, it is unlikely that Okuru skinks found elsewhere on the West Coast have been mistaken for a different species.

We recommend that the species be ranked as ‘Threatened—Nationally Critical’ in the NZTCS with the qualifiers ‘Possibly Extinct’ (PE) and ‘One Location’ (OL). Similarly, we assess O. carinacauda sp. nov. as ‘Critically Endangered (Possibly Extinct)’ in the IUCN Red List, under the following criterions: B1ab(i,ii,iii,v), B2ab(i,ii,iii,v). O. carinacauda sp. nov. is known only by the one location, and if still extant, likely has a continuing decline in both the ‘Extent of Occurrence’ (EOO) and ‘Area of Occupancy’ (AOO) (due to land clearing, invasive species, and potentially flooding). No known mature individuals known to be alive; accordingly, the following Red List criterions are also met: C2a(i,ii) and D1.

Figures

FIGURE 2. Map of type locality of the Okuru skink, Oligosoma carinacauda sp. nov. Okuru, Haast, South Westland, New Zealand.

FIGURES 3. a–c. Oligosoma carinacauda sp. nov. type specimen NMNZ RE005417. Photographs by Jean-Claude Stahl (Te Papa Tongarewa / Museum of New Zealand). (a) dorsal; (b) lateral; and (c) ventral views of holotype.

FIGURE 4. Habitat loss at type locality. The type specimen was found in sphagnum in the centre of this photograph. The land has since been drained and converted into pastureland. Photograph by Trent Bell.

FIGURE 5. (a) Duneland and (b) pakihi wetland habitats adjacent to the type locality—possible habitat types for O. carinacauda sp. nov. Photographs by Tony Jewell.