Petalidium saxatile Swanepoel, K.G.Dexter, E.Tripp & A.E.van Wyk, 2025

Petalidium saxatile Swanepoel, K.G.Dexter, E.Tripp & A.E.van Wyk , sp. nov. ( Figs 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 )

Diagnosis: —A woody dwarf shrub up to 0.5 m tall, morphologically most similar to Petalidium canescens and P. setosum , differing by having indumentum on vegetative parts consisting of simple (weak and robust), bifurcate, stellate-dendritic, dendritic and stalked glandular trichomes, lacking sessile glands ( vs. indumentum strigose with in addition widely spaced long, robust, simple trichomes [ P. canescens ]; long, robust, simple and stalked glandular trichomes, sessile glands present [ P. setosum ]); leaf lamina conduplicate ( vs. flat or subconduplicate [ P. canescens ]; flat or irregularly curved, twisted, widely undulate, recurved or incurved towards margins [ P. setosum ]), usually narrower, up to 22 mm wide ( vs. up to 100 mm [ P. canescens (broad-leaved form)]; up to 50 mm [ P. setosum ]), with 3 or 4 principal lateral veins each side ( vs. 4–6 [ P. canescens ]; 3–7 [ P. setosum ]); corolla expanded portion longer, 7.7–9.1 mm long ( vs. ca. 7.5 mm [ P. canescens ]; ca. 3.4 mm [ P. setosum ]), anterior lobe inside (adaxially) bright yellow, lateral and upper lobes pink or brown-pink, darker towards bases ( vs. all lobes similarly coloured: violet-red but anterior lobe darker [ P. canescens ]; purple, burgundy or carmine, anterior lobe sometimes yellow towards apex [ P. setosum ]), nectar guides absent ( vs. present).

Type: — NAMIBIA. Kunene Region: Farm Driefontein 716, ca. 8 km south-southwest of homestead, southern tributary to Springbok River , rocky area above river bed, 2013BD, 869 m a.s.l., 20 March 2022, Swanepoel 645 ( holotype WIND!; isotypes PRE!, PRU!) .

Hemispherical woody dwarf shrub up to 0.5 m tall; all vegetative parts with sparse to matted white indumentum, trichomes short, eglandular, simple, bifurcate, stellate-dendritic and sparsely branched dendritic with branches tapering, also with longer bottlebrush-like dendritic ones and widely-spaced more robust, longer, stalked glandular trichomes and much longer robust multi-cellular bulbous-based simple, eglandular trichomes up to 3 mm long in addition. Stems single or multi-stemmed from just below or above ground level, erect to procumbent, older distal stems cylindrical, bark smooth or longitudinally fissured and peeling in long narrow strips, cream- or grey-white; young stems quadrangular, green, glabrescent, cystoliths visible, linear, linear-oblanceolate or narrowly elliptic. Leaves opposite and decussate on new shoots, fascicled on older stems; petiole 1–18 mm long; lamina ovate, lanceolate [ sensu Lindley’s definition ( Beentje 2016)] or elliptic, conduplicate, 13–70 × 4–22 mm, green to grey-green (due to white indumentum), apex acute or obtuse, base attenuate or cuneate, margins entire, midrib and the 3 or 4 principal lateral veins prominently raised adaxially, longer robust multi-cellular bulbous-based simple trichomes sometimes absent, cystoliths visible adaxially, linear. Flowers in axillary compact dichasia with sterile branches; bracts foliaceous, linear, linear-lanceolate [ sensu Lindley’s definition ( Beentje 2016)] or linear-oblanceolate, sessile, 19.0–25.0 × 0.8–3.5 mm, indumentum similar to vegetative parts but trichomes scattered (not matted), long robust multi-cellular bulbous-based simple trichomes mostly on margins; pedicels (below bracteoles; “peduncle” of some authors) up to 1 mm long; bracteoles narrowly ovate, symmetrical, papery, ca. 10.7 × 4.4 mm, apex attenuate, pale green or green, stramineous when dry, venation reticulate, dark green, abaxially with indumentum of scattered short simple trichomes and small, short-stalked glandular ones towards apex with in addition long robust multi-cellular bulbous-based simple trichomes, otherwise glabrous, adaxially sparsely strigose towards apex, otherwise glabrous, margin lanate towards apex, cystoliths visible both sides, small, linear, dense. Calyx ca. 8.4 mm long including basal tube of ca. 1.0 mm deep, lobes 4, regular, narrowly triangular, acute, unequal, 6.6–7.4 × 0.8–1.5 mm, anticous lobe bifid for ca. 1.1 mm; strigose both sides. Corolla with narrow unexpanded portion of tube cylindrical, laterally slightly flattened, 23.3–26.5 mm long with lobes straightened, narrow portion 10.0– 13.3 mm long, 1.8–2.3 mm diam., outside glabrous, inside puberulous distally on anterior side otherwise glabrous, expanded portion at slight angle to anterior side of narrow portion, 7.7–9.1 mm long, outside puberulous, inside of anterior part with scattered long stiff white simple trichomes towards mouth, inside otherwise glabrous, herringbone pattern prominently transversely 6-ribbed inside; lobes ascending-spreading or in same plane with respect to corolla tube axis, sometimes recurved towards apex, anterior lobe tapering, obovate or rectangular, apex emarginate, 6.5–7.1 × 4.3–5.6 mm, lateral and upper lobes tapering or ovate, apices rounded, lateral lobes 5.8–6.4 × 3.9 mm, upper lobes 6.7–7.0 × 2.9–3.6 mm, connate for 35–40% of their length, lobe margins entire or recurved towards apex, anterior lobe bright yellow inside (adaxially), lacking nectar guides, lateral and upper lobes pink or brown-pink, darker towards bases with traces conspicuous, paler than associated lobes, all lobes discolorous, outside (abaxially) much paler, glabrous inside (adaxially) except for few long stiff white simple trichomes towards bases. Stamens didynamous, inserted dorsally in throat, fused portion ca. 1.7 mm long, free parts slightly tapering towards apex, glabrous, long filaments ca. 6.2 mm long, short filaments ca. 4.7 mm long, outer filament with basal ridge from point of insertion on corolla (“trace”) decurrent to ca. 9 mm from base of tube, puberulous; filament curtain reduced ( sensu terminology of Manktelow 2000); anthers 2-thecous, thecae oblong, equal, ca. 3 mm long excluding short basal spur, pink-white with few to scattered small, stalked glandular trichomes towards apex. Gynoecium 17.0– 20.5 mm long; ovary ovoid, laterally compressed, ca. 1.6 × 1.3 mm, inserted in fleshy disc, glabrous; style filiform, 15.0– 18.6 mm long, puberulous, stigma lobes linear, unequal, longer lobe ca. 1.2 mm long, shorter lobe ca. 0.6 mm long. Capsule flattened ovoid, ca. 8 × 4 mm, tawny, glossy, glabrous; seeds cordate, ca. 3.7–4.2 × 2.8–3.0 mm, densely covered with white hygroscopic trichomes.

Phenology: —Flowers and fruit have been recorded from March to June (late summer to winter).

Distribution and habitat: —At present, Petalidium saxatile is only known from the Kaokoveld, northwestern Namibia, in the vicinity of Palmwag and southwards to the Bergsig area, with an outlier population on the farm Welbedacht to the south of Khorixas ( Fig. 3 View FIGURE 3 ). Petalidium saxatile occurs in rocky areas and among stones comprising Etendeka Group basalt ( Milner et al. 1994) on arid hillsides and along drainage lines at elevations of 860–1130 m a.s.l., 70–150 km inland from the Atlantic Ocean. Average annual rainfall in the area is from less than 100 to 200 mm and falls in summer ( Atlas of Namibia Team 2022). For other species of Petalidium mainly associated with substrates derived from Etendeka Group basalt, see under “Notes” below.

Conservation status: — Petalidium saxatile has been recorded at four localities in the vicinity of Palmwag and Bergsig and at one outlier locality represented by the gathering Liebenberg 4954 (see note under “Additional specimens examined”) where it is occasional to locally common. Although a brief search at various other localities with seemingly suitable habitat did not reveal any plants, it is probably more widespread than currently known. The extent of occurrence (EOO) has been calculated as 3337 km ² and the area of occupancy (AOO) as 20 km ², based on a cell width of 2 km as recommended by the IUCN Standards and Petitions Committee (2024). Due to its limited geographical range (AOO < 500 km ²), with only five known localities and habitat under pressure from prolonged drought conditions, Petalidium saxatile is here provisionally assessed as Endangered EN B2a,b(iii) ( IUCN 2012).

Etymology:— The specific epithet refers to the habitat in which Petalidium saxatile is most often found; derived from the Latin ‘ saxatilis ’, dwelling or found among rocks ( Stearn 2004).

Notes: — Petalidium saxatile is morphologically most similar to P. canescens and P. setosum . Hence these three species were compared in the diagnosis above. Some of the morphological features to distinguish between P. canescens , P. saxatile , and P. setosum are provided in Table 1; also see Figs 4 View FIGURE 4 & 5 View FIGURE 5 .

In the case of herbarium specimens, the new species may be mistaken not only for P. canescens and P. setosum , but also for other Petalidium species from Namibia’s Kunene Region that have compact inflorescences and indumentum consisting of dendritic trichomes. Notably, these include P. halimoides Moore (1880: 228) and P. lanatum Clarke (1899: 90) . However, Petalidium halimoides and P. lanatum have relatively small corollas, 12–13 mm long ( vs. ca. double the length, 23–26 mm) and the indumentum on the bracts lacks long, simple trichomes ( vs. long simple trichomes present, visible to the naked eye). All the mentioned taxa are from the group composed of plants with irregular, four-parted calyces ( Obermeijer 1936, Tripp et al. 2017).

Our phylogenetic analyses show that P. saxatile is genetically distinct from P. canescens and other species of Petalidium found nearby, forming a reciprocally monophyletic clade in our phylogenetic analysis ( Fig. 6A View FIGURE 6 ). Further, a network analysis that aimed to assess a history of hybridisation or incomplete lineage sorting in P. saxatile and close relatives shows that P. saxatile clearly forms a cohesive genetic cluster that is distinct from other species ( Fig. 6B View FIGURE 6 ). We take these results as supportive of the idea that P. saxatile is a ‘good species’ with respect to the general lineage concept of species (De Quieroz 1998), although further phylogenetic investigations that include P. setosum would be useful to confirm this.

The Etendeka Tableland, a prominent dissected plateau in arid northwestern Namibia, aligns closely with the core distribution area of P. saxatile ( Detay & Detay 2017, Atlas of Namibia Team 2022). This landscape formed around 132 million years ago due to volcanic activity, specifically lava flows from the Etendeka Group basalts, associated with the initial seafloor spreading that eventually created the South Atlantic Ocean ( Goudie & Viles 2015). Although the region is known to be floristically diverse, no comprehensive plant checklist has yet been compiled. A limited ecological study of several mesas (flat-topped, isolated hills) in the area recorded 331 plant species, representing over 7% of Namibia’s known plant species, despite the study covering only 0.1% of the country’s land area ( Burke 2003). Recently, we described P. etendekaense Swanepoel, E.A.Tripp & A.E.van Wyk in Swanepoel et al. (2024: 37), a species exclusive to the Etendeka Tableland. Known populations of P. etendekaense and P. saxatile do not overlap, thus showing allopatric distributions. Other Petalidium species endemic or near-endemic to the Etendeka Tableland include P. giessii , P. luteoalbum , and P. ovatum ( Schinz 1890: 198) Clarke (1899: 90) . Another notable endemic species from this region is Oberholzeria etendekaensis Swanepoel, M.M.le Roux, M.F.Wojc. & A.E.van Wyk in Swanepoel et al. (2015: 11) ( Fabaceae ), a succulent biennial or short-lived perennial shrublet that belongs to a monotypic genus. This suggests the Etendeka Tableland may serve as a local subcentre of plant endemism within the larger Kaokoveld Centre of Endemism, a topic that merits further research.

Additional specimens examined ( paratypes): — NAMIBIA, Kunene Region: On plain in Aub River valley , 1913 DD, 1120 m, 4 June 2000, Gindrig & Hennig 104 ( PRE!, WIND!) ; 4.7 km from Palmwag on road C43 to Sesfontein , 1913 DD, 901 m, 4 May 2022, Dexter & Loiseau 7655 ( E!, LUBA!, WIND!) ; 4.7 km from Palmwag on road C43 to Sesfontein , 1913 DD, 901 m, 11 May 2022, Swanepoel 646 ( WIND!) ; Achab River , 2013 BB, 6 June 1999, Hearn 28 ( WIND!) ; Farm Welbedacht 394, 2014DB, May 1949, Liebenberg 4954 ( PRE, WIND!) .

The outlier locality mentioned on the label of the specimen Liebenberg 4954 ( WIND), cited above, may be incorrect. A duplicate of this collection in Herb. PRE provides the same locality, but also includes the name “Fransfontein,” a village located just north of Khorixas. We interpret this additional information as likely intended to help identify the location of the farm Welbedacht. During a visit in November 2024 to the farm Welbedacht 394, south of Khorixas, one of us ( WS) found no outcrops of Etendeka Group rocks, which serve as the substrate at all other known sites of P. saxatile . Furthermore, an extensive search of the farm for Petalidium species revealed no trace of P. saxatile , but only the presence of P. halimoides .