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| Crassothonna agaatbergensis Swanepoel
in Swanepoel & De Cauwer, 2019.
Photographs by W. Swanepoel.
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Abstract
Crassothonna agaatbergensis, here described as a new species, is known only from the northern part of the Skeleton Coast (part of the Namib Desert) in the Kaokoveld Centre of Endemism, northwestern Namibia. These perennial shrublets grow on basalt of the Agaatberg Mountain under harsh desert conditions. Diagnostic characters for C. agaatbergensis include the partially buried, globose, obovoid or ampulliform caudex and the inconspicuous rays which are much shorter than the involucre. A comparison of some of the more prominent morphological features to differentiate between C. agaatbergensis and its possible nearest relatives, C. clavifolia and C. protecta, is provided. Based on IUCN Red List categories and criteria, a conservation assessment of Endangered (EN D) is recommended for the new species.
Keywords: Agaatberg, endemism, flora, Gariep Centre of Endemism, Kaokoveld Centre of Endemism, Namib Desert, succulent, taxonomy
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| FIGURE 1. Crassothonna agaatbergensis Swanepoel. A. caudex (obovoid), leaves, flowers and roots; B. caudex elongated into a long branch.
Photographs by W. Swanepoel.
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| FIGURE 2. Crassothonna agaatbergensis Swanepoel: capitulum at anthesis. A. side view; B. view from above.
Photographs by W. Swanepoel.
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Crassothonna agaatbergensis Swanepoel, sp. nov.
Diagnosis:— Succulent shrublet up to 150 mm high, related to C. clavifolia and C. protecta, differing from C. clavifolia by leaves being cylindrical, botuliform, or in shorter leaves often clavate, 10–70 mm long, 2–7 mm diam. (3–6 times as long as broad) [vs. leaves in habitat grape-like, up to 15 mm long, 6–8 mm diam. (ca. twice as long as broad)], inflorescences simple or once to several times forked, bracts leaf-like, capitula 1–10, rays very short (a fifth to a quarter as long as involucre) [vs. inflorescences simple, bracts scale-like, capitula solitary, rays long (twice as long as involucre)]; from C. protecta by the caudex being globose, obovoid or ampulliform [vs. caudex slender, bottle-shaped or sausage-like], rays very short (a fifth to a quarter as long as involucre), not rolled backwards, with 1 or 2 green lines [vs. rays long (as long as involucre), soon rolled backwards, with 3 or 4 green lines].
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| FIGURE 3. Habitat of Crassothonna agaatbergensis on the Agaatberg Mountain.
Photograph by W. Swanepoel.
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Distribution and habitat:— At present Crassothonna agaatbergensis is known only from the Agaatberg Mountain 8 km to the east of Cape Fria, in the Skeleton Coast National Park (Figs. 3 & 4). This part of the Skeleton Coast National Park falls within the Namib Desert zone of the Kaokoveld Centre of Endemism, a biogeographical region known for its many restricted-range plants and animals, and extending from northwestern Namibia to southwestern Angola (Van Wyk & Smith 2001). Crassothonna agaatbergensis occurs approximately 8 km from the coast at elevations of 195– 225 m a.s.l. Average annual rainfall in the area is less than 50 mm, occurs in summer and is highly erratic. However, the area regularly receives fog from the bordering Atlantic Ocean (Mendelsohn et al. 2002). The Agaatberg Mountain, especially the higher ridges, clearly receives more precipitation from coastal fog than the surrounding hills and outcrops. This is evident by the occurrence of pockets of moss at the base of rocks and vertical areas, and the density of lichens which is markedly greater on the summits of the highest ridges. The new species occurs on the summits of two of the highest ridges of the Agaatberg Carbonatite Complex (Guj et al. 2011), separated by approximately 0.5 km, in small colonies of about twenty plants each (ca. seventy plants known). It grows on basalt in soil-filled rock fissures and among rocks on level and low vertical areas. Environmental conditions in the general area are extremely harsh, with low rainfall, high temperature variation and strong winds (calm for only 14% of the time as measured at Möwe Bay to the south (Mendelsohn et al. 2002)).
Etymology:— The specific epithet refers to the Agaatberg Mountain 8 km to the east of Cape Fria, in the Skeleton Coast National Park, Namibia.
Notes:— The nearest relatives of Crassothonna agaatbergensis appear to be C. clavifolia and C. protecta, species from which it differs in habit, branches, leaf and floral characters. Distribution ranges of the three species do not overlap; C. agaatbergensis occurs in the northern parts of the Namib Desert, in the Skeleton Coast National Park, C. clavifolia in the southern Namib in the Gariep Centre of Plant Endemism both in Namibia and South Africa (Van Wyk & Smith 2001) and C. protecta in the north-central Namib in Namibia to the Northern and Western Cape Provinces of South Africa (Nordenstam 2012). Some of the more prominent morphological features to distinguish between the three species are provided in Table 1.
Wessel Swanepoel and Vera De Cauwer. 2019. Crassothonna agaatbergensis (Asteraceae), A New Species from the Skeleton Coast, Namibia. Phytotaxa. 427(3); 209-215. DOI: 10.11646/phytotaxa.427.3.4
