Costulodonta bidens, Herbert, 2020

Herbert, David G., 2020, Revision of the aperturally dentate Charopidae (Gastropoda: Stylommatophora) of southern Africa - genus Afrodonta s. lat., with description of five new genera, twelve new species and one new subspecies, European Journal of Taxonomy 629, pp. 1-55 : 30-31

Costulodonta bidens gen. et sp. nov.

urn:lsid:zoobank.org:act:C5C0F93F-7276-41F1-9B91-AB8C0578CB65

Figs 10 E–H, 11, 18K–L

Diagnosis

Shell small, spire flat or at most slightly raised; protoconch for the most part sculptured by close-set axial riblets; teleoconch sculpture of close-set, compound axial riblets and microscopic spiral threads; aperture lacking parietal and columellar dentition; palatal region with two relatively small denticles, one at mid-whorl, the other basal. Shell translucent, corneous-brown to straw-brown when fresh; diameter up to 1.65 mm.

Etymology

From the Latin bis, bi-: two, and dens: a tooth; with reference to the apertural dentition.

Material examined

Holotype

SOUTH AFRICA • KwaZulu-Natal, Giant’s Castle Game Res. , Yellow-Wood Forest ; 29.300° S, 29.467° E; 15 Jul. 1997; Natal Parks Board (now Ezemvelo KwaZulu-Natal Wildlife ) leg.; montane Podocarpus forest, in leaf-litter; diameter 1.65 mm, height 0.83 mm; NMSA V5678/T4264 .

Paratypes

SOUTH AFRICA • 23 specimens; same collection data as for holotype; NMSA P1018/T4265 .

Other material

SOUTH AFRICA • 2 specimens; KwaZulu-Natal, Giant’s Castle, environs of Main Cave ; 29.2856° S, 29.5179° E; 24 Sep. 2004; A. Moussalli and D. Stuart-Fox leg.; montane forest, in leaf-litter; NMSA W3106 .

Description

Shell small, diameter up to 1.65 mm, H/D ratio ±0.50; spire flat or at most slightly raised; whorls tightly coiled; last adult whorl slightly descendant; suture indented, periphery evenly convex. Protoconch comprising apical cap plus approx. 0.75 whorl; diameter ±330 μm; initially smooth, but for the most part sculptured by close-set axial riblets, with indistinct traces of irregular spiral threads. Teleoconch of up to 3.5 whorls; sculptured by distinct, close-set, compound axial riblets with 3–4 finer intermediary axial threads; intervals between riblets 1–2 times riblet width at whorl periphery; spiral sculpture of microscopic threads, strongest below suture. Umbilicus of moderate width. Aperture lunate, somewhat broader basally; parietal and columellar dentition lacking; palatal region with two relatively small denticles, one at mid-whorl, the other basal, set back approx. ⅛ whorl behind outer lip (sometimes weak). Shell translucent, corneous-brown to straw-brown when fresh.

Distribution and conservation

A narrow-range endemic ( Fig. 11 ), known only from the Drakensberg foothills (‘Little Berg’) in the Giant’s Castle area, KwaZulu-Natal, at approx. 1700–1800 m a.s.l.; in leaf-litter of northern afrotemperate forest. The area falls within the Giant’s Castle Game Reserve, which is part of the Maloti-Drakensberg World Heritage Site. It is thus afforded a high degree of protection.

Remarks

Amongst its congeners, Costulodonta bidens gen. et sp. nov. is rendered distinctive on account of its relatively simple apertural dentition. Superficially the shell shows considerable resemblance to that of ‘ Trachycystis ’ contabulata Connolly, 1932, but that species is larger (diameter up to 2.4 mm) and lacks apertural dentition. However, it does have a similarly sculptured protoconch and the two species may in fact be related. Though known to occur in the forests of the broader Giant’s Castle area ( Herbert & Kilburn 2004), ‘ T ’ contabulata has, to date, not been found to co-occur with C. bidens gen. et sp. nov.

Figures

Fig. 10. Shells of Costulodonta gen. nov. species. A–D. C. acinaces (Connolly, 1933) gen. et comb. nov., Van Reenen, KwaZulu-Natal, diameter 1.39 mm (NMSA A9183). E–H. C. bidens gen. et sp. nov., holotype, diameter 1.65 mm (NMSA V5678/T4264). I–K. C. burnupi (Connolly, 1933) gen. et comb. nov., Mkolombe Mtn, Weenen, KwaZulu-Natal, diameter 1.32 mm (NMSA A9187).

Fig. 11. Distribution of Costulodonta gen. nov. species. C. acinaces (Connolly, 1933) gen. et comb. nov. (green square), C. bidens gen. et sp. nov. (turquoise squares), C. burnupi (Connolly, 1933) gen. et comb. nov. (orange square) C. pluridens gen. et sp. nov. (pink squares), C. trilamellaris (Melvill & Ponsonby, 1908) gen. et comb. nov. (royal blue square). Contour at 1000 m.