Afrocymbella symmetrica sp. nov. (LM Figs 2–23, SEM Figs 24–31)

Description:— Valve length 34–49 μm, valve breadth 7.5–9.5 μm. Length to breadth ratio: 5.8. Valves distinctly dorsiventral, slightly heteropolar, semirhomboid-lanceolate (Figs 2–31). Dorsal margin distinctly arched; ventral margin with wide gibbous center portion, typically 1/3 length of valve, otherwise weakly convex (Figs 24, 26). Valve apices acutely rounded and slightly protracted, with headpole and footpole nearly symmetrical, often only differentiated by APF and slightly greater breadth in footpole (Figs 25, 26). Internally, each column of the APF poroids partially surrounded by knobby jointed ridge of silica (Fig. 29). From inside, valve apices with small to indistinguishable pseudosepta (Figs 29, 31). Axial area narrow, curved, ventrally displaced (Fig. 24). Central area small, asymmetric and diamond-shaped, notably wider on dorsal side (Figs 2–24). Distinct single stigmoid present on dorsal side of central nodule; externally slightly elongated (Fig. 27), internally forming antler-like intermissio (Fig. 30). Raphe slightly sinusoidal (Figs 24, 25). External proximal raphe ends slightly expanded into small pores, deflected towards the dorsal (primary) margin (Fig. 27). External distal raphe terminal fissures ventrally deflected (Figs 25, 26). Internal proximal raphe ends covered by nodular or flap-like development from primary side of valve (Figs 28, 30). Internal distal raphe terminal fissures bisect the APF and terminate into small lobed helictoglossa at some distance from valve apices (white arrow in Fig. 29). Striae almost parallel in the middle, strongly radiate towards the apices, and continue uninterrupted onto valve mantle. Stria density in middle portion 15–17 in 10 μm dorsally, often slightly coarser ventrally (15–16 in 10 μm) and increasing slightly (17–18 in 10 μm) towards valve apices. Areolae elongate slits, becoming smaller and nearly circular at edge of valve mantle, 26–30 in 10 μm. Internally, areolae ellipsoid without velum (Figs 28, 30, 31).

Type:— UNITED REPUBLIC OF TANZANIA. Lake Tanganyika central basin, core NPO4-KH1-1K, Drive 4; mud, sampled at 43–44 cm (estimated to be ~ 50 ka), 6°35’27.0” S 30°06’00.0” E, J. R.Stone, 30th March 2019 (holotype designated here, circled specimen ANS-GC17192! = Fig. 9, isotypes circled specimens ANS-GC17193!, BM-101977!). Cleaned type materials: ANS-GCM 33025 .

Registration:https://zoobank.org/NomenclaturalActs/2d7a7068-a78b-425f-a9e6-5d41e121a43b

Etymology:— The specific epithet ‘ symmetrica ’ refers to the symmetrical outline that characterizes this species.

Distribution:— Afrocymbella symmetrica sp. nov. was first observed in sediment assemblages from Lake Tanganyika, dating back to about 60–50 ka from the core NPO4-KH1-1K (4K, Fig. 1) and was not observed in the more recent sediments from this core. Initially, we thought it may have been putatively extinct, but have subsequently observed living specimen from modern samples in Tanzania collected in shallow water near Kigoma.