Gigadiscina lessardi, Mergl & Massa, 2005

Mergl, Michal & Massa, Dominique, 2005, A new giant discinoid brachiopod from the Lower Devonian of Algeria, Acta Palaeontologica Polonica 50 (2), pp. 397-402 : 398-400

Gigadiscina lessardi sp. nov.

Fig. 1A–E.

Holotype: External mould of ventral valve (Fig. 1A), coll. Massa−Mergl ( ZCM S 03734 / MAME 001 a).

Type horizon: Devonian, Pragian (Siegenian), equivalent of the Upper Tadrart Formation.

Type locality: Southern Algeria, Tamesna Basin, In Rahir−Tim Amzi section.

Etymology: After French geologist Dr. L. Lessard, who made the pioneering field study in the Tamesna area.

Material.—Fourteen complete shells mostly preserved as corroded internal moulds, partially with remains of original phosphatic shell.

Diagnosis.—Species of Gigadiscina characterised by posteriorly weakly tapering shell, large pedicle foramen with small and short pedicle tube and rudimentary listrium; differs from G. anomala by finer and denser concentric rugellae (8–12 per mm versus 3–5 per mm in G. anomala ).

Fig. 1. A–E. Gigadiscina lessardi sp. nov. from Pragian (Lower Devonian) of Tamesna Basin, In Rahir−Tim Amzi section. A. Holotype, ventral valve, latex Ą cast of external mould, ZCM S 03734a/ MAME 001a. B. Incomplete ventral valve, latex cast of external mould, ZCM S 03734b/ MAME 001b. C. Complete shell, ventral valve (C 1), side view (C 2), dorsal valve (C 3), and detail of ventral valve exterior (C 4), ZCM S 03735/ MAME 002. D. Complete shell, side view (D 1), dorsal valve (D 2), ventral valve (D 3), and detail of intervascular ridges in ventral mould (D 4), ZCM S 03736/ MAME 003. E. Ventral valve, internal mould, ZCM S 03737/ MAME 0004. F. Gigadiscina anomala ( Kayser, 1892) from Pragian (Lower Devonian) of Siegen, Germany. Complete shell, side view (F 1), ventral valve (F 2), BMNH B 42906.

Description.—The shell is 55–60 mm wide and thin−walled in a relation to the shell size. The outline is almost circular with less curved posterolateral margins. Commissure is rectimarginate. The shell width equals shell length.

The dorsal valve is asymmetrical and highly conical, with a rounded apex. The dorsal apex is subcentral (slightly anterior). The anterior slope is steep and gently convex in lateral profile, the posterior slope is similarly steep, straight near the apex but it tends to be concave near the posterior margin. Lateral slopes are straight.

The ventral valve is flat, with dorsally curved posterior and posterolateral margins. The ventral apex is subcentral, situated between the posterior third and the midlength, embedded into flat surface of the valve. The pedicle foramen is large, located at the bottom of weakly depressed circular area. The pedicle foramen continues internally into a very short, narrow, and posteriorly extend pedicle tube. The interiors of both valves are devoid of well−defined visceral areas. Fine radially arranged vascular canals are impressed on the internal surface of the shell.

The ornament consists of fine concentric rugellae with rounded crests. The size of rugellae is almost uniform over the whole surface, with more distant rugellae only in the apical region. There are 12–16 rugellae per 5 mm in the anteromedian part of the shell. The beak region of the ventral valve is covered by inconspicuous rugellae; the earliest coarser rugellae appear 5–6 mm from the apex. The rugellae are evenly spaced over the whole surface with the anteromedian sector of the shell exhibiting weak irregularities. New rugellae originate exclusively by an intercalation.

Occurrence.—All specimens of the new species were collected in In Rahir−Tim Amzi section (L. Lessard’s sample H 1035) in the Tenere Desert of the Tamesna Basin, south of the Ahaggar Massif in South Algeria. In this area, the Lower Palaeozoic comprises a sequence of Cambrian to Devonian strata in a narrow SW−NE trending belt. The basal transgressive Devonian unit bearing the new species is probably of Pragian (Siegenian) age and it rests unconformably on Silurian shale.