Gigadiscina sp.

Mergl, Michal & Massa, Dominique, 2005, A new giant discinoid brachiopod from the Lower Devonian of Algeria, Acta Palaeontologica Polonica 50 (2), pp. 397-402 : 400-401

Gigadiscina sp.

Fig. 2B .

Material.—One ventral valve, preserved with original shell in dark−grey siltstone with common sandy grains and fragments of the phosphatic shells, collected from the core of BP borehole in Libya, Tripoli area (borehole BP, well C1–8, core 11, depth 54.15 ft.). The siltstone is referred to Middle to Upper Devonian. The specimen is kept in the Natural History Museum, London (BMNH BD 3385).

Description.—The single available valve is flat, broadly oval, 35 mm long and 32 mm (estimated) wide, with the maximum width at the anterior third. The posterior half of the valve is slightly extended, but less than in G. lessardi sp. nov. and G. anomala ( Kayser, 1892) . The pedicle foramen is small (it is poorly preserved), located immediately posterior to the ventral apex, which lies at about two−fifths of the valve length. The external surface is covered by regular concentric rugellae, anteromedianly numbering 17–18 per 5 mm. Rugellae are low, massive, having thickened bases, separated from one another by equally wide or slightly wider flat interspaces. Unlike the interspaces, the variation of rugellae size is limited. Micro−ornament consists of fine, deep circular pits of uniform size, 2–3 µm in diameter. Narrow radial bands without pitting are observable in posterolateral part of the valve. They are 20 to 50 µm broad, with smooth surface or covered by irregular drapes and scattered circular pits.

Remarks.—The species differs from other species of the genus by finer concentric ornamentation and less extended posterior half of the ventral valve.

Figures

Fig. 2. A. Gigadiscina cf. collis (Clarke, 1913) from Upper Devonian of Falkland Islands, near Port Howard. External mould of ventral valve, BNMH B 60298. B. Gigadiscina sp. from Middle–Upper Devonian of Libya, Tripoli area (borehole BP, well C1−8). Ventral valve BMHN BD 3385b, external mould (B1), ventral valve BMNH BD 3385a, detail of ornament (B2), detail of microornament (B3), and detail of pitted radial bands alternating with smooth bands (B4).