Gigadiscina, 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 : 397-398

Genus Gigadiscina nov.

Type species: Gigadiscina lessardi sp. nov.: Lower Devonian , Pragian (Siegenian); Tamesna Basin, Algeria .

Diagnosis.—Discinoid genus characterised by large, convexo−planar, subcircular shell, covered by fine regular concentric rugellae; micro−ornament of uniform, very small deep circular pits over entire surface; dorsal valve highly and obtusely conical, with subcentral apex; ventral valve planar, with subcentral apex; pedicle foramen subcircular, resting at the bottom of shallow circular depression; the foramen leads into very short pedicle tube toward the posterior margin; listrium rudimentary, broad and very short.

Differential diagnosis.—Differs from Orbiculoidea , Acrosaccus , and Lochkothele in having convexo−planar (rather than biconvex) shell, circular pedicle foramen, with very short pedicle tube and rudimentary, very short and broad listrium (rather than narrow pedicle track with long pedicle tube and long listrium); differs from Schizotreta , Oehlertella , and Lingulodiscina by subcentral (rather than submarginal) dorsal apex; differs from Lindstroemella and Discinisca by circular posteriorly closed pedicle foramen that is open in the latter two genera; differs from Roemerella by large size, planar (rather than concave) ventral valve and circular pedicle foramen in depressed area (rather than elongate on elevated area); differs by very large shell from other genera of Discinidae apart of Discinisca .

Remarks.—A few previously described species can be referred to the new genus, mostly from siliciclastic sequences of Lower Devonian age. Kayser (1892) described two large discinid species from Siegen in Germany, associated with common Siegenian brachiopod fauna but only one, “ Discina ” anomala , is referred to the new genus. Valves of G. anomala ( Kayser, 1892) are remarkably large, with length over 70 mm; ventral valve is planar, with the subcentral pedicle foramen. The second species described by Kayser, i.e., Discina siegenensis Kayser, 1892 is biconvex, almost 40 mm wide shell with broad listrium and probably belongs to genus Orbiculoidea d’Orbigny, 1847 .

Some discinoids from the siliciclastic sequences of the Lower Devonian of the Malvinokaffric Realm have similar morphology. Boucot et al. (2001) described a brachiopod fauna with Malvinokaffric affinity from the Parecis Basin of Brazil. As with other sites within the Malvinokaffric Realm ( Rowell 1965b), the large discinids are significant element of this fauna, and represent a low diversity Orbiculoid Community within a very shallow intertidal to subtidal environment of benthic assemblage 1 ( Boucot 1975). Clarke (1913) described another giant discinoid Orbiculoidea collis Clarke, 1913 from the Parana Basin. Rowell (in Boucot et al. 2001) referred several large valves to this species from the Parecis Basin. Figured material ( Boucot et al. 2001: fig. 5) is remarkably similar to Gigadiscina lessardi but because the morphology of the ventral valve of O. collis is unknown, the affinity of the latter is somewhat obscure. Reed (1925) noted a find of a giant discinoid referred by him to Orbiculoidea collis Clarke, 1913 , from the Bokkeweld Beds of South Africa.

Another giant discinoid is known from the Lower Devonian of the Falkland Islands. The ventral valve was collected by R. Baker in 1931 from ferruginous sandstone between Port Howard and Manybranch Harbour. The specimen ( BMNH B 60298) was determined as Orbiculoidea aff. collis Clarke, 1913 , and is referred by us to the new genus. Although the basic morphology is similar, this specimen differs from the species Orbiculoidea falklandica Rowell, 1965 from the Falkland Islands and also from the Parecis Basin of Brasilia ( Boucot et al. 2001). The latter species is smaller than Orbiculoidea collis , about half the size, it has low conical ventral valve, and low dorsal valve. The species Orbiculoidea sp. cf. O. bainii Sharpe, 1856 from the Horlick Formation (Lower Devonian) of the Ohio Range, Antarctica ( Rowell 1965b) may also be referred to the new genus. It is unusually large, having a convexo−planar shell, and a large depressed area encircling the pedicle foramen.

Further species, which may be referred to the new genus, is a ventral valve ( Fig. 2B ) from borehole core in Tripoli area of Libya. This specimen ( BMNH BD 3385) is described herein as Gigadiscina sp.

The species Orbiculoidea magnifica Mergl, 2001 from the Lower Devonian (Lochkov Formation, Lochkovian) of the Prague Basin ( Mergl 2001) is also referred to the new genus. Is has convexo−planar shell, elongate, weakly tapering posterior part of the shell, but its pedicle foramen is more elongate with longer listrium, and the pedicle tube was probably significantly longer than in Gigadiscina lessardi . The species O. magnifica represents the earliest (Early Devonian) member of this discinoid stock, with a preserved pedicle tube and reduced but distinct listrium.

The origin of the new genus can be traced among the small− to medium−sized, strongly inequivalved and almost convexo−planar discinoids of the late Silurian. These are well represented by Orbiculoidea rugata (J. de C. Sowerby in Murchison, 1839) and related species. This group of late Silurian discinoids differs from Orbiculoidea d’Orbigny, 1847 (typified by O. forbesii ) by their convexo−planar shell and by having a shorter, broadly triangular listrium instead of the narrowly spindle−shaped listrium present in Orbiculoidea .

Species referred.— Discina anomala Kayser, 1892 : Siegenian, Germany ( Kayser 1892). Gigadiscina lessardi sp. nov.: Siegenian, Tamesna Basin, Algeria; Orbiculoidea sp. cf. O. bainii Sharpe, 1856 : Lower Devonian, Antarctica ( Rowell 1965b); Orbiculoidea collis Clarke, 1913 : Lower Devonian, Parana Basin, Brazil ( Clarke 1913; Boucot et al. 2001); Orbiculoidea (Roemerella) cf. collis Clarke, 1913 : Lower Devonian, South Africa ( Reed 1925); Orbiculoidea magnifica Mergl, 2001 : Lochkovian, Prague Basin, Bohemia ( Mergl 2001).

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).