Schizocrania striata (J. DE C. SOWERBY, 1839)

Mergl, Michal & Nolčová, Lucie, 2016, Schizocrania (Brachiopoda, Discinoidea): Taxonomy, Occurrence, Ecology And History Of The Earliest Epizoan Lingulate Brachiopod, Fossil Imprint 72 (3 - 4), pp. 225-238 : 228-229

Schizocrania striata (J. DE C. SOWERBY, 1839)

Pl. 3, Figs 17–22

1839 Orbicula striata ; J. de C. Sowerby in Murchison, p. 610, pl. 5, fig. 12.

1854 Orbicula striata, SOW. ; Murchison, pl. 20, fig. 3.

1859 D. (Orbicula) striata, SOW. ; Murchison, pl. 20, fig. 3.

1866 Discina striata SOW. (sp.); Davidson, p. 67, pl. 6, figs 1–4.

1867 D. striata, SOW. [ Orbicula, Sil. Syst. ]; Murchison, pl. 20, fig. 3.

1872 D. striata, SOW. [ Orbicula, Sil. Syst. ]; Murchison, pl. 20, fig. 3.

1902 Orbicula striata J. DE C. SOWERBY ; Blake, p. 6.

1963 Schizocrania striata (J. DE C. SOWERBY) ; Holland et al., p. 154.

1978 Schizocrania striata (J. DE C. SOWERBY) ; Cocks, p. 26.

1980 Schizocrania striata (SOWERBY) ; Lockley and Antia, text-figs 2A–C.

2006 Schizocrania striata (J. DE C. SOWERBY) ; Mergl, p. 217, fig. 3A–I.

2008 Schizocrania striata (J. DE C. SOWERBY) ; Cocks, p. 22.

2010 Schizocrania striata (J. DE C. SOWERBY) ; Mergl, p. 368, fig. 3J–L.

M a t e r i a l. One dorsal valve (3RO 85502, original label VH 12788), and one ventral valve (3RO 72443, original label VH 13755).

D e s c r i p t i o n. Shell thin-walled, considering its size, 20 mm wide, circular. Dorsal valve moderate and evenly convex in transverse and axial profiles, without any sign of fold or sulcus, and without flattening in posterolateral part. Lateral and anterior margins evenly curved. Dorsal apex low, placed near posterior margin, but poorly preserved due to compression and moderate breakage of valve.

The ventral valve is flat, circular, with dorsally inflexed surface adjacent to the pedicle notch. Posterior margin extends backwards slightly, breaking the otherwise regularly rounded outlines of the lateral and anterior margins. The apex is flat, situated at one-fourth of VvL. The posterior margin has developed deep V-shaped emargination of the pedicle notch, occupying 20 % of VvL and 12 % of VvW. Details of the pedicle notch are unknown. Interior of the ventral valve bears fine and short (10% of VvL), median ridge extending below the apex.

The ornamentation of the dorsal valve consists of fine elevated radial costellae regularly spaced over the entire surface. Density of costellae depends on their distance from the apex. There are 7 costellae per 1 mm at 5 mm distance from the apex. New costellae are intercalated on the flat floor of the interspaces, where the interspaces are approximately 150 µm wide. Intercalated new costellae appear anteriorly to prominent concentric growth bands, and rapidly attain the same size as the primary costellae. Concentric ornamentation consists of low ridges distinct on the floors of the interspaces. The intersections of radial costellae with low concentric fila form fine but distinct reticulate ornamentation.

The ventral valve exterior bears prominent, thin, high, radially arranged costellae of uniform shape and size. The first costellae emerges from the apex, and with the newly formed costellae, evenly cover the entire surface of the valve. Costellae are also evenly spaced in the posterolateral sectors of the valve. They are absent only on inflexed shell surface close to the pedicle notch. New costellae originate by implantation at flat interspaces when adjacent bordering costellae are 0.8 mm apart. There are 9 costellae per 5 mm near the anterior margin of the 20 mm long valve. Costellae gently project outside the shell margin. It is likely that originally there was a non-mineralized organic sheet between their tips. Interspaces have a flat floor. The concentric ornamentation is distinct on the floor of interspaces, consisting of low broad bands separated by weak furrows. There are some 13–15 growth lines at 10 mm distance from the apex. Concentric bands become narrower with shell growth. Drape-like structures extending and vanishing backwards are distinct along sides of the interspaces. The structure indicates a tension and folding of the first-formed organic sheet of the shell near tough and already mineralized costellae.

R e m a r k s. The two available shells are referred to Schizocrania striata (J. DE C. SOWERBY, 1839) , because there is no distinct difference between them and specimens from Shropshire, England. Schizocrania verneuilii ( DAVIDSON, 1848) has the radial costellation absent in the posterolateral sectors of the valve ( Mergl 2006: fig. 34O, P).

O c c u r r e n c e. In the Barrandian area, the species is very rare in the top of the Požáry Formation (Pridoli) (locality: Klonk stratotype (bed No. 7)) and the Kotýs Limestone, Lochkov Formation (Lochkovian) (locality: Svatý Jan pod Skalou (Solway’s quarries)). The species is present in the Gorstian and Ludfordian (Upper Ludlow Beds) of Shropshire, England ( Lockley and Antia 1980, Mergl 2006), and in the Homerian to Ludfordian of Gotland ( Mergl 2010).