Grebenella laciniatus, Modzalevskaya, 2003

Modzalevskaya, Tatiana L., 2003, Silurian and Devonian brachiopods from Severnaya Zemlya (Russian Arctic), Geodiversitas 25 (1), pp. 73-107 : 98-99

Grebenella laciniatus n. sp. ( Figs 28 ; 29 )

HOLOTYPE. — N 78/12991, CNIGR Museum , St. Petersburg ( Fig. 28 E-H), conjoined valves; L = 6.0 mm, W = 8.0 mm, T = 4.1 mm.

ETYMOLOGY. — From laciniosus (Latin): shredded.

TYPE LOCALITY. — Loc. 20399, source of the left tributary of the Snegovaya River, Komsomolets Island.

FORMATION. — Rusanov or Al’banov formations.

AGE. — Lower Devonian.

MATERIAL EXAMINED. — Shelly limestone, two conjoined and 10 separated valves from loc. 20399 (source of the left tributary of the Snegovaya River) and loc. 21581 (southern bank of Krasnaya Bay), Komsomolets Island, Severnaya Zemlya ( Männik et al. 2002: fig. 1); Rusanov or Al’banov formations.

MEASUREMENTS (IN MM). — L = 4.8-8.5, W = 6.6-8.0, T = 3.6-4.3.

DIAGNOSIS. — Small shells, transversally oval in outline and ventribiconvex in lateral profile; dorsal fold with deep narrow furrows on the flanks; plicae weakly developed.

DESCRIPTION

Shells small, transversally oval, ventribiconvex with developed sinus and fold, and with smooth flanks. Ventral valve is the most convex in its umbonal part. Apex acute, hanging over the dorsal valve, delthyrium partly closed by deltidial plates, interarea triangular, low. Cardinal extremities rounded, sinus is well limited, arises from the beak, widening anteriorly, anterior tongue pronounced. Dorsal valve with sharply outlined fold bordered by deep narrow furrows. Fold originates at the beak. Broad plicae on flanks arise at about one third of valve length. Concentric growth lamellae are rhythmical, and bear the impressions of spines on their anterior edges in the mould. Ventral umbonal chambers filled by callus. Dental plates high, long, strongly thickened, widely situated and divergent; teeth massive. Ventral muscle field triangular with a triangular furrow near its anterior margin. Callus also well developed in dorsal valve, where thickened crural plates are widely divergent posteriorly and rest on short septum at the valve floor. Crura extend to near the valve floor. Dental sockets well limited medially by inner socket ridges ( Fig. 29 ).

COMPARISON

The flattened dorsal valve, transverse outline, and weakly developed plications suggest the comparison with Grebenella impalpabilis T. Modzalevskaya, 1994 from the Tolbat Beds, Tajmyr ( Modzalevskaya & Cherkesova 1994: 87, pl. X, figs 11-14). However, the fewer plicae, comparatively weak dorsal fold with deep narrow furrows on the flanks support assignment of Severnaya Zemlya specimens to a new species.

Figures

FIG. 28. — Grebenella laciniatus n. sp.; A, ventral view; B, dorsal view; C, lateral view; D, posterior view of a complete shell (N 79/12991), loc. 21581, Severnaya Zemlya or Pod’’mnaya formations; E, ventral view; F, dorsal view; G, lateral view; H, anterior view of a complete shell (holotype N 78/12991); I, ventral interior with dental lamellae and muscle field (N 80/12991), Severnaya Zemlya or Pod’’emnaya formations; J, enlarged external surface of a ventral valve, showing concentric growth lines with the impressions of the spines (N 81/12991), loc. 20399, Rusanov or Al’banov formations. Scale bars: A-D, 5 mm; E-H, 3 mm; I, J, 2 mm.

FIG. 29. — Selected transverse serial sections, illustrating in the development of internal structures in Grebenella laciniatus n. sp. (N 82/12991). Measurements: length 6.8 mm, width?, thickness?; loc. 20399, Rusanov or Al’banov formations. Numerical designations are the same as on Fig. 11. Scale bar: 1.6 mm.