Greenfieldia orbiculata, Modzalevskaya, 2003

Modzalevskaya, Tatiana L., 2003, Silurian and Devonian brachiopods from Severnaya Zemlya (Russian Arctic), Geodiversitas 25 (1), pp. 73-107 : 101

Greenfieldia orbiculata n. sp. ( Figs 22 E-H; 31)

HOLOTYPE. — N 62/12991, CNIGR Museum , St. Petersburg ( Fig. 22 E-H), conjoined valves; L = 11.0 mm, W = 11.0 mm, T = 9.0 mm.

ETYMOLOGY. — From orbiculatus (Latin): rounded.

TYPE LOCALITY. — Loc. 2, Matusevich River, October Revolution Island.

FORMATION. — Samojlovich Formation.

AGE. — Wenlock, Homerian-Sheinwoodian.

MATERIAL EXAMINED. — 44 well preserved conjoined valves from loc. 2, beds 61 and 66, Matusevich River, October Revolution Island ( Männik et al. 2002: figs 2, 5); Samojlovich Formation.

MEASUREMENTS (IN MM). — L = 6.8-11.8, W = 6.8- 11.0, T = 4.6-9.0.

DIAGNOSIS. — Shells small with rounded outline and strongly biconvex in lateral profile; lacking dorsal fold, anterior margin almost rectimarginate.

DESCRIPTION

Shells strongly biconvex with rounded outline. Ventral beak high and incurved, sinus weakly developed. Dorsal fold absent or restricted to narrow furrow; anterior margin almost rectimarginate or very slightly uniplicate. Interior with high, long dental plates, which are parallel and straight. Teeth deeply inserted into dental sockets ( Fig. 31 ). Dorsal interior with perforated cardinal plate. High, vertical crura arise from outer plates and extend to the middle of the shell, becoming convex medially and merging into primary lamellae. The coils are directed laterally and comprise nine whorls. Jugal saddle directed anteriorly to the middle of the shell.

VARIATION

Young specimens have a flattened anterior margin. Outline varies from rounded to oval, ventral sinus and dorsal furrow are absent or weakly developed.

COMPARISON

In development of a ventral sinus and lack of a dorsal fold the specimens from Severnaya Zemlya are most similar to Greenfieldia uberis tanta Modzalevskaya, 1980 described from the Dolgij Formation on Dolgij Island ( Modzalevskaya 1980: 92, pl. II, figs 4, 5). The main differences are the smaller size and more rounded outline of the Severnaya Zemlya specimens.

Figures

FIG. 22. — A-D, Hyattidina remota n. sp.; A, ventral view; B, dorsal view; C, anterior view of a complete shell (holotype N 58/12991); D, lateral view of a complete shell (N 59/12991), loc. 13, bed 48, Golomyannyj Formation; E-H, Greenfieldia orbiculata n. sp.; E, ventral view; F, dorsal view; G, lateral view; H, anterior view of a complete shell (holotype N 62/12991), loc. 2, bed 66, Samojlovich Formation; I, Meristina? norilica (Nikiforova in Nikiforova & Andreeva, 1961), ventral view of a complete shell (N 61/12991), loc. 13, bed 27, Srednij Formation; J-M, Dubaria tenera Nikiforova & T. Modzalevskaya, 1968; J, ventral view; K, dorsal view; L, lateral view; M, anterior view of a complete shell (N 57/12991), loc. 13, bed 89, Vodopad Formation. Scale bars: A-C, E-H, 5 mm; D, J-M, 3 mm; I, 6 mm.

FIG. 31. — Selected transverse serial sections, illustrating the development of internal structures in Greenfieldia orbiculata n. sp. (N 63/12991). Measurements: length 10.9 mm, width 9.1 mm, thickness 9.2 mm; loc. 2, bed 66, Samojlovich Formation. Numerical designations are the same as on Fig. 11. Scale bar: 2.7 mm.