Idiostrophia valdari Ross, 1972

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 112

Idiostrophia valdari Ross, 1972

Pl. 28, Fig. 12; Pl. 29, Figs. 1 –6; Table 37

1972 Idiostrophia valdari Ross ―p. 25, Pls. 6–8; Pl. 18, Figs. 16–20.

Holotype. USNM 167191 , complete shell; Orthidiella zone , Antelope Valley Limestone; Meiklejohn Peak , Nevada ( Ross 1972).

Material. 96 generally poorly preserved specimens from the samples containing trilobites A84136 , A84137 , A84165 , A84292 and A84360 a and from samples F2875, F3478, F3797, JH-15, JH-35, JH-36, JH-49, JH-52, JH- 57, JH-89, JH-105, JH-131, JH-139 and JH-188. The figured specimens are TSGF16773 , TSGF16901–16905 and TSGF17079 .

Description of Spitsbergen material. Shells small, slightly biconvex and thin-walled. Posterolateral sides sharply deflected to form flattened areas similar to lunules. Umbo slightly or moderately obtuse. Shell thickest just anterior of 35% of valve length. L/W ratio 0.86–1.15. Largest measured specimen (a ventral valve) 5.1 mm long and 4.9 mm wide. Proximal part of shell without ornamentation, but 7–10 strong angular costae abruptly developing at 1.8–3.0 mm of valve length. Weak median furrow apparently present on anterior part of two specimens.

Dorsal valve subcircular to subtriangular and slightly convex. Some specimens with steep posterolateral sides. Socked plates converge on valve floor at about 34% of valve length. Median septum extending to about 54% of valve length. Adductor scars elongate-oval.

Ventral valve subtriangular to subpentagonal and slightly to moderately convex. Muscle field sometimes directly on valve floor (generally in smaller specimens), sometimes raised on a low spondylium simplex (generally in larger specimens). Spondylium narrow and slightly V-shaped with short anterior median septum. Pedicle callist often developed. Valve floor often without steep posterolateral sides.

Remarks. The present specimens may differ from the American specimens of Idiostrophia valdari Ross, 1972 in their general lack of a dorsal median groove, but this difference may be explained by the poor preservation and small size of most Svalbard specimens. Additionally, the costae develop 1–2 mm more proximally than in the type material.

Occurrence. 75–90 and 95 m above base of Olenidsletta Member and middle part of Profilbekken Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen. Orthidiella zone (Dapingian) in Antelope Valley Limestone, Nevada ( Ross 1972).

Figures

FIGURE 1 Locality maps of the Svalbard Archipelago. a) Locations of sites yielding Lower Palaeozoic fossils mentioned in this paper. b) Geological map of the Basissletta plain, northeastern Ny Friesland; redrawn from Fortey & Bruton (1973). The base map is based on aerial photos made available by the Norwegian Polar Institute and on field observations made by the authors in 2008.