Numericoma sp.

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 69

Numericoma sp.

Pl. 18, Figs. 5–12, Table 21

1975 Ephippelasma spinosum Biernat ―Krause & Rowell, p. 61–64, Figs. 52–54; Pl. 8, Figs. 7–22; Pl. 10, Figs. 1–4 , 7, 8; Pl. 11, Fig. 2 .

Material. 22 ventral valves from samples F3797, JH-45 and JH-52. The figured specimens are TSGF 16839, TSGF 16840, TSGF 16850 and TSGF 16852.

Description. Shell transversely oval to subcircular with broad, nearly straight to gently convex posterior margin. L/W ratio 0.63–0.81. Anterior commissure weakly unisulcate. Largest measured specimen 0.7 mm long and 0.9 mm wide. Larval shell moderately convex and transversely oval to subcircular, 0.17–0.22 mm long, with dense micro-pitting. Pits deep, small, subequal in size, up to about 1 µm in diameter. Postlarval ornamentation of rather dense, even, distinct rugae; approximately 8–20 rugae per 0.2 mm.

Ventral valve moderately high conical. Pseudointerarea poorly defined laterally, straight-sloped catacline to steeply procline. Pseudointerarea about half as wide as valve and generally without median division, although both a weak interridge and especially a weak intertrough can occur. Valve 69–86% as high as long. Anterior and lateral slopes gently convex. Foramen small, circular, located just behind apex and completely enclosed within larval shell. External tube normally absent. Cardinal muscle scars distinctly impressed on thickening of valve wall a little over halfway down sides of pseudointerarea in one specimen. Cardinal muscle track distinct in same specimen. Mantle canal system with anterolaterally directed vascula lateralia. Apical process a short, low septum connecting internal pedicle tube along posterior valve margin to anterior valve margin. Internal pedicle tube lacking in smallest specimens.

Remarks. No dorsal valves were identified among the material. However, some fragments may have been misidentified as the co-occurring Numericoma ? proclina sp. nov.

Krause & Rowell (1975) placed their specimens from Nevada within Ephippelasma spinosum Biernat, 1973 (now Numericoma ? spinosa ). In his review of the species E. spinosum , however, Holmer (1989) excluded the American specimens on the basis of their shorter dorsal pseudointerarea and more asymmetric and longer dorsal median septum. The specimens from Spitsbergen and Nevada are further distinguished by the ornamentation of the valves, which is more strongly developed in the Spitsbergen specimens than in N.? spinosa .

Occurrence. 84, 90 and 94 m above base of Olenidsletta Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.

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.

FIGURE 2 Stratigraphical ranges and diversity of brachiopod species at Basissletta, Spitsbergen, and relative sea levels and percentages of linguliform brachiopods in the total brachiopod fauna. White shading in the last graph indicates stratigraphical intervals without brachiopods. Thicknesses of units are according to Fortey & Bruton (1973). Lithological log and sea level curve are from Hansen & Holmer (2010).

FIGURE 3 Rose histograms for the umbonal orientation of the large brachiopod Ectenoglossa? oviforma sp. nov. found in the Olenidsletta Member, Valhallfonna Formation. a) Histogram of 58 specimens from 88 m above the base of the member. b) Histogram of 13 specimens from 94 m above the base of the member.