Numericoma, Popov, 1980

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 72-73

Numericoma ? proclina var. 1

Pl. 19, Fig. 15; Pl. 20, Figs. 1 –10; Tables 24–25

Material. 40 dorsal and 4 ventral valves from samples F3366, F4997, F5032, JH-23, JH-95 and JH-184. The figured specimens are TSGF 16939–16941 and TSGF 17040.

Description. Shell transversely oval with nearly straight or gently convex posterior margin. Shell widest at about mid-valve length. L/W ratio 0.69–0.95. Largest measured specimen 1.3 mm long and 1.3 mm wide. Larval shell subcircular, 0.15–0.20 mm long, with dense micro-pitting. Pits deep and circular, variable in size, mostly about 2 µm in diameter. Postlarval ornamentation consisting of coarse growth lines developing into thin concentric ridges distally.

Dorsal valve slightly concave to slightly convex, with weak or absent sulcus. Pseudointerarea strongly anacline, concave or planar, 33–60% as wide as valve. Median groove rather shallow to moderately deep, 17–40% as wide and 5–12% as long as valve, with weakly concave or planar anterior margin. Median buttress weak. Median septum moderately high and thin, triangular, beginning at 0.1–0.2 mm of valve length, with apex located at 53– 83% and front located at 72–86% of valve length. Median septum with septal rod and asymmetrically folded upper part. Two to four spines along anterior edge of septum. Cardinal muscle scars moderately large, subtriangular, poorly impressed. Anterocentral muscle scars obscure. Mantle canal system not impressed.

Ventral valve low to moderately conical with gently convex lateral and anterior slopes and rounded apex. One specimen about 40% as high as long. Pseudointerarea procline or catacline with weak intertrough or interridge. Foramen moderately large, circular, located within posterior part of larval shell. Foramen generally not extended as a short tube. Apical process forming a low ridge.

Remarks. This variety differs from the typical form in its higher mean L/W ratio, relatively lower ventral valves, absent or weak sulcus, very short median groove, and generally shorter median septum with fewer spines. Because very few specimens (all of which are damaged) are available from the lower part of its distribution, it is not possible at present to test whether there is a continuous evolutionary development from the typical form to this form. Therefore, it is possible that the specimens assigned to this variety constitute a new species. However, they probably represent an evolutionary/ecological change within N.? proclina sp. nov.

Occurrence. 110, 125 and 140 m above base of Olenidsletta Member and 21, 26, 52 and 54 m above base of Profilbekken 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.