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 : 69-72

Numericoma ? proclina sp. nov.

Pl. 18, Figs. 13–15; Pl. 19, Figs. 1 –14; Tables 22–23

Derivation of name. Latin ‘ proclino’, sloping in anterior direction; refers to the procline ventral pseudointerarea.

Holotype. Pl. 19, Figs. 9–11; TSGF16830 , ventral valve; 94 m above base of Olenidsletta Member, Valhallfonna Formation, sample JH-52; Profilstranda, Basissletta, Ny Friesland, Spitsbergen.

Material. 56 dorsal and 99 ventral valves from samples F3043, F3475, F3478, F3518, F3613, F3664, F3671, F3797, F5690, JH-15, JH-27, JH-35, JH-45, JH-52, JH-136, JH-140 and JH-141. The paratypes are TSGF16827– 16829 , TSGF16831–16833 and TSGF16835–16837 .

Plate 18

Scaphelasma lamellosum Krause & Rowell, 1975

1–4. TSGF16973 , ventral valve exterior, oblique lateral view, larval shell, and detail of larval micro-ornamentation. Valhallfonna Formation, Profilbekken Member, 26 m above base. Coll. J. Hansen, 04.08.2008, sample JH-184 .

Numericoma sp.

Valhallfonna Formation, Olenidsletta Member.

5–7. TSGF16840 , ventral valve exterior, oblique posterolateral view, and detail of larval micro-ornamentation. 90 m above base. Coll. J. Hansen, 20.07.2008, sample JH-45 .

8–9. TSGF16839 , ventral valve exterior and larval shell. 90 m above base. Coll. J. Hansen, 20.07.2008, sample JH-45 .

10–11. TSGF16852 , oblique posterolateral view of ventral valve and interior with apical process. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

12. TSGF16850 , oblique anterior view of ventral valve exterior. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52. Numericoma ? proclina sp. nov .

Valhallfonna Formation, Olenidsletta Member.

13–14. TSGF16829 , paratype, dorsal valve exterior, oblique anterolateral view. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

15. TSGF16836 , paratype, dorsal valve exterior. 90 m above base. Coll. J. Hansen, 20.07.2008, sample JH-45 .

Diagnosis. Broadly oval or subtriangular Numericoma with procline ventral pseudointerarea undivided or divided by a weak interridge; folded, rather delicate dorsal median septum with upper septal rod and rather few, short spines.

Description. Shell transversely oval or subtriangular with straight to weakly convex posterior margin. Shell widest at 31–50% of valve length. L/W ratio 0.65–0.85. Largest measured specimen 2.8 mm long and 3.4 mm wide. Anterior commissure unisulcate. Larval shell subcircular to transversely oval, 0.13–0.18 mm long, with dense micro-pitting. Larval micro-ornamentation consisting of deep circular pits up to 2 µm in diameter. Ornamentation consisting of fine growth lines and thin lamellate ridges, 9-17 ridges per 0.2 mm.

Dorsal valve planar to gently convex, with shallow to moderate sulcus developing slightly anterior to umbo. Valve deepest at 43% of valve length. Pseudointerarea short, concave to planar, moderately steep or steep anacline, 47–76% as wide as valve. Median groove moderately deep and well-defined, 15–29% as wide and 7–14% as long as valve, with concave to convex anterior margin. Cardinal muscle scars poorly impressed, diamond-shaped. Each muscle scar 15–18% as wide as valve, together occupying central 57% of valve width. Median buttress weak and short. Median septum moderately high, triangular, with apex located at 42–78% of valve length and front located at 76–92% of valve length. Median septum with short upper septal rod; asymmetrically folded anterior part with up to nine short spines along anterior edge. Anterocentral muscle scars generally obscure, small, subcircular, located at 46% of valve length. Mantle canal system not impressed.

Ventral valve low to moderately conical, 50–66% as high as long. Anterior and lateral slopes nearly straight or gently convex. Posterior slope straight or weakly concave. Larval shell moderately convex, possibly slightly bilobed in transverse profile. Foramen small, located just behind apex within posterior part of larval shell, generally not forming a tube. Pseudointerarea steeply procline or more rarely catacline, generally divided by a weak interridge, 39–62% as wide as valve. Pseudointerarea occasionally with a weak intertrough. Apical process a broad, low ridge or thickening of the valve floor anterior to the foramen. Vascula lateralia straight, anterolaterally directed, generally indistinct. Cardinal muscle track strongly developed along margins of pseudointerarea.

Remarks. This species is distinguished from most species included in the genus by having a broadly oval outline, a steeply procline ventral interarea and a weak interridge. N. campanula Mergl, 2002 from the Darriwilian of Bohemia is further distinguished by its simple median septum with upper septal rod. N. electa Popov in Nazarov & Popov, 1980 from the Dapingian of Kazakhstan has a densely spiny median septum. N. latior ( Biernat, 1973) from the Middle Ordovician of Poland is distinguished by having a much shorter dorsal median septum with a broad, strongly developed surmounting plate. N. ornata Popov in Nazarov & Popov, 1980 from the Dapingian of Kazakhstan differs by having many more spines on the dorsal median septum. N. perplexa Holmer, 1989 from the Darriwilian of Sweden and South China is distinguished by having a very short and strongly folded median septum and a short pedicle tube. N. simplex Holmer, 1989 from the Darriwilian of Sweden and Kazakhstan differs by having a shorter, more massive and spiny median septum. N.? spinosa ( Biernat, 1973) from the Dapingian and Darriwilian of Poland, South China and Sweden has a transversely oval outline like the Spitsbergen species but differs in most other general characters and in having a shorter and higher median septum. N. vulcanogena Mergl, 1996 from the Dapingian of Bohemia has a transverse outline and a weak interridge like the Spitsbergen species but is distinguished by being apsacline and by having a short, massive, folded median septum.

Occurrence. 73, 75, 85–94, 97 and 99 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.