Phragmorthis noda, Hansen & Holmer, 2011

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 107-108

Phragmorthis noda sp. nov.

Pl. 27, Figs. 7–15; Table 36

Derivation of name. Latin ‘ nodus ’, node; refers to the node on the dorsal median ridge.

Holotype. Pl. 27, Fig. 10; TSGF16898 , ventral valve; 67 m above base of Profilbekken Member, Valhallfonna Formation, sample JH-95; Profilbekken meltwater stream, Basissletta, Ny Friesland, Spitsbergen.

Material. Two complete specimens and 31 dorsal and 57 ventral valves from samples F4721, F4923, F4927, F4942, F4950, F5123, F5268, JH-23, JH-25 and JH-95. The paratypes are TSGF16895–16897 , TSGF16899 and TSGF17057 .

Diagnosis. Phragmorthis with mucronate outline; weak ventral sulcus; flattened dorsal valve; nearly catacline and planar ventral interarea; low dorsal median ridge with node at crossing of muscle-bounding ridge.

Description. Shell small, subquadrate, planoconvex to ventribiconvex and mucronate, normally slightly lobate. L/W ratio 0.36–0.75. Cardinal extremities acute (45–80˚). Sulcus gentle or sometimes angular, narrow, developing close to umbo. Anterior commissure moderately unisulcate to rectimarginate. Largest specimen 2.6 mm long and 5.2 mm wide. Larval shell 0.13 mm long. Ornamentation coarsely multicostellate to fascicostellate; 1–2 finer radial ribs between each stronger rib and 5–7 rounded costellae per mm at 1-mm valve length. 14–22 costellae along valve margin. Normally two or four distinct anterolateral costae diverging at about 40-50°. Ornamentation impressed on valve floor.

Dorsal valve planar or weakly convex. Interarea short (13–22% of valve length), plane or slightly concave and anacline. Notothyrium wide and open. Simple, ridge-like cardinal process low and separated from median ridge. Rod-like brachiophores long and diverging at 80–130˚ anterolaterally. Notothyrial platform broad, well defined and raised above valve floor on septalium. Notothyrial platform about 40% as long as wide and 25–31% as long as valve. Fulcral plates absent. Median ridge low along most of its length, dividing valve floor and extending to anterior valve margin. Dorsal muscle field variably impressed. Muscle-bounding ridge forming triangle with longest point along anterior part of median ridge at 75-81% of valve length. Node often strongly developed and raising high above valve floor where muscle-bounding ridge meets median ridge forming a short septum or pillar. Muscle field widest at 37–60% of valve length.

Ventral valve subpyramidal, 47–76% as deep as long. Greatest convexity located at umbo in lateral profile, becoming nearly straight in anterior half. Most specimens with weak sulcus in anterior part of valve. Anterior flank of valve nearly straight. Interarea high, planar or slightly concave (rarely convex), generally catacline tending toward apsacline, rarely steeply procline. Interarea height equalling 44–81% of valve length. Delthyrium narrow to moderately wide (17–25% of valve width), V- or U-shaped and open. Teeth high, simple, thin, supported by recessive dental plates. Small crural fossettes present. Muscle field raised on a broad platform or pseudospondylium reaching 31–65% of valve length. Median ridge short, broad, often supporting pseudospondylium. Diductor scars slightly longer then adductor scars. Pedicle furrow very thin, separating thin, cordate adductor scars slightly elevated above diductor scars. Bases of vascula lateralia weakly impressed, slightly diverging from adductor scars, and bounding thickened median part of valve floor.

Remarks. Phragmorthis noda sp. nov. is one of the oldest known species of that genus. It is morphologically most similar to the Dapingian P. antiqua Ross, 1972 from Nevada and P. mucronata Williams & Curry, 1985 from Ireland. However, it is distinguished from P. antiqua by the nearly catacline and planar ventral interarea, the mucronate outline and the flattened dorsal valve. P. mucronata has a moderately convex dorsal valve and a free spondylium and lacks the distinct, high node on the anterior part of the dorsal median ridge. The Sandbian P. buttsi Cooper, 1956 from the USA differs by its strongly inflated shell, lack of a ventral sulcus, obtuse cardinal extremities and by the median septum which is high along its entire length. The Upper Ordovician P. conciliata Popov, 1986 from Kazakhstan has obtuse cardinal extremities, is longer, has a high dorsal median septum, and has a pseudospondylium that is 21–26% as long as the valve. The Katian P. crassa Cooper, 1956 from Virginia differs by having obtuse cardinal extremities, a subquadrate outline and a ventral crest. P. longisepta Xu & Liu, 1984 from China has a markedly higher L/W ratio, a strong dorsal median septum and nearly rectangular cardinal extremities.

Occurrence. 17, 21, 30, 45–57, 67 and 70 m above base of Profilbekken Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.