Semitreta pustulosa, 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 : 62-63

Semitreta pustulosa sp. nov.

Pl. 15, Figs. 18–20; Pl. 16, Figs. 1 –10; Table 18

Derivation of name. Latin ‘ pustula ’, a blister-like prominence; refers to the ornamentation.

Holotype. Pl. 16, Figs. 3 –5; TSGF16822 , ventral valve; 67 m above base of Profilbekken Member, Valhallfonna Formation, sample JH-95; Profilbekken meltwater stream, Basissletta, Ny Friesland, Spitsbergen.

Material. Four dorsal and 11 ventral valves from samples F4997 and JH-95. The paratypes are TSGF16818– 16821 and TSGF16824 .

Diagnosis. Semitreta with dense pustules covering at least part of the exterior; dorsal median ridge extending to two-thirds of valve length; ventral valve deeper than long; gently convex ventral pseudointerarea; apical process absent but lateral walls with apical thickening.

Description. Shell transversely oval to subtriangular with narrow, gently to moderately convex posterior margin. L/W ratio 0.70–0.90. Largest measured specimen with a half width of 0.85 mm. Larval shell subcircular, 0.15– 0.17 mm long, with micro-ornamentation consisting of very fine and dense pits. Pits subcircular, rather deep and somewhat variable in size, up to 2.5 µm in diameter. Postlarval shell with fine growth lines and dense, fine pustules. Pustules most strongly developed distally on dorsal valve.

Dorsal valve very slightly to slightly convex with moderate sulcus. Pseudointerarea concave, steeply anacline, about 38–63% as wide as valve. Median groove deep, small, with slightly concave anterior margin, 15–21% as wide as valve. Median buttress low and wide. Cardinal muscle scars weakly impressed and closely spaced. Median ridge subtriangular, extending from pseudointerarea to about two-thirds of valve length, with apex located at about 55% of valve length. Anterior edge of median ridge much steeper than posterior. Anterocentral muscle scars located at about 35% of valve length. Mantle-canal system indistinct.

Ventral valve high conical with nearly straight anterior and lateral slopes, 129–175% as high as long. Pseudointerarea poorly defined, gently convex, apsacline to slightly procline, with or without weak interridge. Foramen small, enclosed in larval shell at apex of valve. Foramen not extended as tube. Apical process absent, but lateral walls with apical thickening.

Remarks. Within the genus, Semitreta pustulosa sp. nov. is distinguished by having densely pustulose ornamentation, by lacking an apical process and by having a deeper-than-long ventral valve. The other two species from Spitsbergen, S. basisslettaensis sp. nov. and S. spitsbergensis sp. nov., are also distinguished by having a longer dorsal median ridge.

Occurrence. 54 and 67 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.

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.