Semitreta basisslettaensis, 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 : 59-62

Semitreta basisslettaensis sp. nov.

Pl. 15, Figs. 2 –17; Tables 16–17

Derivation of name. Named after the Basissletta plain, where it was collected.

Holotype. Pl. 15, Figs. 13–16; TSGF16815 , ventral valve; basal bed of Olenidsletta Member, Valhallfonna Formation, sample JH-152; N79°51.034’ E17°41.429’, Profilstranda beach, Basissletta, Ny Friesland, Spitsbergen. Plate 15

Hisingerella maniformis sp. nov.

Valhallfonna Formation, Profilbekken Member.

1. TSGF17011 , paratype, ventral valve interior. 26 m above base. Coll. J. Hansen, 04.08.2008, sample JH-184 .

Semitreta basisslettaensis sp. nov.

2–5. TSGF16915 , paratype, dorsal valve exterior, oblique lateral view, detail of larval micro-ornamentation, and larval shell. Kirtonryggen Formation, Nordporten Member, top beds. Coll. J. Hansen, 29.07.2008, sample JH-153 .

6–7. TSGF16814 , paratype, dorsal valve interior, oblique lateral view. Valhallfonna Formation, Olenidsletta Member, basal bed. Coll. J. Hansen, 29.07.2008, sample JH-152 .

8–9. TSGF16912 , paratype, dorsal valve interior, oblique posterolateral view. Kirtonryggen Formation, Nordporten Member, top beds. Coll. J. Hansen, 29.07.2008, sample JH-153 .

10. TSGF16816 , paratype, ventral valve exterior. Valhallfonna Formation, Olenidsletta Member, basal bed. Coll. J. Hansen, 29.07.2008, sample JH-152 .

11–12. TSGF16817 , paratype, ventral valve exterior, oblique lateral view. Valhallfonna Formation, Olenidsletta Member, basal bed. Coll. J. Hansen, 29.07.2008, sample JH-152 .

13–16. TSGF16815 , holotype, ventral valve exterior, oblique lateral view, lateral view of larval shell, and detail of larval micro-ornamentation. Valhallfonna Formation, Olenidsletta Member, basal bed. Coll. J. Hansen, 29.07.2008, sample JH-152 .

17. Ventral valve interior. Specimen lost. Valhallfonna Formation, Olenidsletta Member, basal bed. Coll. J. Hansen, 29.07.2008, sample JH-152.

Semitreta pustulosa sp. nov.

Valhallfonna Formation, Profilbekken Member.

18–20. TSGF16819 , paratype, dorsal valve exterior, larval shell, and detail of postlarval ornamentation. 67 m above base. Coll. J. Hansen, 24.07.2008, sample JH-95 .

Material. 23 dorsal and 17 ventral valves from samples JH-152 and JH-153. The paratypes are TSGF16814 , TSGF16816 , TSGF16817 , TSGF16912 and TSGF16915 .

Diagnosis. Moderately high or high conical Semitreta with weak to moderately developed subtriangular apical process; steeply apsacline to procline ventral pseudointerarea with weak interridge; well-impressed median buttress; subtriangular dorsal median ridge or low septum.

Description. Shell transversely oval to subcircular with narrow, gently convex posterior margin. L/W ratio 0.75–0.93. Largest measured specimen 1.4 mm in length and 1.0 mm in half-width. Larval shell subcircular, with densely pitted micro-ornamentation. Pits circular, rather shallow and highly variable in size, reaching a maximum diameter of 3.5 µm. Postlarval shell with fine or moderately fine growth lines.

Dorsal valve slightly convex. Sulcus present. Pseudointerarea anacline, 9% as long and 35–48% as wide as valve. Median groove 19–32% as wide as valve, with convex or straight anterior margin. Median buttress well developed. Median ridge or septum variably developed, subtriangular, with apex located at 48–67% of valve length; beginning at 0.3 mm of valve length and ending at 67–87% of valve length. Cardinal muscle scars obscure or weakly impressed, small, closely spaced and subtriangular. Anterocentral muscle scars normally not impressed but otherwise small, round and located at 41% of valve length. Mantle canal system obscure.

Ventral valve moderately high to high conical. Pseudointerarea poorly defined, weakly concave to straight, steeply apsacline to procline, bisected by weak interridge. Valve 50–107% as high as long. Anterior and lateral slopes nearly straight to gently convex. Foramen located on poorly developed tube on apex. Mantle canal system baculate, with weak to moderately impressed vascula lateralia. Apical process weakly or moderately developed, subtriangular, bounded by vascula lateralia.

Remarks. Brock & Holmer (2004) redefined Semitreta to include Hansotreta in their synonymy based on the fact that the features of Hansotreta apparently conformed to the current understanding of Semitreta . In their list of included species, they did not include the two Chinese species described by Liu et al. (1983). The Chinese species probably do not belong to the genus, but they can be compared with the species from Spitsbergen. S. bassisslettaensis sp. nov. is distinguished from both S. spitsbergensis sp. nov. from Spitsbergen and S. acrobela ( Krause & Rowell, 1975) from Nevada, USA by lacking an apical process that completely occludes the apex and by having a markedly more low conical ventral valve. S.? jiangxiensis Liu in Liu et al., 1983 from the Middle Ordovician of China differs by having a very short dorsal ridge or septum reaching only slightly anterior to the midvalve and a strongly transverse-oval outline. S. kotujensis Ushatinskaya, 1994 from the Upper Cambrian of Siberia is distinguished by having a narrow, high conical ventral valve, a geniculate dorsal valve, and a poorly developed apical process. S. lauriei Brock & Holmer, 2004 from the Floian of Australia is distinguished by having a gently concave upper half of the anterior slope of the very high ventral valve and by having a pseudointerarea that is always apsacline and provided with an intertrough. S.? magna ( Gorjansky, 1969) from the Lower Ordovician of Estonia differs by having an intertrough and a gently but distinctly concave anterior slope. S. maior Biernat, 1973 from the Tremadocian of Poland differs by having an apsacline ventral pseudointerarea and a very weakly developed dorsal median ridge. S.? shangraoensis (Liu in Liu et al., 1983) from the Ordovician of China is distinguished by having a short dorsal septum with a thickened front and a weak median buttress. Sutton et al. (2000) described a Semitreta ? sp. from the Tremadocian and Floian of Great Britain that differs by having an apsacline ventral pseudointerarea with a weak intertrough.

Occurrence. Uppermost beds of Nordporten Member, Kirtonryggen Formation and basal bed of Olenidsletta Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.

Figures

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).