Semitreta spitsbergensis, 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 : 63-65

Semitreta spitsbergensis sp. nov.

Pl. 16, Figs. 11–18; Pl. 17, Figs. 1 –7; Table 19

Derivation of name. Named after Spitsbergen.

Holotype. Pl. 17, Figs. 5–6; TSGF16930 , ventral valve; 97 m above base of Olenidsletta Member, Valhallfonna Formation, sample JH-136; Profilstranda, Basissletta, Ny Friesland, Spitsbergen.

Material. 85 dorsal and 125 ventral valves from samples F2-scratch, F2634, F3475, F3477, F3478, F3518, F3524, F3527, F3577, F3596, F3598, F3664, F3673, F3675.5, JH-12–15, JH-17, JH-35, JH-38, JH-49, JH-54, JH- 55, JH-131, JH-136, JH-155, JH-184, JH-188 and TS92.3. The paratypes are TSGF16925–16929 and TSGF17042 .

Plate 16

Semitreta pustulosa sp. nov.

Valhallfonna Formation, Profilbekken Member, 67 m above base. Coll. J. Hansen, 24.07.2008, sample JH-95.

1–2. TSGF16820 , paratype, dorsal valve interior, oblique lateral view .

3–5. TSGF16822 , holotype, ventral valve exterior, detail of postlarval ornament, and larval shell .

6–8. TSGF16821 , paratype, anterior view of ventral valve, detail of larval micro-ornamentation, and detail of postlarval ornamentation .

9. TSGF16824 , paratype, oblique posteroventral view of ventral valve .

10. TSGF16818 , paratype, ventral valve interior .

Semitreta spitsbergensis sp. nov.

Valhallfonna Formation, Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136.

11–14. TSGF16926 , paratype, dorsal valve exterior, oblique lateral view, larval shell, and detail of larval micro-ornamentation. 15 . TSGF16925 , paratype, dorsal valve interior .

16–17. TSGF16927 , paratype, dorsal valve interior, oblique anterolateral view .

18. TSGF17042 , paratype, ventral valve interior .

Diagnosis. High conical Semitreta with apical process completely occluding apex; catacline or procline ventral pseudointerarea; weakly convex anterior ventral slope; low, triangular median septum.

Description. Shell transversely oval to subcircular with gently convex posterior margin. Pseudointerarea 40– 53% as wide as maximum valve width. L/W ratio 0.78–0.99. Largest specimen 3.1 mm long and 3.6 mm wide. Larval shell subcircular, 0.15–0.18 mm long, with micro-ornamentation consisting of very fine and dense pits. Pits deep, about 1 µm in diameter. Postlarval shell often with coarse growth lines.

Dorsal valve nearly planar to moderately convex, with broad, slightly angular sulcus. Sulcus generally moderately developed. Valve highest at 33–34% of valve length. Pseudointerarea concave, strongly anacline, with wide but short median groove. Median groove 3–10% as long and 14–20% as wide as valve. Cardinal muscle scars strongly developed, anterolaterally directed, subangular to elongate-oval, relatively closely spaced. Median buttress high and well defined. Median septum low, simple, triangular. Septum beginning at 0.2–0.4 mm and ending at 55–92% of valve length, with apex located at 47–65% of valve length. Anterocentral muscle scars raised, elongateoval, located at 42–50% of valve length. Mantle canal system occasionally well impressed, pinnate.

Ventral valve as high conical as long, with weakly convex anterior slope. Pseudointerarea poorly defined and straight, catacline or procline, with weak interridge. Foramen small, enclosed in larval shell at apex of valve, commonly located on short tube. Apical process forming platform, completely occluding apex and surrounding internal pedicle tube laterally and anteriorly. Vascula lateralia deeply impressed, running anterolaterally from posterior part of tube.

Remarks. Semitreta acrobela ( Krause & Rowell, 1975) from the Dapingian of Nevada, USA differs from the new species by having a more pointed dorsal umbo and a shorter median ridge, reaching two-thirds of the valve length, as well as a straight anterior slope of the ventral valve. S. basisslettaensis sp. nov. has a more low conical ventral valve and weak or moderately developed apical process. S. jiangxiensis Liu in Liu et al., 1983 from the Middle Ordovician of China differs by having a poorly developed apical process and a strongly transverse-oval shell outline. S. kotujensis Ushatinskaya, 1994 from the Upper Cambrian of Siberia differs by having a narrow, high conical, geniculate dorsal valve and a poorly developed apical process. S. lauriei Brock & Holmer, 2004 from the Floian of Australia is distinguished by having a ventral valve that is gently concave in the upper half of the anterior valve slope and an apsacline pseudointerarea with an intertrough. S.? magna ( Gorjansky, 1969) from the Lower Ordovician of Estonia is distinguished by having an intertrough and a gently but distinctly concave anterior slope. S. maior Biernat, 1973 from the Tremadocian of Poland has a poorly developed apical process and an apsacline ventral pseudointerarea. S. shangraoensis (Liu in Liu et al., 1983) from the Ordovician of China has a dorsal septum with a broad front and a weak median buttress. Semitreta ? sp. described by Sutton et al. (2000) from the Tremadocian and Floian of Great Britain has an apsacline ventral pseudointerarea with a weak intertrough.

Occurrence. 80–101 m above base of Olenidsletta Member and 26 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.