Schizotreta marginalis, 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 : 39-44

Schizotreta marginalis sp. nov.

Pl. 7, Figs. 14–17; Pl. 8, Figs. 1 –17; Pl. 9, Figs. 1 –8; Table 7

?1980 Schizotreta sp. 1 ―Popov in Nazarov & Popov, pp. 115–116, Pl. 31, Figs. 3 –6. Plate 8

Schizotreta marginalis sp. nov.

Valhallfonna Formation.

1–2. TSGF16954 , paratype, dorsal valve exterior and detail of ornamentation. Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

3–5. TSGF17029 , paratype, dorsal valve exterior, oblique lateral view, and detail of micro-ornamentation thereof. Profilbekken Member, 26 m above base. Coll. J. Hansen, 04.08.2008, sample JH-184 .

6–7. TSGF17028 , paratype, dorsal valve exterior and larval shell. Profilbekken Member, 26 m above base. Coll. J. Hansen, 04.08.2008, sample JH-184 .

8–10. TSGF16947 , paratype, ventral valve exterior, close view of larval shell, and detail of postlarval micro-ornamentation. Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

11–13. TSGF16948 , paratype, ventral valve exterior, oblique anterolateral view, and detail of postlarval micro-ornamentation. Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

14. TSGF16949 , paratype, ventral valve exterior. Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

15–16. TSGF16951 , paratype, dorsal valve interior and oblique anterolateral view thereof. Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

17. TSGF17024 , paratype, dorsal valve interior. Profilbekken Member, 26 m above base. Coll. J. Hansen, 04.08.2008, sample JH-184 .

Derivation of name. Latin ‘ marginis ’, marginal; refers to the location of the dorsal umbo at the posterior valve margin.

Holotype. Pl. 7, Figs. 16–17; TSGF16953 , dorsal valve; 97 m above base of Olenidsletta Member, Valhallfonna Formation, sample JH-136; Profilstranda at Basissletta, Ny Friesland, Spitsbergen.

Material. One conjoined specimen, 805 dorsal and 841 ventral valves and 1710 undifferentiated fragments from samples containing the trilobites A84134 , A84137 , A84271 and A84365 and from samples F*-scratch, F3478, F3518, F3797, F5114, F5690, TS92.3, JH-12, JH-14, JH-15, JH-23–25, JH-35, JH-45, JH-52, JH-95, JH- 131, JH-136, JH-140, JH-141, JH-143, JH-155, JH-157, JH-175 and JH-184. The paratypes are TSGF16947– 16949 , TSGF16951 , TSGF16952 , TSGF16954 , TSGF17024 , TSGF17028 and TSGF17029 .

Diagnosis. Small Schizotreta with marginal dorsal umbo; gently to moderately convex dorsal valve; generally elongate-oval outline; and micro-ornamentation of subequal pits.

Description. Shell slightly to moderately ventribiconvex, convexity decreasing with increasing size. Outline elongate-oval to subcircular or subangular with greatest width at 38–69% of valve length. Valves thin-walled. L/W ratio 0.95–1.79, increasing with valve size. Largest valve 3.7 mm long and 3.2 mm wide. Larval shell smooth, subcircular, 0.2–0.5 mm long. Postlarval shell with subequal micro-pitting generally in radial rows, about 7–11 rows per 0.1 mm, and macro-ornamentation of growth lines and lamellae. Lamellae usually growing higher distally; generally 5–6 lamellae per 0.2 mm proximal to larval shell or 7–27 within 1 mm of apex and 11–16 per mm within 1-2 mm anterior of apex. Edge of higher lamellae curved posteriorly.

Dorsal valve slightly to strongly convex with maximum depth at 18–53% of valve length. Umbo marginal. Pseudointerarea absent. Median groove wide (17% of valve width), shallow to deep and nearly vertical in orientation. Median ridge weak, absent in small specimens, extending close to anterior valve margin. Muscle field generally obscure. Anterior adductor muscle scars rather large, elongate-oval or diamond-shaped and located at about mid-length of valve, bounding vascula media. Oblique internal muscle scars small and located posterolaterally to anterior adductor scars. Posterior adductor scars of similar size and located posterior and slightly more lateral with respect to oblique internal scars. Vascula media subparallel at least to mid-length of valve. Vascula lateralia impressed along valve edge just anterior of pseudointerarea.

Ventral umbo submarginal to subcentral, located at 5–38% of valve length. Anterior valve slope slightly concave to gently convex. Foramen U-shaped in subadult specimens but becoming submarginal and tear-shaped with listrium closing most of pedicle track. Anterior end of foramen or slit notching larval shell. Pedicle opening about 15% as wide as valve. Pedicle tube short. Anterior adductor scars broad and located at about 65% of valve length. Vascular system apparently baculate. Vascula lateralia diverging rather strongly from anterior adductor scars, trunks narrowing significantly.

Remarks. The large number of specimens from the Valhallfonna Formation reveals a surprising degree of variability, suggesting the co-occurrence of several species. However, this hypothesis is not supported by our morphological study. The plasticity of the species makes comparison with other species difficult, although the marginal dorsal umbo is a consistent feature distinguishing it from most other species. The variability of many features, such as the location and size of the pedicle opening, is correlated with ontogeny and thus should probably be avoided as a diagnostic feature for distinguishing species. Schizotreta sp. 1 ( Nazarov & Popov 1980) from the lower Darriwilian of Kazakhstan has at least a nearly marginal dorsal umbo like that of the Spitsbergen specimens and otherwise appears conspecific. Of the other species having a marginal dorsal umbo or for which the location of the dorsal umbo is unknown, S. posteroconvexa Cooper, 1956 from the Sandbian of Tennessee has no preserved dorsal valve but is subcircular and has a low conical ventral valve with a subcentral apex. S. prilepensis Mergl, 2002 from the upper Darriwilian or lower Sandbian of Bohemia is based on one ventral valve with too few distinct features for a satisfactory comparison. It appears to have more weakly developed filae, but more and better material is needed for

Plate 9

Schizotreta marginalis sp. nov.

Valhallfonna Formation, Olenidsletta Member.

1–2. TSGF16876 , paratype, dorsal valve exterior and detail of micro-ornamentation. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

3. TSGF16875 , paratype, ventral valve exterior. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

4. TSGF16873 , paratype, interior of ventral valve fragment. 97 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

5–6. TSGF16872 , paratype, dorsal valve interior with visible muscle scars and detail of valve floor. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

7–8. TSGF16950 , paratype, ventral valve interior and oblique anterolateral view thereof. Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

Conotreta convexa sp. nov.

Valhallfonna Formation, Olenidsletta Member.

9–11. TSGF16845 , paratype, dorsal valve exterior, oblique lateral view, and detail of larval shell. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

12–13. TSGF16846 , paratype, dorsal valve interior, oblique lateral view. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

14–15. Dorsal valve interior, oblique lateral view (specimen lost in SEM). 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52.

16–17. TSGF16848 , paratype, dorsal valve interior, oblique lateral view. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

18–19. TSGF17043 , holotype, ventral valve exterior, oblique posterolateral view. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

a detailed comparison. S. vaneki Mergl & Ferrova, 2009 from the Devonian of the Czech Republic has a marginal dorsal umbo like that of S. marginalis sp. nov. but has much fewer lamellae, is about twice as large, and has a superficial listrium. The Dapingian Schizotreta sp. from England described by Williams (1974) has the ventral umbo located at about 35% of valve length, and the rugae are stronger and less densely distributed than in S. marginalis . Specimens of S. marginalis with the ventral umbo located at 35% of valve length do occur but are not very common. The Darriwilian species Schizotreta sp. in Mergl (2002) from Bohemia has a marginal dorsal umbo but differs in having fine filae with broad interspaces and a minute pedicle opening. Schizotreta sp. ( Percival et al. 2009) from the Darriwilian of New Zealand has a very small and short pedicle track and more even, distinct concentric ridges. Schizotreta sp. ( Popov et al. 2002a) from the Upper Ordovician of Kazakhstan is planoconvex and subcircular, and the ventral apex is located at 10% of valve length. Schizotreta ? sp. ( Popov et al. 2008) from the Dapingian of Iran is distinguished by having dense, small pits between well-separated radial rows of larger micropitting, but no other differences can be discerned from the provided illustration and description.

Occurrence. 64, 73–75, 84–100, 113 and 140 m above base of Olenidsletta Member and 17–21, 26, 35 and 67 m above base of Profilbekken Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen. Possibly also from lower Darriwilian of Kazakhstan (see Nazarov & Popov 1980).

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.