Conotreta, Walcott, 1889

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 46-49

Conotreta ? devota Krause & Rowell, 1975

Pl. 10, Figs. 8–16; Pl. 11, Figs. 1 –13; Tables 10–11

1975 Conotreta ? devota Krause & Rowell, new species ―Krause & Rowell, pp. 35–38, Pl. 4, Figs. 1 –16; Pl. 11, Figs. 4 –6; Pl. 12, Fig. 5.

Material. 115 dorsal and 114 ventral valves from samples F3675.5, F5765, JH-14, JH-45, JH-52, JH-136 and JH- 184. The figured specimens are TSGF 16849, TSGF 16851, TSGF 16917, TSGF 16918 and TSGF 16920–16924.

Description of Spitsbergen material. Shell transversely oval to subangular with nearly straight to gently convex posterior margin. L/W ratio 0.69–0.88. Greatest width at 44–63% of valve length. Largest measured specimen 1.2 mm long and about 1.6 mm wide. Larval shell subcircular to transversely oval, 0.13–0.20 mm long, with micropitting. Pits circular, dense, superficial, up to 2 µm in diameter. Dorsal postlarval ornamentation consisting of lamellae separated by two to five growth lines. Lamellae declining anteriorly; 2–5 lamellae and 5–8 growth lines in proximal 0.2 mm of valve length anterior to larval shell. Ventral postlarval ornamentation of dense, even rugae and occasionally low lamellae; 6–7 rugae in proximal 0.2 mm of valve length anterior to larval shell.

Plate 10

Conotreta convexa sp. nov.

Valhallfonna Formation, Olenidsletta Member.

1–3. TSGF17043 , holotype, ventral valve exterior, larval shell, detail of larval micro-ornamentation. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

4–6. TSGF16847 , paratype, ventral valve exterior, oblique posterolateral view, and oblique anterolateral view. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

7. TSGF16844 , paratype, ventral valve interior. 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

Conotreta ? devota Krause & Rowell, 1975

Valhallfonna Formation.

8–11. TSGF16924 , dorsal valve exterior, oblique anterolateral view, larval shell, and detail of larval micro-ornamentation. Profilbekken Member, 26 m above base. Coll. J. Hansen, 04.08.2008, sample JH-184 .

12–14. TSGF16917 , dorsal valve exterior, oblique anterolateral view, and larval shell. Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

15–16. TSGF16851 , dorsal valve exterior and detail of larval micro-ornamentation. Olenidsletta Member, 94 m above base. Coll. J. Hansen, 21.07.2008, sample JH-52 .

Dorsal valve slightly to moderately convex, generally with weak sulcus in anterior part. Valve highest at 30– 43% of valve length. Pseudointerarea concave, anacline or orthocline, 40–59% as wide as valve. Median groove usually moderately deep, 17–35% as wide and 9–18% as long as valve, with straight or slightly concave anterior margin. Median buttress narrow. Thin, triangular median ridge or low septum with apex located at 56–80% of valve length and front at 72–93% of valve length. Spine occasionally present at apex of septum. Cardinal muscle scars moderately large, weakly to moderately impressed, subtriangular, located rather close together. Anterocentral muscle scars generally not impressed but otherwise located at about 32% of valve length. Mantle canal system obscure.

Ventral valve subconical with rounded apex. Pseudointerarea slightly concave or straight procline with broad, weak interridge. Interridge 18–26% as wide as valve. Valve 12% as high as long in one specimen. Anterior valve slope gently convex. Larval shell with sulcus. Foramen within larval shell, not extended as a tube. Internal tube not developed. Apical process a small, distinct, very short ridge. Vascula lateralia diverging anterolaterally at about 60°.

Remarks. Krause & Rowell (1975) tentatively referred Conotreta ? devota Krause & Rowell to the genus based on its apparent lack of a pinnate mantle canal system and its relatively low ventral valve with a rounded apex. We agree that C? devota should probably be assigned its own genus, but we retain it in Conotreta at this time. The material from Spitsbergen shows some morphological trends from the oldest to the youngest strata. Specimens from the lower part of the succession generally have no more than a low dorsal median ridge, whereas specimens from the upper part consistently have a low septum. The dorsal valve also becomes more convex.

Specimens from Spitsbergen rarely, and only among those from the upper part, have anterior spines on the median septum, in contrast to specimens described from the type locality in Nevada. However, this character is somewhat correlated to ontogeny, and most of the Spitsbergen specimens are smaller than those from the type locality. Therefore, we consider the Spitsbergen specimens to be conspecific with the Nevada specimens .

Occurrence. 83, 90, 94, 97 and 99 m above base of Olenidsletta Member and 26 m above base of Profilbekken Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland , Spitsbergen. Orthidiella zone in lower part of Antelope Valley Limestone , Meiklejohn Peak , Nevada .

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.