Conotreta convexa, 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 : 44-46

Conotreta convexa sp. nov.

Pl. 9, Figs. 9–19; Pl. 10, Figs. 1 –7; Tables 8–9

Derivation of name. Latin ‘ convexus ’, convex; refers to the rather convex dorsal valve.

Holotype. Pl. 9, Figs. 18–19; Pl. 10, Figs. 1–3 ; TSGF17043 , ventral valve; 97 m above base of Olenidsletta Member, Valhallfonna Formation, sample JH-136; Profilstranda at Basissletta, Ny Friesland, Spitsbergen.

Material. 39 dorsal and 39 ventral valves from the sample containing the trilobite A84192 and from samples F3475, F3478, F3576, F3743, F3746, F3789, JH-27, JH-52, JH-54, JH-136, JH-140 and JH-157. The paratypes are TSGF16844–16848 .

Diagnosis. Conotreta with slightly to moderately convex dorsal valve; moderately conical and procline ventral valve with straight to weakly convex slopes; low, triangular median septum, often lacking septal rod; narrow interridge and broad, robust apical process.

Description. Shell subcircular to transversely oval with gently convex posterior margin. L/W ratio 0.75–0.95. Largest measured specimen 3.6 mm long and 4.5 mm wide. Larval shell subcircular, 0.18–0.20 mm long, with micro-ornamentation of dense pitting. Pits circular or subcircular, moderately deep (in well-preserved specimens) and variable in size, up to 2 µm in diameter. Postlarval ornamentation of fine growth lines.

Dorsal valve generally moderately convex, often with weak sulcus in front. Pseudointerarea concave, orthocline to anacline, short and 40–54% as wide as valve. Median groove triangular, well defined and moderately deep, with straight to slightly concave anterior margin, 18–25% as wide and 9–10% as long as valve. Cardinal muscle scars strongly impressed, subcircular to subtriangular. Median buttress well developed. Median septum low, triangular, with apex at 52–71% of valve length and front at 73–88% of valve length. Median septum occasionally with rod and rodal spine. Anterocentral muscle scars rarely impressed but otherwise at about 38% of valve length. Mantle canal system obscure.

Ventral valve low to moderately high conical with acute apex. Valve 29–46% as high as long. Pseudointerarea catacline to moderately procline and poorly defined laterally, divided by narrow interridge. Pseudointerarea apsacline in some of the stratigraphically youngest specimens. Interridge 11–18% as wide as valve. Posterior slope gently concave to gently convex. Anterior and lateral valve slopes nearly straight or weakly convex. Larval shell moderately convex. Small foramen enclosed by larval shell at apex and forming only a rudimentary tube. Internal tube not developed. Apical process broad and robust in large specimens and extending slightly anterior as a low ridge.

Remarks. Conotreta was most recently revised by Holmer (2000), who included seven species and tentatively assigned nine others. Of the seven, C. shidertensis Popov & Holmer, 1994 from the Floian of Australia was later transferred to Ottenbyella by Brock & Holmer (2004) based on its catacline-to-procline ventral pseudointerarea with distinct interridge, characteristic anterior bulge of the larval shell, foramen on a short external pedicle tube, straight vascula lateralia, and low dorsal ridge without a septal rod. C. convexa sp. nov. also has a procline pseudointerarea, relatively distinct interridge, and rudimentary external pedicle tube and usually lacks an upper septal rod. In general, however, its features are closer to Conotreta than to Ottenbyella , so it is placed in the former genus. The species differs from Conotreta apicalis Cooper, 1956 from the Upper Ordovician of Alabama by its more low conical ventral valve with straight to weakly convex anterior slope, its generally lower dorsal median septum, and its absent or very weakly impressed anterocentral muscle scars. C. chernovi Popov, 2000b from the Katian of Kazakhstan has an undivided ventral pseudointerarea and a narrow apical process. C.? conoidea Reed, 1917 from the Ashgillian of Scotland is more high conical, has a more convex Anterior ventral slope, and has a distinct ventral mantle canal system. C.? cuspidata Cooper, 1956 from the Upper Ordovician of Virginia differs by its catacline ventral pseudointerarea and weak rugae. C. davidsoni ( Reed, 1917) from the Sandbian of Scotland has a strongly defined median ridge dividing the well-defined, broad, slightly concave ventral pseudointerarea. C. huanghuaensis Zeng in Wang et al., 1983 from the Hirnantian of China has a strongly defined ventral interridge and weakly defined dorsal cardinal muscle scars. C. lepton Williams & Curry, 1985 from the Middle Ordovician of Ireland has a catacline ventral pseudointerarea, a gently convex dorsal valve, and a very short median septum. C. miboshanensis Fu, 1982 from the Ordovician of China has a very short dorsal median septum and a rather low conical ventral valve. C. mica Gorjansky, 1969 from the Dapingian to Darriwilian of Russia, Sweden and Nevada has distinctly elongate dorsal cardinal muscle scars and a high median septum. C. millardensis Popov et al., 2002b from the Tremadocian of Utah has a nearly catacline ventral pseudointerarea and a distinct rod on the rather high dorsal septum. C. multisinuata Cooper, 1956 from the Upper Ordovician of Virginia has rather well-impressed anterocentral muscle scars, catacline or nearly catacline ventral pseudointerarea, and a strongly impressed ventral mantle canal system. C.? nicholsoni ( Davidson, 1868) from the Upper Ordovician of Scotland has a short dorsal median ridge and a nearly catacline ventral pseudointerarea. C.? orbicularis Holmer, 1986 from the Katian of Sweden has a more low conical ventral larval shell, no apical process, a flat dorsal valve, and a faint median buttress. C. parva Bednarczyk, 1986 from the Upper Tremadocian or lowermost Floian of Poland has a broad, well-developed rim on the dorsal valve floor and a well-defined, rather narrow median groove. C.? plana Cooper, 1956 from the Upper Ordovician of Virginia has a subtriangular outline and an indistinct interridge. C. rusti Walcott, 1889 from the Katian of New York has a weaker apical process and a conical ventral larval shell, and its pseudointerarea is about 75% as wide as the valve. C. siljanensis Holmer, 1989 from the Dapingian and Darriwilian of Newfoundland and Sweden has a high median septum, a conical ventral larval shell, and a distinct external pedicle tube. The specimens from the Dapingian of Western Newfoundland referred to C. siljanensis by Robson & Pratt (2001) differ by having an internal pedicle tube and by the three to four spines on the high median septum. C. tchaganensis Popov, 1975 from Kazakhstan has a ventral valve more than half as deep as long and a strongly defined apical process. Conotreta ? sp. ( Nikitina et al. 2006) from the Darriwilian of Kazakhstan has a ventral apex well anterior of the foramen, and its apical process completely occludes the apex.

Occurrence. 75, 80, 92–97, 110 and 113 m above base of Olenidsletta 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 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).

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.