Eurytreta subtriangularis, 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 : 53-56

Eurytreta subtriangularis sp. nov.

Pl. 13, Figs. 4 –13; Tables 12–13

Derivation of name. Latin ‘ sub ’ and ‘ triangulum ’, nearly triangular; refers to the rounded triangular outline of both valves.

Holotype. Pl. 13, Figs. 7–8; TSGF16993 , dorsal valve; 97 m above base of Olenidsletta Member, Valhallfonna Formation, sample JH-136; Profilstranda at Basissletta, Ny Friesland, Spitsbergen.

Material. 30 dorsal and 64 ventral valves from samples F3475, F3673, JH-52 and JH-136. The paratypes are TSGF16989–16992 .

Diagnosis. Eurytreta with distinctly subtriangular outline of both dorsal and ventral valves; distinct interridge; ventral apex located well anterior of umbo; rather weak dorsal median ridge.

Description. Shell subtriangular in outline with gently convex posterior margin. Largest measured specimen 1.5 mm long and 1.5 mm wide. Ornamentation consisting of fine growth lines. Larval shell with dense micro-pitting. Pits subcircular, shallow to moderately deep, 0.7–2 µm in diameter.

Dorsal valve slightly to moderately convex, generally with weak sulcus. L/W ratio 0.82–1.00. Maximum width at 50–65% of valve length; maximum height at 24–44% of valve length. Larval shell subcircular, 0.17–0.18 mm long. Pseudointerarea concave, slightly to moderately anacline, 43–64% as wide as total valve width and occupying 8–14% of valve length. Median groove deep, 20–41% as wide as valve, with concave anterior margin. Median ridge low, often subtriangular, rarely developing into a low septum. Median ridge beginning at 0.2–0.4 mm of valve length and ending at 76–88% of valve length. Apex of median septum (when developed) located at 56–70% of valve length. Median buttress broad, poorly developed. Cardinal muscle scars egg-shaped, variably impressed, located relatively close together. Anterocentral muscle scars subcircular, located at 44% of valve length, rarely impressed. Mantle canal system obscure.

Plate 13

Cyrtonotreta spinosa sp. nov.

Valhallfonna Formation, Olenidsletta Member.

1–2. TSGF16803, exterior of ventral valve fragment and detail of postlarval ornamentation. 90 m above base. Coll. J. Hansen, 20.07.2008, sample JH-45.

3. TSGF16808, interior of ventral valve fragment with part of apical process preserved. 90 m above base. Coll. J. Hansen, 20.07.2008, sample JH-45.

Eurytreta subtriangularis sp. nov.

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

4–6. TSGF16989 , paratype, exterior of dorsal valve, oblique anterolateral view, and larval shell .

7–8. TSGF16993 , holotype, interior of dorsal valve and oblique anterolateral view thereof .

9. TSGF16990 , paratype, lateral profile of ventral valve .

10–11. TSGF16991 , paratype, ventral valve exterior and detail of larval shell .

12–13. TSGF16992 , paratype, ventral valve interior and oblique anterolateral view thereof .

Ventral valve moderately high conical, 30–70% as high as long. Outline subtriangular in ventral view, with L/ W ratio 0.93–1.09. Pseudointerarea concave, catacline to apsacline, short and poorly defined, with distinct, narrow interridge. Anterior valve slope gently to moderately convex. Foramen located on very short, posteriorly directed tube within larval shell, but generally not at highest part of valve. Apex located at 24–34% of valve length. Maximum width located at 63–69% of valve length. Larval shell subcircular, conical. Apical process poorly developed, triangular, bounded by anterolaterally directed vascula lateralia, with semicircular depression or divided into three parts.

Remarks. Compared to the new species, Eurytreta belti ( Davidson, 1868) from the Tremadocian of Wales has a better-developed apical process and a transversely oval outline with a mean L/W ratio of 0.82. E. bisecta ( Matthew, 1901) from the Lower Ordovician of Utah differs by its transversely oval outline and steeply procline ventral pseudointerarea without a distinct interridge. E. campaniformis Krause & Rowell, 1975 from the Dapingian of Nevada is distinguished by its ventral pseudointerarea having a nearly straight slope, its apical foramen, and the subrectangular outline of its dorsal valve. E. chabakovi ( Lermontova, 1933) from the Floian of the South Urals differs in being transversely oval with the foramen at or very close to the apex. E. curvata ( Walcott, 1902) from the Lower Ordovician of Nevada, USA differs in being transversely oval with a weak interridge and pustulose ornamentation. E. discors Popov in Koneva & Popov, 1988 from the Upper Cambrian of Kazakhstan is distinguished by having a transversely oval outline and a procline ventral pseudointerarea. E. evanda Popov in Koneva & Popov, 1988 from the Lower Ordovician of Kazakhstan differs in being transversely oval with a procline ventral pseudointerarea. E.? exigua Popov in Koneva & Popov, 1988 from the Lower Ordovician of Kazakhstan is distinguished by having a subcircular to transversely oval outline and a procline ventral pseudointerarea. E. fillmorensis Holmer et al., 2005 from the Tremadocian of Utah differs in having a transversely oval dorsal valve, an indistinct interridge, and an apical or nearly apical foramen. E. intermedia Biernat, 1973 from the Darriwilian of Poland is distinguished by having a transversely oval outline, an intertrough, and a moderately high dorsal median septum. E. kendyktassica Popov in Nazarov & Popov, 1980 from the Darriwilian of Kazakhstan differs in being transversely oval with a procline ventral pseudointerarea. E. minor Biernat, 1973 from the Tremadocian of Kazakhstan differs by its transversely oval outline, procline ventral pseudointerarea, and lack of an interridge. E. sabrinae ( Callaway, 1877) from the Tremadocian of England is distinguished by its much stronger dorsal median ridge and its transversely oval to subcircular outline. E? stapeleyensis ( Williams, 1974) from the Darriwilian of England differs in having a catacline to procline ventral pseudointerarea and a transversely oval outline. E. sublata Popov in Koneva & Popov, 1988 from the Lower Ordovician of Kazakhstan differs by having a catacline to procline pseudointerarea, a transversely oval outline and a weak interridge. Eurytreta sp. ( Popov et al. 2008) from the Tremadocian of Iran is distinguished by being subcircular and by the apex of the dorsal median ridge, which is located closer to mid-valve length.

Occurrence. 85, 92, 94 and 97 m above base of Olenidsletta Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.