Pelonomia sulcata, 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 : 84-85

Pelonomia sulcata sp. nov.

Pl. 22, Figs. 10–15; Pl. 23, Figs. 1–4 ; Table 28

Derivation of name. Latin ‘ Sulco ’, a furrow; refers to the generally well-defined dorsal sulcus.

Holotype. Pl. 22, Figs. 10–11; TSGF17063 , complete specimen; 97 m above base of Olenidsletta Member, Valhallfonna Formation, sample JH-136; Profilstranda, Basissletta, Ny Friesland, Spitsbergen.

Material. 15 articulated specimens and 191 dorsal and 256 ventral valves from the samples with trilobites A84135 , A84162 , A84165 , A84173 , A84182 , A84270 , A84271 , A84278 , A84292 , A84309 , A84316 , A84339 , A84362 a, A84363 , A84365 and A84373 and from samples F3028, F3395, F3425, F3475, F3485, F3505, F3527, F3535, F3664, F3675 ½, F3678, F3747, F3748, F3749, F3797, F3857, JH-3, JH-6, JH-7, JH-12, JH-13, JH-15, JH- 26, JH-27, JH-44, JH-45, JH-52, JH-53, JH-55, JH-129, JH-131, JH-136, JH-139, JH-190 and JH-191. The paratypes are TSGF17058 , TSGF17060–17062 , TSGF17083 and TSGF17084 .

Diagnosis. Pelonomia with well-defined dorsal sulcus; generally subequally parvicostellate, with 8–12 costellae per mm at 1-mm valve length; rows of tubercles absent or poorly developed along midline of dorsal valve floor.

Description. Shell small, subangular or semicircular, widest at hinge line, planoconvex. L/W ratio 0.49–0.74 (0.97). Largest measured specimen 7.9 mm long and 10.8 mm wide. Shell about one-third as deep as long. Angle of cardinal extremities normally 65–90˚. Anterior margin weakly unisulcate. Shell material coarsely pseudopunctate. Ornamentation parvicostellate, generally with little differentiation between costellae. Interspaces usually present between costellae. Dense, fine growth lines, low filae and capillae also present; 7–12 narrow costellae per mm at 1- mm valve length and 19 per 2.5 mm at 2.5-mm growth stage. About 24–28 filae per mm. Costellae impressed on valve floor.

Dorsal valve nearly planar, with broad, generally well-developed sulcus. Dorsal interarea short, strongly anacline to hypocline, 65% as long as ventral valve. Notothyrium covered by large, low chilidium. Valve floor outside muscle field with moderately dense, circular pustules. Pustules generally ordered in rows between impressions of external costellae but also in concentric rows spaced at the same distance as the radial rows. Brachiophores delicate, diverging at about 100–110˚. Muscle field generally not impressed, otherwise reaching about 25% of valve length. Median ridge absent, but pustules becoming slightly larger and occasionally developing into two or three weak, radial rows of tubercles in larger specimens. Low peripheral rim occasionally present in larger specimens.

Ventral valve slightly convex, normally crested and nearly 10% longer than dorsal valve. Ventral interarea apsacline, planar or slightly concave, 11% as long as valve. Delthyrium wide. V-shaped and open. Pseudodeltidium partly covering delthyrium. Pustules as in dorsal valve. Teeth small, triangular. Dental plates short and divergent. Bilobed ventral muscle field raised above valve floor and reaching 14–23% of valve length. Ventral valve floor often with low peripheral rim located 15–25% of valve length from margin.

Remarks. The density of costellae evolves significantly from 9–12 costellae per mm on specimens in the middle part of the succession to 7–9 on the stratigraphically youngest specimens.

We consider both P. sulcata sp. nov. and Pelonomia ? sp. (Benedetto in Benedetto & Cech (2006)) from Argentina to be unambiguous members of Pelonomia because the median radial rows of tubercles are occasionally developed on the dorsal valves of Pelonomia sulcata .

Compared to P. sulcata , the type species, P. delicatula (Billings, 1865) , has a more unequicostellate ornamentation and strong, radial rows of tubercles along the midline of the dorsal valve floor. Pelonomia ? sp. from Argentina is distinguished by its lack of a fold and sulcus and by its 6–7 costellae per mm at the 1-mm growth stage. The youngest specimens of P. sulcata , which approximately equal in age to the species from Argentina, generally have 8–9 costellae and thus appear to be closely related to that species.

Occurrence. 25, 75–100, 110 and 130–133 m above base of Olenidsletta Member and 15, 17 and 20 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.

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.