Mirilingula, Popov, 1983

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 37

Mirilingula ? svalbardensis sp. nov.

Pl. 6, Figs. 12–20; Pl. 7, Figs. 1–3

Derivation of name. Refers to the Svalbard archipelago.

Holotype. Pl. 6, Figs. 19–20; TSGF16969 , ventral valve fragment; 21 m above base of Profilbekken Member, Valhallfonna Formation, sample JH-23; Profilbekken meltwater stream, Basissletta, Ny Friesland, Spitsbergen.

Material. Eight fragments of dorsal valves and 11 fragments of ventral valves from the sample with A84363 and from samples JH-23 and JH-136. The paratypes are TSGF16968 , TSGF16971 , TSGF16972 , TSGF16978 and TSGF16979 .

Diagnosis. Lingulellotretid with strongly acuminate ventral valve and strong ventral flexure lines; undivided and highly elevated dorsal pseudointerarea; unthickened visceral areas; a low, thin dorsal median ridge; and no sulcus.

Description. Shell slightly dorsibiconvex, strongly unequivalved and rather thin-walled. Posterior margins of ventral valve diverging at about 45–50˚. Postlarval ornamentation consisting of growth lines. Micro-ornamentation absent on larval and postlarval shell.

Dorsal umbo broadly rounded with partly resorbed larval shell. Valve slightly to moderately convex in lateral profile, elongate-oval to triangular in outline. Pseudointerarea orthocline, highly elevated above and completely separated from valve floor. Pseudointerarea lacking flexure lines and median groove, covered with strong growth lines. Valve floor with scattered, coarse pitting.

Ventral valve slightly convex and strongly acuminate, subtriangular or egg-shaped, with slightly emarginated umbo. Pseudointerarea orthocline, long, highly elevated above and completely separated from valve floor. Central part of pseudointerarea slightly lower than lateral parts. Flexure lines distinct and nearly straight. Growth lines strongly developed on pseudointerarea, curving posteriorly near pedicle groove. Pedicle groove deep, partly covered in posterior one-half to two-thirds of its length, becoming completely covered in anterior one-third to one-half of its length, where a pedicle tube forms on the valve floor. Pedicle slit pointed anteriorly. Visceral area obscure.

Remarks. Mirilingula ? svalbardensis is the youngest known representative of the family Lingulellotretidae . Mirilingula has not previously been recorded from strata younger than the Tremadocian. The species is here assigned to Mirilingula on the basis of its highly raised, undivided dorsal pseudointerarea, lack of visceral platforms, low dorsal median ridge, and general shell outline. However, the lack of a shallow sulcus in the dorsal and ventral valves and the presence of distinct flexure lines in the ventral pseudointerarea sets it apart from all other known species within the genus.

Occurrence. 95 and 97 m above base of Olenidsletta Member and 21 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.