Dictyonites mugilis, 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 : 81-83

Dictyonites mugilis sp. nov.

Pl. 22, Figs. 4 –9; Fig. 4

Derivation of name. Latin ‘ mugio ’, low; refers to the very low ventral valve.

Holotype. Pl. 22, Fig. 4; TSGF16764 , ventral valve; 73 m above base of Olenidsletta Member, Valhallfonna Formation, sample JH-141; Profilstranda, Basissletta, Ny Friesland, Spitsbergen.

Material. 140 specimens, mostly fragments, from the sample with trilobite A84365 and from samples JH-23, JH-45, JH-48, JH-52, JH-53, JH-131, JH-136 and JH-141. The paratypes are TSGF16765 , TSGF17034 , TSGF17035 , TSGF17037 and TSGF17038 .

Diagnosis. Dictyonites with very low, subconical ventral valve and nearly planar dorsal valve; strong transverse grooves on homeodeltidium; ventral larval shell anteriorly separated from postlarval shell by two ridges; ventral valve generally weakly sulcate.

Plate 22

Acanthambonia sp.

Valhallfonna Formation.

1–2. TSGF16988, fragment of valve exterior and detail of ornamentation. Olenidsletta Member, 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136.

3. TSGF16864, fragment of valve exterior. Profilbekken Member, 67 m above base. Coll. J. Hansen, 24.07.2008, sample JH- 95.

Dictyonites mugilis sp. nov.

Valhallfonna Formation, Olenidsletta Member.

4. TSGF16764 , holotype, ventral valve exterior. 73 m above base. Coll. J. Hansen, 28.07.2008, sample JH-141. 5 . TSGF17034 , paratype, larval shell and homeodeltidium of ventral valve. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

6. TSGF17037 , paratype, detail of pitted ornamentation. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136. 7 . TSGF17038 , paratype, detail of perforate ornamentation. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136. 8 . TSGF17035 , paratype, detail of distal-valve ornamentation. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136. 9 . TSGF16765 , paratype, dorsal valve exterior. 73 m above base. Coll. J. Hansen, 28.07.2008, sample JH-141 .

Pelonomia sulcata sp. nov.

Valhallfonna Formation, Olenidsletta Member.

10–11. TSGF17063 , holotype, dorsal view of complete specimen and oblique posterolateral view thereof. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

12. TSGF17083 , paratype, dorsal valve exterior. Middle unit. Coll. J. Hansen, 17.07.2008, sample JH-190 .

13–14. TSGF17058 , paratype, oblique anterolateral view and ventral view of dorsal valve interior. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

15. TSGF17062 , paratype, dorsal valve interior. 97 m above base. Coll. J. Hansen, 28.07.2008, sample JH-136 .

Description. Shell small and very low, subpyramidal, with semicircular outline and nearly straight posterior margin. Posterolateral extremities rounded. Largest measured specimen 2.5 mm long and originally about 3.4 mm wide. L/W ratio about 0.67–0.74. Larval shell about 0.45 mm long. Postlarval ornamentation consisting of dense hexagonal or diamond-shaped pits arranged in divaricating and commonly also radiating rows. Pits increasing in size distally but becoming strongly transverse close to valve edge. Largest pits about 0.35 mm wide. Ornamentation often perforated in central part of shell but generally poorly developed in distal parts of larger specimens.

Dorsal valve nearly planar, with weak sulcus or slightly plicosulcate. Larval shell gently unisulcate, separated from postlarval shell by two slightly raised rims.

Ventral valve very low, subpyramidal, with slightly concave to gently convex anterior slope. Sulcus often weak, angular. Pseudointerarea slightly procline with well-developed homeodeltidium. Homeodeltidium with strong, transverse grooves. Larval shell subangular, separated from postlarval shell by two ridges. Larval shell normally with angular, moderate sulcus.

Remarks. Dictyonites mugilis sp. nov. differs from the similar specimens from the Dapingian of Nevada described and illustrated by Krause & Rowell (1975), which have a conical or convex dorsal valve, only one anterior ridge separating the ventral larval shell from the postlarval shell, and a ventral valve that is markedly more high conical. The Swedish upper Darriwilian Dictyonites fredriki Holmer, 1989 , which is one of the two previously established species, is distinguished by having an ornamentation dominated by perforation, only one ridge separating the ventral larval shell from the postlarval shell, a moderately high conical ventral valve, and a homeodeltidium with much weaker transverse grooves. The type species, D. perforata , is distinguished by having a moderately high, pyramidal ventral valve, a dominantly perforated ornamentation, a ventral larval shell that is not separated from the postlarval shell by two ridges, and a much less deeply impressed division of the ventral larval shell. Holmer (1989) also figured a fragment of an unnamed species from the Swedish mid-Darriwilian. This fragment is distinguished from the new species by the same characters as D. fredriki , except for its ornamentation, which consists of pits.

Occurrence. 73, 89, 90, 93, 94, 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.