Homotreta cf. gorjanskii ( Pelman, 1973 )

Kouchinsky, Artem, Alexander, Ruaridh, Bengtson, Stefan, Bowyer, Fred, Clausen, Sébastien, Holmer, Lars E., Kolesnikov, Kirill A., Korovnikov, Igor V., Pavlov, Vladimir, Skovsted, Christian B., Ushatinskaya, Galina, Wood, Rachel & Zhuravlev, Andrey Y., 2022, Early-middle Cambrian stratigraphy and faunas from northern Siberia, Acta Palaeontologica Polonica 67 (2), pp. 341-464 : 358-361

Homotreta cf. gorjanskii ( Pelman, 1973)

Fig. 11 .

Material.—Several tens of calcium phosphatic dorsal and ventral valves, including figured SMNH Br151212–151222, from samples 2/0, 2/10, and 8/32, Kuonamka and Olenyok formations, Bol’shaya Kuonamka and Malaya Kuonamka rivers, Siberia, Russia. Tomagnostus fissus – Acadoparadoxides sacheri Zone , lower Mayan stage (Drumian Stage).

Description.—Shell rounded in outline, covered with fine concentric growth lines with occasional sets of drapes. Ventral valve conical with marginal umbo. Brephic shell, ca. 200 μm in diameter, broadly oval in outline, 150–180 μm long and 180–210 μm wide, covered with scattered pits, 0.5– 2.0 μm in dimeter, becoming less distinct towards the margin of the brephic shell. Circular foramen, 25–30 μm in diameter, opens at the tip of a conical extension of brephic shell at a distance from its posterior margin. Ventral pseudointerarea steeply procline. Apical foramen in a short tube, directed posteriorly and upward. External pedicle opening enclosed within brephic shell. Dorsal valve gently convex, rounded with a marginal umbo. Dorsal pseudointerarea with a broad triangular median groove and broad propareas. Dorsal interior with faint broad median ridge. Brephic shell broadly oval in outline, covered with pits, ca. 1 μm in diameter.

Remarks.—In Homotreta gorjanskii , the foramen closes at the boundary between the brephic and adult shells and is directed posteriorly, whereas in Homotreta cf. gorjanskii , the foramen appears to be enclosed within the brephic shell and directed posteriorly and upwards. The ventral brephic shell in H. cf. gorjanskii has asymmetrically spaced tuberсles and a more prominent cone-like extension from the foramen. The maximum diameter of the pits on the brephic shell of H. cf. gorjanskii is twice as large as the maximum diameter of pits in H. gorjanskii .

Homotreta cf. gorjanskii probably represents a new species, but the systematics of the Siberian forms of the family Acrotretidae is currently under revision and it is left under open nomenclature here.

Figures

Fig. 11. Lingulid brachiopod Homotreta cf. gorjanskii (Pelman, 1973) from the middle Cambrian Olenyok Formation, Malaya Kuonamka River, Siberia, Russia; sample 2/10. A–E. Ventral valves, SMNH Br151212–151216, respectively.A2, close-up of brephic shell,A3, enlargement showing pitted surface; E1, lateral view; E2, posterior view. F–K. Dorsal valves, SMNH Br151217–151222, respectively. G, brephic shell. Scale bar: 250 µm (A1, B–F, H–K), 125 µm (A3, G), and 12.5 µm (A2).