Triplicatella sp.

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 : 407-409

Triplicatella sp.

Fig. 46 .

Material.—Five silicified internal moulds of conchs with casts of inner surfaces of opercula (situated in closed position with respect to the aperture of the conch), including figured specimens SMNH X11178–11181, from sample 19/65.5, and probably several hundred silicified moulds of conchs without traces of opercula belonging to the same species, from samples 19/66.5, 19/66.75, 19/3B, 19/4B, Kuonamka Formation, Khorbusuonka River, Siberia, Russia. Tomagnostus fissus – Acadoparadoxides sacheri Zone , lower Mayan stage (correlated with the lowermost Drumian Stage).

Description.—Conchs rounded trapezoidal in cross-section, gently curved in sagittal plane, moderately expanding from the initial part, at ca. 25° in the dorsal view and ca. 20° in lateral view. The aperture carries a bilaterally symmetrical cast of a folded operculum, rounded trapezoidal to oval in the frontal view. The operculum, 0.7–1 mm wide is flattened to slightly concave externally. It carries two deep folds forming a V-shaped groove diverging at ca. 40° on the surface viewed in dorso-ventral orientation of the conch. Invagination of the dorsal margin of the operculum produces two distinct embayments in the dorsal margin of the conch mould. The folds continue ventrally, converging and shallowing towards the ventral margin of the operculum.

Remarks.—The form described above is similar to that figured by Kouchinsky et al. (2011: fig. 11A), although a silicified operculum and conch therein may not belong to the same species, and to Triplicatella sinuosa figured by Kouchinsky et al. (2015a: fig. 29C). Four species of Triplicatella Conway Morris in Bengtson et al., 1990, are described, such as Triplicatella disdoma Conway Morris in Bengtson et al., 1990 (type species), Triplicatella sinuosa Skovsted, Peel, and Atkins, 2004 , Triplicatella peltata Skovsted, Peel, and Atkins, 2004 , and Triplicatella papilio Kouchinsky in Kouchinsky et al., 2015a, are known from acid isolated material while a fifth species, Triplicatella opimus ( Yu, 1974) with preserved soft parts, is known from the Chengjiang fauna of South China ( Liu et al. 2020, 2021).

Orthothecida sp. indet.

Fig. 40L–N .

Material.—Several tens of calcium phosphatic internal moulds, including SMNH X11147–11149, from 19/5.5, 19/11.75, 19/12.75. Erkeket Formation, Khorbusuonka River, Siberia, Russia. Lower Botoman stage (correlated with the lower Cambrian Stage 4).

Description.—Moderately expanding conchs, slightly dorso-ventrally curved at the initial part. Aperture rounded trapezoidal and straight. In dorsal view, the apical angle is ca. 30°. Apical part rounded blunt, not separated.

Orthothecid operculum type A Fig. 47 .

Material.—Several tens of phosphatised opercula, including figured SMNH X11182–11189, from sample 21/21, Tyuser Formation, Lena River, Siberia, Russia. Dokidocyathus regularis Zone, Tommotian stage (correlated with the upper part of Cambrian Stage 2).

Description.—Bilaterally symmetrical operculum with a sub-circular outline and flattened outer side. Initial part circular flattened situated in a shallow depression of the outer side of the operculum, about twice as close to the dorsal side of the operculum as to the ventral one. A bilaterally symmetrical pair of radial folds beginning at the initial part divide the outer surface into two unequal sectors (lobes) situated at ca. 90° to each other. The opposite surface of the operculum carries a circular ridge with two closely situated prongs, elevated lateral parts and sinus in the ventral portion. The outer side of the ridge is steep towards the margin of the operculum. The inner side slopes towards the initial part.

Remarks.—Most similar to the Conotheca mammilata ? operculum ( Fig. 41 ), but differs from it in having outer side divided by а bilaterally symmetrical pair of radial folds into ventral and dorsal lobes.

Orthothecid operculum type B

Fig. 48 .

Material.—Several tens of phosphatised opercula, including figured SMNH X11190–11195, from samples 11/2B and 11/16.05, basal Erkeket Formation, Khorbusuonka River and 21/21, Tyuser Formation, Lena River. Siberia, Russia; Dokidocyathus regularis Zone, Tommotian stage (correlated with the upper part of Cambrian Stage 2).

Description.—Broadly ovaliform (smaller specimens) to rounded rectangular operculum somewhat extended in sagittal plane, with thick wall and low domal external surface covered with distinct growth increments. Initial part elevated and blunt, slightly shifted towards the dorsal side of the operculum. The external surface flattens laterally. Inner surface concave and smooth separated from the external surface by a thick rim consisting of growth increments. The concavity is somewhat extended in the sagittal plane of the operculum and has a shallow sinus dorsally.

Orthothecid operculum type C Fig. 49 .

Material.—Several tens of phosphatised opercula, including figured SMNH X11196–11202, from samples 11/2B and 11/16.05, basal Erkeket Formation, Khorbusuonka River and 21/21, Tyuser Formation, Lena River. Siberia, Russia; Dokidocyathus regularis Zone, Tommotian stage (correlated with the upper part of Cambrian Stage 2).

Description.—Operculum broadly oval slightly extended in the sagittal plane with a rounded initial part situated closer to the dorsal margin, at ca. 1/3 of the opercular diameter. Domal in the lateral view, with a deep sinus of the dorsal margin. The outer surface covered with concentric growth lines and thicker growth increments at the margin. Inner side concave and smooth, with radial striae and pitted surface. Walls thick consisting of fibrous bundles.

Remarks.—Other undetermined phosphatised orthothecid opercula of different types occur in the Dokidocyathus regularis Zone, Tommotian stage : SMNH X11203 ( Fig. 50A ), from sample 11/16.05, SMNH X11205 ( Fig. 50C ) and SMNH X11206 ( Fig. 50D ), from sample 11/2B (basal Erkeket Formation, Khorbusuonka River) and SMNH X11204 ( Fig. 50B ), from sample 21/21 (Tyuser Formation, Lena River).

Figures

Fig. 46. Hyolith conchs and opercula, Triplicatella sp., from the middle Cambrian Kuonamka Formation, Khorbusuonka River, Siberia, Russia; sample 19/65.5. A–D. Silicified internal moulds of conch closed with operculum, SMNH X11178–11181, respectively.A1, lateral view; A2, apertural view. Scale bar: 1 mm (B) and 500 µm (A, C, D).

Fig. 40. Hyolith conchs from the lower Cambrian Erkeket Formation, Khorbusuonka River (A–D, G, H, K–N) and Tyuser Formation, lower reaches of the Lena River (E, F, I, J), Siberia, Russia; samples 11/16.05 (K), 11/16.1 (A), 11/16.5 (B, C), 11/2B (D), 19/4.25 (H), 19/11.75 (L), 19/12.75 (M, N), 19/14.25 (G), 21/21 (I, J), and 21/51 (E, F). A, B. Conotheca mammilata Missarzhevsky in Rozanov et al., 1969, phosphatised conchs, SMNH X11136 and 11137, respectively. C, D. Conotheca mammilata Missarzhevsky in Rozanov et al., 1969, phosphatised conchs with initial part preserved, SMNH X11138 and 11139, respectively. E, F. Conotheca cf. longiconica (Qian, 1978), phosphatised conchs, SMNH X11142 and 11143, respectively. G, H. Conotheca circumflexa Missarzhevsky in Rozanov et al., 1969, phosphatised conchs, SMNH X11140 and 11141, respectively.H2, close-up showing transversal fibers in the wall. I–K. Egdetheca aldanica Missarzhevsky in Rozanov et al., 1969, calcium phosphatic internal moulds, SMNH X11144–11146, respectively. K1, lateral view; K2, apical view.L–N. Orthothecida sp. indet., calcium phosphatic internal moulds, SMNH X11147–11149, respectively. M1, lateral view; M2, dorsal view. Scale bar: 1 mm (A), 500 µm (B–D, G–K), 250 µm (E, F, L–N), and 50 µm (H2).

Fig. 47. Hyolith opercula “Orthothecid operculum type A”, from the lower Cambrian Tyuser Formation, lower reaches of the Lena River, Siberia, Russia; sample 21/21. A–H. Phosphatised opercula, SMNH X11182–11189, respectively. B1, outer surface; B2, oblique dorsal; D1, inner surface; and D2, oblique lateral views. Scale bar 500 µm.

Fig. 41. Hyolith opercula from the lower Cambrian Tyuser Formation, lower reaches of the Lena River (A, B, G) and Erkeket Formation, Khorbusuonka River (C–F, H, I), Siberia, Russia; samples 11/16.05 (C, H), 11/16.1 (I), 11/2B (D–F), and 21/21 (A, B, G). A–I. Phosphatised opercula tentatively attributed to Conotheca mammilata Missarzhevsky in Rozanov et al., 1969, SMNH X11150–11158, respectively. A1, B1, C1, D1, inner surface; A2, ventral view; A3, lateral view; B2, oblique lateral view;A4, C2, oblique dorsal view on the opercula. Scale bar 500 µm.

Fig. 48. Hyolith opercula “Orthothecid operculum type B”, from the lower Cambrian Erkeket Formation, Khorbusuonka River (A, C–F) and Tyuser Formation, lower reaches of the Lena River (B), Siberia, Russia; samples 11/16.5 (A, C, F), 11/2B (D, E), and 21/21 (B). A–D. Phosphatised opercula, SMNH X11190–11193, respectively. E, F. Phosphatised opercula of presumably the same species, SMNH X11194 and 11195, respectively.A1, D3, F2, outer surface;A2, B2, F3, oblique lateral; D2, E2, lateral;A3, D1, F1, dorsal; B1, E1, inner surface views. Scale bar: 500 µm (A–E, F2, F3) and 250 µm (F1).

Fig. 49. Hyolith opercula “Orthothecid operculum type C”, from the lower Cambrian Erkeket Formation, Khorbusuonka River (A–D) and Tyuser Formation, lower reaches of the Lena River (E–G), Siberia, Russia; samples 11/2B (A–D) and 21/21 (E–G). A–G. Phosphatised opercula, SMNH X11196–11202, respectively. E1, inner surface and transverse broken view; E2, enlargement showing fibrous composition of the wall; F1, ventral view; A1, C1, F2, outer surface; A2, C3, dorsal view; A3, lateral view; C2, oblique lateral view; D1, oblique inner surface view; D2, enlargement showing pitted inner surface. Scale bar: 500 µm (A, C, D1, E1, F) and 250 µm (B, D2, E2, G).

Fig. 50. Hyolith opercula from the lower Cambrian Erkeket Formation, Khorbusuonka River (A, C, D) and Tyuser Formation, lower reaches of the Lena River (B), Siberia, Russia; samples 11/16.05 (A), 11/2B (C, D), and 21/21 (B). A–D. Phosphatised orthothecid opercula of four different types, SMNH X11203–11206, respectively. A1, B3, outer surface; A2, D2, inner surface; B1, C1, D3, lateral view; B2, ventral view; C2, oblique inner surface; C3, D1, dorsal view. Scale bar: 625 µm (D), 500 µm (B), and 250 µm (A1, A2, C).