Conotheca mammilata Missarzhevsky

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 : 399

Conotheca mammilata Missarzhevsky in Rozanov et al.,1969

Fig. 40A–D .

Material.—Several hundred phosphatised conchs from samples 11/2B, 11/16.05, 11/16.1, 11/16.5, 11/18.7, 11/19.6, 11/21.6, 11/28.8, including figured SMNH X11136–11139, from the Erkeket Formation, Khorbusuonka River and sample 21/21, Tyuser Formation, Lena River. Siberia, Russia; Dokidocyathus regularis Zone, Tommotian stage (correlated with the upper part of Cambrian Stage 2).

Description.—Conchs almost straight or gently dorso-ventrally curved, with moderate expansion rate and apical angle up to 15° in dorsal view. Aperture straight and circular. Juvenile conch blunt and somewhat bulbous. External surface with straight thin transverse growth lines. Wall consists of inner layer of transversally oriented fibers and outer layer of longitudinally oriented fibers.

Remarks.—Co-occurring circular orthothecid opercula ( Fig. 41 ) fitting circular straight aperture of Conotheca mammilata conchs may belong to the same species. The outer surface of the operculum is flattened, with a rounded slightly protruding initial part displaced somewhat dorsally. The inner surface of the operculum carries a circumferential bilaterally symmetrical circular ridge, offset slightly from the margin of the operculum and delimiting a circular depression in the middle of the operculum. The outer face of the ridge dips ca. 90º towards the opercular margin. The inner face slopes towards the center of operculum and carries bilaterally symmetrical lateral sets of radial folds. The crest of the ridge undulates: it arches ventrally to the opercular margin, produces bilaterally symmetrical elevated folds laterally and a dorsal pair of closely situated short prongs. The lateral folds are reminiscent of clavicules of hyolithids, whereas prongs can be considered cardinal processes. Although attached opercula are not found and cannot be at present unequivocally paired with conchs in our material, similar opercula with an offset marginal zone are previously described from Conotheca australiensis Bengtson in Bengtson et al., 1990, Conotheca rotunda (Duan, 1984) , and Conotheca brevica Qian, Xie, and He, 2001 . They have similar clavicle-like structures and paired processes on the internal side (Devaere et al. 2014).

Stratigraphic and geographic range.— Tommotian stage (correlated with the upper part of Cambrian Stage 2), Siberian Platform, Russia.

Conotheca circumflexa Missarzhevsky in Rozanov et al., 1969

Fig. 40G, H .

Material.—Twenty phosphatised conchs, including figured SMNH X11140 and 11141, from samples 19/4.25, 19/5.5, 19/8.5, 19/14.25. Erkeket Formation, Khorbusuonka River, Siberia, Russia. Lower Botoman stage (correlated with the lower part of Cambrian Stage 4).

Description.—Conch helically curved about 1/4 revolution clockwise in the direction of growth, with apical angle ca. 20°. Aperture straight and slightly oval. Wall consists of transversally oriented fibrous bundles running around the conch in the inner layer and longitudinally oriented in the outer layer.

Remarks.—See also Kouchinsky et al. (2009: fig. 45 and 2015a: fig. 27F).

Stratigraphic and geographic range.—Atdabanian and Botoman stages (correlated with the Cambrian Stage 3 and lower part of Stage 4), Siberian Platform, Russia.

→ Fig. 39. Sclerites of chancelloriid Archaeopetasus typicus ( Sipin, 1999) from the lower Cambrian Erkeket Formation, Khorbusuonka River, Siberia, Russia; sample 11/2B. A–F. Sclerites preserved as phosphatic coats on their convex and concave sides, SMNH X11130–11135, respectively. A. Positive replica of the convex outer surface of sclerite showing scattered tubercles. B. Negative replica of the concave inner surface of sclerite (note a partially preserved positive replica of the convex outer surface with scattered tubercles in the left). C. Two sclerites merged along their convex outer surfaces; C 1, close-up showing two merged sclerites in lateral view. D. Oblique lateral view on the convex outer surface of sclerite. E. Phosphatic coat representing a negative replica of the concave inner surface of sclerite (E 1), enlargement showing central elevation with pits (E 2). F. Phosphatic coat representing a positive replica of the concave inner surface of sclerite with a central cavity (F 1), close-up showing the central cavity with flattened tubercles (F 2). Scale bar: 500 µm (A, C 2, D, E 1, F 1), 250 µm (B, C 1), and 100 µm (E 2, F 2).

Figures

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. 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.