Conotheca, Missarzhevsky, 1969

Kouchinsky, Artem, Bengtson, Stefan, Clausen, Sébastien & Vendrasco, Michael J., 2015, An early Cambrian fauna of skeletal fossils from the Emyaksin Formation, northern Siberia, Acta Palaeontologica Polonica 60 (2), pp. 421-512 : 445

Conotheca cf. C. mammilata Missarzhevsky in Rozanov et al., 1969

Figs. 25E–G , 26K, 27A–E, H.

Material.— Six specimens from sample 3/42; three specimens, including SMNH X 4620, from sample 3/65; five, including SMNH X 4621 and 4622, from sample 7/27.5; two from sample 7/28.5; five, including SMNH X 4623, from sample 7/29; SMNH X 4663 from sample 7/30, eight from sample 7/31.5, including SMNH X 4624; one from sample 7/33; one from sample 7/34; nine phosphatised conchs with internal moulds, including SMNH X 4625–4627, from sample 7/36.7; several specimens from sample 7/55; one specimen, SMNH X 4628, from sample 7/70, five from sample K1-3B and five from sample 1/7.8. Emyaksin Formation, Malaya Kuonamka and Bol’shaya Kuonamka rivers; correlated with the Repinaella Zone, Atdabanian Stage – Calodiscus -Erbiella Zone, lower Botoman Stage.

Description.—Slightly dorso-ventrally curved conch, with a straight aperture and circular cross-section. The surface is covered with straight transverse growth lines. Apical angle is up to 15°. Juvenile conch is blunt and bulbous. Wall microstructure consists of inner layer of transverse fibres running around the conch, and the outer layer composed of longitudinally oriented fibres ( Fig. 27A, B ).

Remarks.— Conotheca mammilata Missarzhevsky in Rozanov et al., 1969 is known from the Tommotian and Atdabanian Stages of Siberia. In the type material, the apical angle ranges from 8–20° and the presence of both robust and gracile forms was indicated in the original description of C. mammilata by Missarzhevsky (in Rozanov et al. 1969: 113). Specimens with better-preserved phosphatised walls show microstructure similar to that of some other hyoliths described herein ( Fig. 27A–D ; see also Kouchinsky 2000b).

Figures

Fig. 25. Hyolith conchs from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/36.7. A. Hyolithida sp. indet. 1, SMNH X 4656; lateral (A1), dorsal (A2), and apertural (A3) views. B. “Lenatheca groenlandica (Poulsen, 1932)”?, initial part, SMNH X 4640; apertural (B1), dorsal (B2), and lateral (B3) views. C. Microcornus? sp., SMNH X 4655; lateral (C1), apertural (C2), and dorsal (C3) views. D. Loculitheca? sp., SMNH X 4634; D1, oblique apical view, septum; D2, plan view of the conch. E–G. Conotheca cf. C. mammilata Missarzhevsky in Rozanov et al., 1969. E. SMNH X 4625; lateral view. F. SMNH X 4626; dorsal view. G. SMNH X 4627; lateral view. Scale bar 600 μm.

Fig. 27. Hyolith conchs from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River and basal Kuonamka Formation, Malaya Kuonamka River, Anabar Uplift, Siberia. A–E, H. Conotheca cf. C. mammilata Missarzhevsky in Rozanov et al., 1969. A. SMNH X 4623, sample 7/29; longitudinal view on presumably dorsal side. B. SMNH X 4621, sample 7/27.5; longitudinal view on presumably dorsal side. C. SMNH X 4622, sample 7/27.5; longitudinal view. D. SMNH X 4663, sample 7/30; D1, transversal view from the apertural end; D2, longitudinal view. E. SMNH X 4620, sample 3/65; lateral (E1) and dorsal (E2) views. H. SMNH X 4624, sample 7/31.5; longitudinal view. F. Conotheca circumflexa Missarzhevsky in Rozanov et al., 1969, SMNH X 4629; F1, oblique longitudinal view showing transversal section; F2, longitudinal view. G, J. Linevitus? minutus Missarzhevsky in Rozanov and Missarzhevsky, 1966. G. SMNH X 4657, sample 7/31.5; lateral (G1), dorsal (G2), and apertural (G3) views. J. SMNH X 4658, sample 7/70; ventral side with growth increments. →