Orthothecida, Marek, 1966

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 : 451

Orthothecida ? sp. indet. 1

Fig. 23C–I.

Material.—Twenty-five phosphatised conchs, including SMNH X 4613–4617, from sample 7/70, fourteen from sample K1-3B and three from sample 1/7.8; uppermost Emyaksin Formation, Bol’shaya Kuonamka and Malaya Kuonamka rivers; Calodiscus -Erbiella Zone, lower Botoman Stage. Several conch fragments, including SMNH X 4618 and 4619, from samples 7/33 and 7/31.9; Emyaksin Formation, Bol’shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

I. Fragment of a hyolith conch with phosphatised microstructure, SMNH X 4664, sample 6/18; dorsal (I 1), lateral (I 2), and apertural (I 3) views; I 4, close-up of the apical end. K. Hyolithida sp. indet. 2, pyritised conch, with operculum attached, SMNH X 4659, sample 1/9; oblique apertural (K 1) and lateral (K 2) views. Scale bar 600 μm, except A, B, I 4, 300 μm; I 1 –I 3, J, 1200 μm.

Description.—Almost straight or slightly dorso-ventrally curved conchs, up to several mm long. The dorso-lateral part is semi-circular in transverse section, passing via rounded margins into a flattened ventral part. In dorsal view, the apical angle is ca. 15°. The conch wall includes two horizontal layers of fibres (microstructure is described in detail by Kouchinsky 2000b). Fibres in the inner layer run transversely on the dorsal side and branch towards the aperture laterally (Fig. 23F, H). The fibres may branch and anastomose intensively and are less regularly arranged on the ventral side of the conch, although their typical orientation there is transverse. The outer layer consists of longitudinally oriented fibres on the dorsal side (Fig. 23I).

Remarks.—The hyoliths are discussed as allathecid(?) hyolith, microstructural type B, forms 1 and 2 by Kouchinsky (2000b: 70, figs. 4, 5).