Tetratheca clinisepta ( Sysoev, 1960 )

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 : 447

Tetratheca clinisepta ( Sysoev, 1960)

Fig. 26F–H.

Material.—Nineteen phosphatised conchs and internal moulds, including SMNH X 4641 and 4642, from sample 7/70 and one additional internal mould, SMNH X 4643, from sample B-247. Uppermost Emyaksin Formation, Bol’shaya Kuonamka River; Calodiscus -Erbiella Zone, lower Botoman Stage.

Description.—Straight conch, rectangular in cross-section and covered with faint straight transversal growth lines. The four sides of the conch are slightly depressed and separated from each other by a longitudinal keel. The apical angle is <5°. A smooth septum, concave towards the apex, is preserved in one of the fragments (Fig. 26 G 2).

Remarks.—In one of the internal moulds, the apertural end is a little flared (Fig. 26 F). Phosphatised casts of pore channels perpendicular to the wall are preserved between inner and outer phosphatic crust delimiting the now-dissolved wall (Fig. 26 G, H).

Stratigraphic and geographic range.—Atdabanian and lower Botoman Stages, Siberian Platform.

→ Fig. 23. Hyolith conchs from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia. A, B. Majatheca tumefacta Missarzhevsky in Rozanov et al., 1969, conchs with phosphatised microstructure of the walls; sample 7/70. A. SMNH X 4605 (Mo 160444 in Kouchinsky 2000b: fig. 3); A 1, close-up of the ventral inner surface at the aperture; A 2, close-up of the dorsal outer surface at the aperture; A 3, dorsal view. B. SMNH X 4606 (Mo 160445 in Kouchinsky 2000b: fig. 3); apertural ( B 1), dorsal ( B 3), and lateral ( B 4) views; B 2, close-up of the dorsal surface with longitudinal striae. C – G. Orthothecida ? sp. indet. 1, conchs with phosphatised microstructure of the walls; sample 7/70. C. SMNH X 4613; ventral view. D. SMNH X 4614; dorsal view. E. SMNH X 4615; dorsal view. F. SMNH X 4616 (Mo 160448 in Kouchinsky 2000b: fig. 4); dorsal ( F 1) and lateral ( F 2) views. G. SMNH X 4617; apertural ( G 1) and ventral ( G 2) views. H, I. Conchs of apparently the same species, Orthothecida ? sp. indet. 1, with phosphatised microstructure of the walls; sample 7/33. H. SMNH X 4618 (Mo 160449 in Kouchinsky 2000b: fig. 4); ventral inner surface. I. SMNH X 4619 (Mo 160447 in Kouchinsky 2000b: fig. 4); lateral ( I 1) and dorsal ( I 2) views. Scale bar: A 1, A 2, B 2, 150 μm; A 3, B 1, B 3, B 4, H, I, 600 μm; C – G, 1200 μm.