Enigmaconus, Mackinnon, 1985

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 : 436

Enigmaconus ? pyramidalis Kouchinsky and Vendrasco sp. nov.

Figs. 15 , 16 .

Etymology: From pyramid, referring to the shape of the internal moulds. Type material: Holotype: SMNH Mo 167676, internal mould ( Fig. 15 ). Paratypes: SMNH Mo 160422, 167677 and 167678, internal moulds ( Fig. 16 ).

Type locality: 70°43’N, 112°52.5’E (section 7 herein, or section A- 54 in Val’kov 1975), Bol’shaya Kuonamka River , eastern flanks of the Anabar Uplift, northern Siberia .

Type horizon: Uppermost Emyaksin Formation (sample 7/70), Calodiscus - Erbiella Zone, lower Botoman Stage .

Material.— Twenty-eight calcium phosphatic internal moulds, including four figured SMNH Mo 160422, 167676–167678 from the type horizon (sample 7/70) and one specimen from sample K1-3B .

Diagnosis.—Bilaterally symmetrical, nearly orthoconic shells with a large centrally placed apex and wide oval aperture with a broad sub-apical sinus. A narrow cleft (cast of pegma-like structure) separates a flattened smooth sub-apical portion from the rest of the mould. The surface of the internal moulds is covered with somewhat inclined and scaly polygons.

Description.—Orthoconic shell, with a broad centrally placed apex and wide oval aperture. A deep narrow pegma-like protrusion (reflected by a cleft on the internal mould) projects ventrally ( Fig. 16A 3 ) and separates a flattened portion from the rest of the shell. Aperture with a broad sub-apical sinus. Almost the entire surface of the internal mould is covered with flattened convex polygons, ca. 20 μm wide, that gradually become smooth towards the top of the mould.

Remarks.—The fossil is discussed as Enigmaconus sp. by Kouchinsky (2000a: 133, fig. 12).

Stratigraphic and geographic range.—Known only from its type locality and horizon.

Figures

Fig. 15. Mollusc Enigmaconus? pyramidalis sp. nov., holotype, internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; SMNH Mo 167676, sample 7/70. A. Upper view. B. Close-up of A, scaly texture on internal mould. C. Lateral view. D. View on the pegma-like sub-apical structure. E. Close-up of C. F. View on the side opposite to D. Scale bar 300 μm, except B, E, 60 μm.

Fig. 16. Mollusc Enigmaconus? pyramidalis sp. nov., paratypes, internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/70. A. SMNH Mo 160422; upper (A 1) and lateral (A 2) views; A 3, view on the pegma-like structure; A 4, view on the side opposite to the pegma-like structure with shell deformation reflected on the internal mould. B. SMNH Mo 167677; upper (B 1) and lateral (B 2) views. C. SMNH Mo 167678; lateral (C ) and upper (C ) views. Scale bar 600 μm, except A , A , 200 μm.

Fig. 3. Helcionellid mollusc Yochelcionella cf. Y. greenlandica Atkins and Peel, 2004 internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/36.7. A. SMNH Mo 160417; lateral (A 1, A5), sub-apical (A 2), upper (A 3), and supra-apical (A 4) views; A 6, polygonal texture on the surface of snorkel mould. B. SMNH Mo 160418; lateral (B 1), sub-apical (B 2), and upper (B 3) views; B 4, enlargement of B 2, with a shallow groove between the snorkel and aperture; B 5, apical part of the mould with tubercles; B 6, close-up of B 5. C. SMNH Mo 167648; lateral (C 1), sub-apical (C ), and upper (C ) views. D. SMNH Mo 167649; lateral view. Scale bar 600 μm, except A , 60 μm; B , 150 μm; B , 300 μm; B , 120 μm.