Pelagiella sp. 1

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 : 436-438

Pelagiella sp. 1

Figs. 17 , 18 , 19A .

Material.—Nine calcium phosphatic internal and external moulds, including figured SMNH Mo 167679–167685 from sample 7/70 and six moulds from sample K1-3B. Uppermost Emyaksin Formation, Bol’shaya Kuonamka and Malaya Kuonamka rivers; Calodiscus -Erbiella Zone, lower Botoman Stage.

Description.—Very rapidly expanding shell consisting of up to ¾ whorl. Aperture wide, asymmetrical, with pinched and strongly drawn-out peripheral side ( Figs. 17A 3 , 19A 2 ). The peripheral surface of the whorl is rounded and distally extended, separated from the spiral and umbilical surfaces by an angle.The spiral surface of the whorl is depressed and concave, whereas the umbilical surface is convex ( Figs. 17A 1 , 19A 1 ). Ornamentation of the shell exterior consists of fine radial striations, 1–3 μm wide, and transversal, narrow ribs. Distance between ribs up to ~50 μm ( Fig. 18 ). The ornamentation of ribs fades out towards the periphery of the

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whorl ( Fig. 18B ). A sinusoidal course of ribbing follows the relief of the umbilical and upper spiral surfaces.

Remarks.—The fossils described here are generally most similar to Pelagiella madianensis ( Zhou and Xiao, 1984) illustrated in Gravestock et al. (2001: pl. 46), but the ornamentation of P. madianensis consists of granules arranged in spiral rows (Parkhaev in Gravestock et al. 2001). The ornamentation of Pelagiella sp. 1 herein consists of comarginal ribs similarly spaced as in the middle Cambrian Pelagiella sp. cf. Costipelagiella zazvorkai from the overlying Kuonamka Formation ( Kouchinsky et al. 2011), but they do not appear to extend to the periferal portion of the whorl. Pelagiella sp. 1 differ from Pelagiella sp. 2 described below in having a strongly extended peripheral part of the aperture and whorl, a more depressed spiral side and more rapidly expanding whorl. Unlike other species of Pelagiella , Pelagiella sp. 1 has a strongly extended peripheral part of the aperture.

Figures

Fig. 17. Pelagiellid mollusc Pelagiella sp. 1 internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/70. A. SMNH Mo 167679; sub-apical (A1), spiral side (A2), and apertural (A3) views; A4, close-up of A2, striation on the surface of internal mould. B. SMNH Mo 167680; umbilical side (B1) and apertural (B2) views. C. SMNH Mo 167681; oblique umbilical (C1) and umbilical side (C2) views. D. SMNH Mo 167682; umbilical side (D1), spiral side (D2), abapertural (D3), and oblique abapertural (D4) views. Scale bar 1200 μm, except A4, 300 μm.

Fig. 18. Pelagiellid mollusc Pelagiella sp. 1, external mould of the outer shell surface, from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/70. A. SMNH Mo 167683, sinusoidal course of transverse ribs of the shell exterior. B. SMNH Mo 167684; B1, transversal ribs fade out distally; B2, close-up of B1. Scale bar: A, B1, 300 μm; B2, 120 μm.

Fig. 19. Comparison of internal moulds of two pelagiellid molluscs Pelagiella sp. 1 (A) and Pelagiella sp. 2 (B, C) from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/70. A. SMNH Mo 167685; sub-apical (A1), apertural (A2), and spiral side (A3) views. B. SMNH Mo 167686; sub-apical (B1), apertural (B2), and spiral side (B3) views. C. SMNH Mo 167687; sub-apical (C1), umbilical side (C2), apertural (C3), and spiral side (C4) views. Scale bar 600 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.

Fig. 2. Derivation of fossiliferous samples (arrows with numbers) and chemostratigraphic correlation of sections 1 and 3 (Malaya Kuonamka River) with sections 6 and 7 (Bol’shaya Kuonamka River). Correlation with carbon isotope features II–VII from stratotypes of south-eastern Siberian Platform (Brasier et al. 1994) according to Kouchinsky et al. (2001). Note that the zero level in all sections corresponds to the water level above which the sections were exposed in 1996. Description of fauna from the lower part of the Emyaksin Formation in complementary outcrops at the Bol’shaya Kuonamka River (below that zero level) will be published elsewhere. Fauna from the overlying Kuonamka Formation is published by Gubanov et al. (2004) and Kouchinsky et al. (2011).

Fig. 1. Map showing location of sections 1, 3, Malaya Kuonamka River; 6, 7, Bol’shaya Kuonamka River.