Pelagiella sp. 2

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 : 438-439

Pelagiella sp. 2

Figs. 19B, C , 20 .

Material.—One calcium phosphatic internal mould, SMNH Mo 160429 and one fragmentary external mould, SMNH Mo 167688 of the same specimen, from sample K1-3B. Two internal moulds SMNH Mo 167686–167687 from sample 7/70. Uppermost Emyaksin Formation, Malaya Kuonamka and Bol’shaya Kuonamka rivers; Calodiscus -Erbiella Zone, lower Botoman Stage.

Description.—Rapidly expanding shell consisting of ¾ whorl. Aperture is asymmetrical oval ( Figs. 19B 2 , C 3 , 20A 2 ). The peripheral surface of the whorl is rounded and passes without angulation into convex spiral and umbilical surfaces of the whorl. Ornamentation of the shell exterior ( Fig. 20B ) consists of fine radial striations, 1–3 μm wide, and transversal narrow ribs that appear on the surface from ca. ½ revolution. Distance between ribs increases with growth and towards periphery of the whorl, up to ~25 μm.

Remarks.—The fossils are discussed as Pelagiella cf. subangulata by Kouchinsky (2000a: 135, fig. 14). On one of the internal moulds, a low polygonal texture is developed unilaterally as a radially extended area transitional between the peripheral and spiral surfaces of the whorl ( Fig. 20A 4 , A 5 ; Kouchinsky 2000a: fig. 14A, arrow E). The texture consists of polygons, up to 50 μm in width, with tubercles, 5–10 μm in diameter, at their centres ( Fig. 20A 4 , A 5 ).

Among other pelagiellids, the forms described herein are distinguished by the ornamentation of the shell exterior with comarginal ribs thought to be diagnostic of Costipelagiella Horný, 1964 . The ribs are twice as densely spaced as in the middle Cambrian C. zazvorkai Horný, 1964 and Pelagiella sp. cf. Costipelagiella zazvorkai from Siberia ( Kouchinsky et al. 2011), although the shell exterior was apparently similarly smooth over the initial half revolution. C. nevadense Skovsted, 2006 from Laurentia and Costipelagiella sp. cf. C. zazvorkai from the middle Cambrian of New Zealand ( MacKinnon 1985) are more laterally compressed, but have similarly spaced ribs, which do not extend, however, to the peripheral portion of the whorl in the former form.

Figures

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. 20. Pelagiellid mollusc Pelagiella sp. 2 from lower Cambrian Emyaksin Formation, Malaya Kuonamka River, Anabar Uplift, Siberia; sample K1-3B. A. SMNH Mo 160429, internal mould; spiral side (A1), apertural (A2), sub-apical (A3), and abapical (A4) views; A5, polygonal network on the spiral side arrowed in A4). B. SMNH Mo 167688; B1–B3, external mould of the outer shell surface. Scale bar: A1–A3, 600 μm; A4, B1, 300 μm; and A5, B2–B3, 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. 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. 4. Helcionellid mollusc Bemella? sp. internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River,Anabar Uplift, Siberia; sample 7/36.7. A. SMNH Mo 167650; lateral (A1), apertural (A2), and sub-apical (A3) views. B. SMNH Mo 167651; lateral (B1) and upper (B2) views. C. SMNH Mo 167652; apertural (C1), lateral (C2), and sub-apical (C3) views. D. SMNH Mo 167653; D1, close-up of apertural margin in D3; upper (D2) and lateral (D3) views. Scale bar 1200 μm, except D1, 300 μm.

Fig. 5. Helcionellid mollusc Parailsanella sp. 1 internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/36.7. A. SMNH Mo 160415; lateral (A 1), sub-apical (A 2), and upper (A 3) views. B. SMNH Mo 167655; lateral (B 1), sub-apical (B 2), and upper (B 3) views. C. SMNH Mo 167656; lateral (C 1), upper (C 2), and sub-apical (C 3) views. D. SMNH Mo 160416; lateral (D 1), oblique sub-apical D ), and upper (D ) views. Scale bar 600 μm.