Yochelcionella, Runnegar and Pojeta, 1974

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 : 425

Yochelcionella cf. Y. greenlandica Atkins and Peel, 2004

Fig. 3 .

Material.—Eight calcium phosphatic internal moulds, including four figured specimens SMNH Mo 160417, 160418, 167648, and 167649 from sample 7/36.7. Emyaksin Formation, Bol’shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Shell slightly cyrtoconic, laterally somewhat compressed, with a sub-apical tube (snorkel) and overhanging apex inclined towards snorkel. In lateral view, the supra-apical margin is convex near the apex, but straightens towards the aperture. Three to four transverse folds run around the inner shell surface (as reflected by internal moulds), more prominent laterally. The aperture is oval, with a length/width ratio of ca. 1.5. The diameter of bulbous apex varies from 0.2 to 0.3 mm. Snorkel, drop-like or oval in cross-section ( Fig. 3A 2 –C 2 ), narrows distally without apparent flaring and projects upwards at ~45° with respect to the apertural plane.

Remarks.—The fossils herein were described as Yochelcionella sp. by Kouchinsky (2000a: 130, fig. 9). Comparison with shells and internal moulds of the same size belonging to twenty known species of Yochelcionella shows that the fossils described herein are most similar to the holotype of Y. greenlandica Atkins and Peel, 2004 from Stage 4 of Laurentia ( Atkins and Peel 2008: fig. 8 A–D), but the latter has a slightly more pronounced overhang of the apex and a relatively slim snorkel.

Figures

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).