Pararaconus sp.

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 : 433-434

Pararaconus sp.

Fig. 13 .

Material.—A single phosphatic internal mould SMNH Mo 160421 from sample 7/70. Uppermost Emyaksin Formation,

→ Fig. 10. Helcionellid mollusc Mackinnonia anabarica Parkhaev, 2005 internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; SMNH Mo 167665, sample 7/70. A. Close-up of E, texture on the surface of internal mould in sub-apical region. B. Sub-apical view. C. Close-up of A, texture at the base of sub-apical train. D. Supra-apical view. E. Lateral view. F. Upper view. G. Texture on the surface of comarginal folds. Scale bar: A, 120 μm; B, F, 600 μm; C, 30 μm; D, E, 300 μm; G, 60 μm.

Bol’shaya Kuonamka River; Calodiscus -Erbiella Zone, low- er Botoman Stage.

Remarks.—The fossil represents a slightly laterally compressed (length/width ratio of the aperture ca. 1.4) orthoconic internal mould with a slightly displaced apex. The surface of the mould is smooth and covered with polygons, 10–20 μm wide ( Fig. 13C, D, F ; see also description of surface textures in Kouchinsky 2000a). The distinguishing characters of both known species of Pararaconus , P. staitorum Runnegar in Bengtson et al., 1990 (type species) and P. paradoxus Parkhaev in Gravestock et al. 2001 from the Abadiella huoi Zone of Australia relate to the presence/absence (respectively) of lateral depressions (buttresses on internal moulds) near the aperture (Parkhaev in Gravestock et al. 2001). The ventral margin of the fossil herein is jagged and the original aperture margin appears to be broken off. Thus without knowing these character states we cannot determine with certainty to which species of Pararaconus this fossil belongs. The fossil is discussed as Obtusoconus sp. by Kouchinsky (2000a: 131, fig. 11).

Figures

Fig. 13. Helcionellid mollusc Pararaconus sp. internal mould from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; SMNH Mo 160421, sample 7/70. A. Lateral view. B. Sub-apical view. C. Close-up of D. D. Polygonal texture on the apical surface of internal mould. E. Upper view. F. Pits on the surface of mould near the aperture. Scale bar: A, B, E, 600 μm; D, 120 μm; C, F, 30 μm.