Parailsanella 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 : 425-427

Parailsanella sp. 1

Figs. 5 , 6 .

Material.—Thirty-nine calcium phosphatic internal moulds, including four figured specimens SMNH Mo 160415, 160416, 167655, and 167656 from sample 7/36.7. Emyaksin Formation, Bol’shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Slightly cyrtoconic, laterally somewhat compressed shells. Supra-apical surface evenly convex; sub-apical surface concave. Aperture broadly oval, with length/ width ratio 1.7 to 2.0. Low and broad comarginal folds, faint on small (juvenile) specimens, are situated on the inner shell surface (as reflected by internal moulds). The apex is ~ 0.3 mm in lateral view, bulbous and pinched at its base.

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Remarks.—The surface of internal moulds, excluding apical area, is commonly covered with pits ~5 μm in diameter and 10–20 μm apart ( Fig. 6A ). In one case, a juvenile mould preserves a shallow polygonal texture with centres of polygons 10–20 μm apart ( Fig. 6B ). Parailsanella sp. 1 is very similar to Capitoconus inclinatus Skovsted, 2004 from Stage 4 of Greenland, but the latter form has a relatively larger globose protoconch and laterally more compressed shell. The fossils described herein are discussed as Ilsanella sp. by Kouchinsky (2000a: 130, fig. 8).

Parailsanella sp. 2

Fig. 7.

Material.—Single calcium phosphatic internal mould SMNH Mo 167658 from sample 7/70. Uppermost Emyaksin Formation, Bol’shaya Kuonamka River; Calodiscus -Erbiella Zone, lower Botoman Stage.

Description.—Slightly cyrtoconic and laterally compressed shell. Aperture is narrow oval, with length/width ratio of 3. The apex is bulbous, ~ 0.3 mm in lateral view. The inner shell surface (as reflected by internal moulds) is covered with comarginal folds.

Remarks.—The form is similar to Capitoconus artus Skovst- ed, 2004 from Stage 4 of Greenland. The latter is preserved, however, with remains of phosphatised shell, with prominent sharp transverse ribs and a more distinct apex (Skovsted 2004: fig. 5). Parailsanella sp. 2 is more compressed laterally and has more distinct narrower comarginal folds than Parailsanella sp. 1 herein.

Genus Figurina Parkhaev in Gravestock et al., 2001

Type species: Figurina figurina Parkhaev in Gravestock et al., 2001; Abadiella huoi Zone , lower Cambrian; Horse Gully section, Yorke Peninsula, Australia.

Figures

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.

Fig. 6. Helcionellid mollusc Parailsanella sp. 1 internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River,Anabar Uplift, Sibe- ria; sample 7/36.7. A. SMNH Mo 167656; close-up of Fig. 5C 2; A 1, pits on the surface of internal mould; A 2, close-up of A 1. B. SMNH Mo 160416; close-ups of Fig. 5D; B 1 , lateral view, polygonal texture on the mould; B 2 , polygons, A B close-up of B 1. Scale bar: A 1, 120 μm; A 2 and B 2, 30 μm; B 1, 300 μm. margin and continuous with the pegma-like structure (Figs. 9C, 11C). Remarks.—The surface of the moulds is covered with 10–40 μm wide, blunt tubercles and bears reticulate microornamentation best seen on the convex parts (folds) of the mould. The tubercles tend to be situated in comarginal rows, similar to what occurs in Mellopegma (Vendrasco et al. 2011b). Where concentric folds are developed, the tubercles are concentrated D along their surface. Reticulation is formed by shallow concave C polygons, ca. 20 μm wide. On the surface of concentric folds, the polygons become deeper and more variable in size, with a diameter typically ranging from 2–10 μm (Fig. 10G). The microornamentation flattens out in the subapical area. In addition to the polygonal texture, some specimens of Mackinnonia show imprints of a laminar shell microstructure in the concave bands between folds on the internal mould (Fig. 10C, G). Fig. 7. Helcionellid mollusc Parailsanella sp. 2 internal moulds from low- er Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar As demonstrated by Runnegar (in Bengtson et al. 1990: fig. Uplift, Siberia; SMNH Mo 167658, sample 7/70; lateral (A), sub-apical 159g), the troughs on internal moulds of Mackinnonia corre- (B), and oblique upper (C) views. D. Close-up of A, showing pits on the spond to areas where the shell was thicker, and where the inner surface of internal mould. Scale bar 1200 μm, except D, 300 μm. layer of shell microstructure would be preserved, whereas the