Mackinnonia anabarica Parkhaev, 2005

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 : 427-431

Mackinnonia anabarica Parkhaev, 2005

Figs. 9–11 .

Material.— Twenty two calcium phosphatic internal moulds, including six figured specimens SMNH Mo 160419, 160420, 167662–167665 from sample 7/70 and fifteen specimens from sample K1-3 B. Uppermost Emyaksin Formation, Bol’shaya Kuonamka and Malaya Kuonamka rivers; Calodiscus -Erbiella Zone, lower Botoman Stage .

Description.—Cyrtoconic, laterally compressed shell, with length/width ratio 1.8 to 2.5. The apex is displaced to near the apertural margin and hooked. The aperture is drop-like

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in dorsal profile, narrowing towards the subapical margin. The subapical margin of the aperture curves upwards and forms a train. The protoconch is not distinct on the internal mould. The inner shell surface (as reflected by internal moulds) bears concentric folds, which are most prominent near the apertural margin of larger specimens and tend to disappear in the subapical region. Smaller (juvenile?) shell has only one fold on the inner surface, near the apertural convex folds correspond to thin regions of shell where the outer layer of shell microstructure could be preserved. Thus, Mackinnonia appears to have had the common molluscan shell microstructure pattern of an outer prismatic shell layer (represented by polygons) and an inner laminar shell layer. This pattern characterises modern monoplacophorans, as well as Mellopegma and other stenothecids ( Vendrasco et al. 2011b).

Parkhaev (in Gravestock et al. 2001) reassigned Mellopegma rostratum Zhou and Xiao, 1984 to Mackinnonia Runnegar in Bengtson et al., 1990. According to Parkhaev in Gravestock et al. 2001) and Parkhaev (2005), M. obliqua Landing and Bartowski, 1996 from Stage 4 of Laurentia, as well as Bemella costa Zhou and Xiao, 1984 and B. anhuiensis Zhou and Xiao, 1984 from North China, should be regarded as junior synonyms of M. rostrata ( Zhou and Xiao, 1984) . However, the illustrations of M. rostrata (as well as B. costa and B. anhuiensis ) are not detailed, and it appears that individuals of this species tend to lack the more frequent and prominent internal ridging that characterises M. davidi . M. rostrata is also curved more than M. davidi , and for these reasons we view the synonymy as questionable. The ornamentation of the internal mould of Mackinnonia anabarica from the lower Botoman Stage of Siberia with tubercles would be the only reliable difference from otherwise similar M. davidi Runnegar in Bengtson et al., 1990 from Cambrian

Series 2 of South Australia ( Bengtson et al. 1990; Gravestock et al. 2001) and M. rostrata from Stage 4 of South and North China ( Zhou and Xiao 1984; Feng et al. 1994) and lower Stage 5 of the Mediterranean region ( Spain; Wotte 2006). It is, however, hard to confirm or deny the lack of tubercles in the original M. rostrata illustrated by Xiao and Zhou (1984) from China. M. rostrata and M. anabarica appear to share the same higher degree of coiling than that illustrated in M. davidi by Bengtson et al. (1990: fig. 159), whereas M. davidi has more frequent and prominent internal ridging.

The fossils described herein were discussed as Mackin-

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nonia sp. by Kouchinsky (2000a: 131, fig. 10). Mackinnonia anabarica Parkhaev, 2005 and Leptostega hyperborea Parkhaev, 2005 (see below) were originally described from sample B-247, which is from an unknown locality, according to Parkhaev (2005). However, the same sample available in the material of Vladimir V. Missarzhevsky and collected by Larisa G. Voronova derives from near the mouth of the Ulakhan-Tyulen Brook, Bol’shaya Kuonamka River (as written on a label). Thus, it is equivalent to section 6 herein, which should be considered the type locality for those two species.

Stratigraphic and geographic range.—Lower Botoman Stage, Siberian Platform.

Figures

Fig. 9. Helcionellid mollusc Mackinnonia anabarica Parkhaev, 2005 internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/70. A. SMNH Mo 167662; lateral (A 1), sub-apical (A 2), and upper (A 3) views. B. SMNH Mo 167663; lateral (B 1), sub-apical (B 2), upper (B 3), and supra-apical (B 4) views. C. SMNH Mo 160419; supra-apical (C 1), lateral (C 2), sub-apical (C 3), and upper (C 4) views. D. SMNH Mo 167664; supra-apical (D ), upper (D ), lateral (D ), and sub-apical (D ) views. Scale bar 600 μm.

Fig. 11. Helcionellid mollusc Mackinnonia anabarica Parkhaev, 2005 internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/70. A. SMNH Mo 160420; A1, lateral view; A2, close-up of A1, microtexture on the surface of internal mould, detailed. B. SMNH Mo 167665 (see also Fig. 10); lateral view. C. SMNH Mo 160419 (see also Fig. 9C); C1, lateral view; C2, close-up of C1, polygons on the surface of internal mould. Scale bar: 300 μm, except A2, 120 μm; C2, 60 μm.