Securiconus cf. simus Jiang, 1980

Kouchinsky, Artem, Bengtson, Stefan, Landing, Ed, Steiner, Michael, Vendrasco, Michael & Ziegler, Karen, 2017, Terreneuvian stratigraphy and faunas from the Anabar Uplift, Siberia, Acta Palaeontologica Polonica 62 (2), pp. 311-440 : 333-334

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https://doi.org/ 10.4202/app.00289.2016

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https://treatment.plazi.org/id/03AD87A8-FF8D-6D30-FF06-F938638F8694

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scientific name

Securiconus cf. simus Jiang, 1980
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Securiconus cf. simus Jiang, 1980 View in CoL

Figs. 7D, F View Fig , 8D View Fig .

Material.—About 50 internal moulds, including figured SMNH Mo160411, 181940, and 181941, from samples 1/45, 1/29.2 (section 1, Fig. 4 View Fig ), K2/12, 3/6.5 (section 3, Fig. 3), 5/10, 5/10B, 5a/1.2 (sections 96-5 and 96-5a, Fig. 2 View Fig ), Medvezhya and lower Emyaksin formations, Anabar Uplift, Siberian

Platform, Russia. Correlated with the upper Fortunian and lower part of Cambrian Stage 2.

Description.—Bilaterally symmetrical, cyrtoconic (up to 1/2 revolution), rapidly expanding shells. Aperture an elongate oval, with length/width ratio ca. 2. Apical part curved with no obvious protoconch boundary and projects far beyond the aperture margin. Subapical side short. Supra-apical side (dorsum) long and evenly convex. Inner surface of the shell (as replicated by internal moulds) smooth or with irregularly spaced and low comarginal folds that cross the dorsum.

Remarks.—The internal moulds are similar in morphology to those identified by Missarzhevsky (1989: pl. 10: 5, 6) as Securiconus simus from lower Medvezhya Formation on the Kotuj River. It is not however clear if the type material of Securiconus simus Jiang, 1980 , from the middle Meishucunian Stage of South China represents the same form. From probably the same locality, the holotype of Securiconus costulatus Missarzhevsky, 1989 , was described Missarzhevsky 1989: pl. 7: 5, 9, sample M421/94) and the topotype material was illustrated by Parkhaev (in Rozanov et al. 2010: pl. 33: 1–4). The latter form is wider and has more regularly and distinctly expressed transverse ribs on internal moulds. Securiconus cf. simus differs from Oelandiella korobkovi in being generally lower and cyrtoconic (up to a half revolution) rather than coiled (up to one revolution or more)

and having a more tapered apical part. Bemella spp. conchs are significantly wider and have a more rounded apex. Forms from the Nemakit-Daldyn Formation described as S. incertus Bokova, 1985 (Kouchinsky 2000: fig. 6G–L), are similar to Bemella ? sp. (Fig. 9A–C, F, G) or Oelandiella ? sp. (Fig. 9D, E) illustrated herein, and their affinity is uncertain.

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