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

Securiconus cf. simus Jiang, 1980

Figs. 7D, F , 8D .

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

Figures

Fig. 7. Helcionellid molluscs, internal moulds, from early Cambrian Medvezhya Formation, Kotuj River, section 1, western Anabar Uplift, Siberia, Russia; samples 1/50 (A, B), 1/46 (C, F), 1/45 (D), and 1/29.5 (E). A–C, E. Oelandiella korobkovi Vostokova, 1962, SMNH Mo181937–181939 (A–C, respectively), SMNH Mo181942 (E). D, F. Securiconus cf. simus Jiang, 1980, SMNH Mo181940 and 181941, respectively. A1–D1, E, F, lateral views; A2, B3–D3, apertural views; A4–C4, D2, subapical views; A3, B2, C2, upper views. Scale bar 1 mm.

Fig. 8. Helcionellid molluscs, internal moulds, from early Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, section 96-5a, eastern Anabar Uplift, Siberia, Russia; sample 5a/1.2. A, B. Oelandiella korobkovi Vostokova, 1962, SMNH Mo181943 and 181944, respectively. C, E. Oelandiella korobkovi? Vostokova, 1962, SMNH Mo181945 and 181947, respectively. D. Securiconus cf. simus Jiang, 1980, SMNH Mo160411 (see also Kouchinsky 2000a: fig. 6A–F). A1, B, C2, D1, E2, lateral views; A2, E1, D2, subapical views; A3, C3, apertural views; E3, D3, upper views; C1, close-up of subapical area with polygons. Scale bar 125 μm (C1) and 1 mm (A, B, C2, C3, D, E).

Fig. 4. The δ13Ccurve for Kotuj River section 1and sampled horizons with fossiliferous samples (arrows with numbers). Shift in stratigraphic column carb at Nemakit-Daldyn–Medvezhya formations contact indicates position of sequence boundary. Stratigraphic ranges and first appearances of fossils (see SOM 1): 2, Oelandiella korobkovi; 3, Securiconus cf. simus; 4, Bemella jacutica?; 8, Anabarella plana; 9, Watsonella crosbyi; 10, Pseudoyangtzespira selindeica; 11, Aldanella crassa; 13, Aldanella attleborensis; 15, Philoxenella spiralis; 16, Barskovia hemisymmetrica; 18, Khairkhania cf. evoluta; 19, Ceratoconus striatus; 21, Salanyella cf. costulata; 23, Purella cristata; 25, Yunnanopleura sp.; 26, Obscurania auriculata; 29, Protoconus cf. crestatus; 32, Siphogonuchites triangularis; 34, Lopochites sp.; 37, first appearance of Halkieria sp.; 39, first appearance of Chancelloria sp.; 40, Tianzhushanella sp.; 42, Khetatheca cotuiensis; 46, Turcutheca sp.; 48, Orthothecid operculum type A; 52, Fomitchella infundibuliformis; 57, Hyolithellus sp.; 62, Selindeochrea missarzhevskyi; 63, Selindeochrea ternaria; 64, Anabarites trisulcatus; 67, Anabarites kelleri; 68, Anabarites biplicatus; 71, Anabarites hexasulcatus; 74, Tiksitheca licis; 75, Cambrotubulus decurvatus; 76, Platysolenites antiquissimus; 77, Blastulospongia sp.

Fig. 2. Composite δ13Ccurve for the Bol’shaya Kuonamka River sections 96-4, 5, and 5a and sampled horizons with fossiliferous samples (arrows with carb numbers). Arrow 4/4 indicates “marker bed with angustiochreids” (Val’kov 1975; Kouchinsky et al. 1999). For section 96-6 see Kouchinsky et al. (2001, 2015a). Shift in stratigraphic column at Manykay–Emyaksin formations contact indicates position of sequence boundary. Stratigraphic ranges and first appearances of fossils (see SOM 1): 1, first appearance of helcionellid molluscs; 2, Oelandiella korobkovi; 3, Securiconus cf. simus; 6, Obtusoconus vostokovae; 7, Obtusoconus cf. mirabilis; 8, Anabarella plana; 13, Aldanella attleborensis; 14, Aldanella cf. sibirica; 22, Purella antiqua; 24, Yunnanopleura accurata; 34, Lopochites sp.; 35, Tianzhushania longa; 37, first appearance of Halkieria sp.; 38, Chancelloria symmetrica; 42, Khetatheca cotuiensis; 44, Conotheca cf. mammilata; 46, Turcutheca sp.; 47, Cupitheca sp.; 50, Orthothecid operculum type C; 51, Protohertzina anabarica; 52, Fomitchella infundibuliformis; 53, Fomitchella aff. infundibuliformis; 54, Fomitchella acinaciformis; 55, Fomitchella aff. acinaciformis; 56, Hyolithellus vladimirovae; 57, Hyolithellus sp.; 60, Selindeochrea cf. tecta; 61, Selindeochrea tripartita; 62, Selindeochrea missarzhevskyi; 63, Selindeochrea ternaria; 64, Anabarites trisulcatus; 65, Anabarites latus; 66, Anabarites natellus; 67, Anabarites kelleri; 68, Anabarites biplicatus; 69, Anabarites tristichus; 70, Anabarites compositus; 71, Anabarites hexasulcatus; 72, Anabarites hariolus; 73, Aculeochrea rugosa; 74, Tiksitheca licis; 75, Cambrotubulus decurvatus; 76, Platysolenites antiquissimus.