Bemella jacutica

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 : 336-338

Bemella jacutica ? (Missarzhevsky in Rozanov and Missarzhevsky, 1966)

Fig. 10 .

Material.—Ten phosphatic internal moulds, including figured SMNH Mo181955–181958, from samples 1/50 and 1/6 section 1, Fig. 4 ), 3/12, 3/12.2, and K2/25 (section 3, Fig. 3), Kugda-Yuryakh (1/6) and Medvezhya formations, western Anabar Uplift, Siberian Platform, Russia. Correlated with the upper Fortunian and lower part of Cambrian Stage 2.

Description.—Bilaterally symmetrical, low cyrtoconic coiled less than 1/2 revolution), rapidly expanding shells.

Aperture broad oval in outline, with length/width ratio ca. 1.5 and with laterally straight margins. Apical part declined and projects beyond the apertural margin, apparent protoconch rounded and blunt, hemispherical, separated by a shallow constriction on internal mould. Subapical side short and concave, dorsum long and evenly convex. The inner surface of the shell (as replicated by internal moulds) smooth and carries widely spaced irregularly distributed low comarginal folds that cross the dorsum.

Remarks.— Bemella Missarzhevsky in Rozanov et al., 1969 was described from relatively large mechanically extracted

Fig. 11. Mollusc Helcionellida indet., internal moulds, from early Cambrian Emyaksin (A, D–J) and basal Kugda-Yuryakh (B, C) formations, Anabar → Uplift, Siberia, Russia; samples 5a/34.5 (A), 5/10 (D–F, I, J), and 5/10B (G, H), sections 96-5 and 96-5a; K2/60 (B), section 3; and K1a/67 (C), section 2. A–G, I, J. SMNH Mo181959–181965 (A–G, respectively), SMNH Mo 181966 (I), and SMNH Mo 181967 (J). H. SMNH 167610. A 1, B 4, C 2, E 2, F 3, upper; A 2, B 3, D 1, E 3, F 2, I 1, H, subapical; B 1, C 1, D 2, E 1, F 1, G, I 2, J 3, lateral; I 3, J 2, apertural views; B 2, apical area with polygons enlarged. Scale bar 125 μm (B 2), 500 μm (A, B 1, B 3, B 4, C, D 1, I, J) and 1 mm (D 2, E–G, H).

internal moulds from the Medvezhya Formation on the northwest flank of the Anabar Uplift. Bemella jacutica ? described herein has diagnostic features of the genus, but has a subdued ornamentation and probably represent small specimens of the type species B. jacutica from the same area and level. Parkhaev (in Gravestock et al. 2001) and Parkhaev and Demidenko (2010) have revised Bemella . The genus is generally similar to Igorella Missarzhevsky in Rozanov et al., 1969, which was also first described from the Medvezhya Formation from mechanically prepared, mm-sized internal moulds. According to Parkhaev and Demidenko (2010) and Missarzhevsky (1989), Bemella is distinguished by having a relatively lower shell than Igorella . Bemella jacutica ? is different from Bemella simplex Yu, 1979 , from South China described by Parkhaev and Demidenko (2010) in having a higher shell with a more elongate, hooked apical part. Helcionellida indet. are different from Bemella jacutica ? in wider shells (Fig. 11A–C) or in a more pointed hooked initial part (Fig. 11D–J).

Figures

Fig. 10. Mollusc Bemella jacutica? Missarzhevsky in Rozanov et al., 1969, internal moulds, from early Cambrian Kugda-Yuryakh (1/6) and Medvezhya formations, Kotuj River, western Anabar Uplift, Siberia, Russia; samples 3/12.2 (A), 3/12 (B), and K2/25 (D), section 3; and 1/6 (C), section 1. A–D. SMNH Mo181955–181958, respectively. A1, B2, C3, D3, subapical; A2, B1, C1, D4, lateral; A3, B3, C2, D1, upper; D2, apertural views. Scale bar 500 μm.

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.