Barskovia hemisymmetrica Golubev, 1976

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 : 351-353

Barskovia hemisymmetrica Golubev, 1976

Fig. 24A–C, F, G.

Material.—17 internal moulds, including figured SMNH Mo182284–182286 and 182289, from samples 1/26.3, 1/29.2, 1/6 (section 1, Fig. 4 ) and K2/21, K2/50, 3/20.5 (section 3, Fig. 3), Kugda-Yuryakh (1/6) and Medvezhya formations, Anabar Uplift and SMNH Mo182290 from sample 1571/2, from Leningradskaya River, Tajmyr, Siberian Platform, Russia. Correlated with Cambrian Stage 2.

Description.—Shell up to 1.3 mm in diameter, sinistrally coiled, up to 1.5 moderately expanding whorls regularly with broadly oval, nearly circular cross-section. The inner shell surface (as replicated on the internal moulds) is smooth, conch wall transected by dense network of orthogonal canals.

Remarks.—Other such sinistral forms, as Nomgoliella Missarzhevsky, 1981 , are characterized by a more tightly coiled shell and ovaliform aperture, whereas internal moulds of Nekolenia Vasil’eva, 1998, are more loosely coiled. This may be a preservational difference between shells and internal moulds. Parkhaev (2002: 34) synonymized Barskovia with Nekolenia, and both genera can probably be also synonymized with Nomgoliella . Synonymization can be justified by the facts that more and less tightly coiled forms such as Khairkhania rotata and K. evoluta appear in Khairkhania (see above) and that the shells appear to be more tightly coiled than their internal moulds. Although a circular aperture is considered diagnostic for Barskovia , broadly oval outlines are present in both B. hemisymmetrica and Philoxenella spiralis in our material (Fig. 23).

Stratigraphic and geographic range.— Upper Fortunian and Cambrian Stage 2 of the Siberian Platform and probably western Mongolia (depending on proposed synonymies).

→ Fig. 21. Mollusc Aldanella attleborensis (Shaler and Foerste, 1888) , internal moulds, from early Cambrian Medvezhya Formation, Kotuj River, western Anabar Uplift, Siberia, Russia; samples 3/10 (A, C–F), K2/22 (B), and K2/24 (G), section 3. A–G. SMNH Mo182265–182271, respectively. A 1, C, D 1, E, F, G 1, adapical side; A 2, D 2, apertural views; B, oblique adapical view; G 2, apex enlarged. Scale bar 50 μm (G 2), 500 μm (A–F), 1 mm (G 1).

Figures

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.