Aldanella cf. sibirica Parkhaev and Karlova, 2011

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

Aldanella cf. sibirica Parkhaev and Karlova, 2011

Fig. 23.

Material.—About 80 internal moulds, including figured SMNH Mo182273–182283, from samples 5a/17.5, 5a/18.5, 5a/34.5, 5a/34.75 (section 96-5a, Fig.2 ), Emyaksin Formation from the Anabar Uplift, Siberian Platform, Russia. Correlated with Cambrian Stage 2.

Description.—Dextrally coiled shell up to 1.5 mm in diameter, up to 2.5 whorls, slowly expands, with moderately projecting spire. Apertural cross-section broadly oval. Upper and lower surfaces of the last whorl convex, lateral periphery gently rounded. Protoconch not separated on the internal molds. Inner shell surface (as replicated on internal moulds) smooth, initial part with rounded polygonal impressions 20–40 μm in diameter (Fig. 23H 1), apex gently pointed in some specimens.

Remarks.— Aldanella sibirica was first described by Parkhaev and Karlova (2011) from basal Erkeket Formation, section near Boroulakh on the Olenyok River, Olenyok Uplift of northern Siberian Platform, from the Dokidocyathus regularis Zone of the Tommotian Stage. The species shows variation in cross-sectional shape of the whorl, from narrower elliptical to broadly and irregularly oval. Such variation is consistent with morphological variability in internal moulds of Aldanella described by Landing (1988) from Avalonian North America. Internal moulds of A. sibirica are morphologically similar to those of A. attleborensis , as pointed out by Parkhaev and Karlova (2011: 1151), who mentioned rare “morphologically intermediate specimens”. A distinct spiral ornamentation of the shell exterior of A. sibirica (Parkhaev and Karlova 2011: pl. 11: 9) is absent in A. attleborensis ( Fig. 22A herein). In the latter species, a fine spiral striation occurs on the surface of internal moulds (Fig. 20B, D). Spiral ornamentation is not developed in our material of Aldanella cf. sibirica represented by internal moulds. These are similar to specimens of A. sibirica in their broad cross-section of the last whorl and are relatively smaller than A. attleborensis .

Figures

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.

Fig. 22. Mollusc Aldanella attleborensis (Shaler and Foerste, 1888), SMNH Mo182272 (cast of the outer shell surface) from early Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, eastern Anabar Uplift, Siberia, Russia; sample 5/10 from section 96-5. A. Adapical side with transverse sculpture. B. Protoconch enlarged. Scale bar 250 μm (A) and 125 μm (B).