Khetatheca cotuiensis (Sysoev, 1959a)

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 : 389

Khetatheca cotuiensis (Sysoev, 1959a)

Figs. 50A–C, F–H , 53C .

Material.—About 100 internal and external moulds, including figured SMNH X5841–5843, 5846–5848, 5866, from samples 1/45, 1/39.3, 1/29.5, 1/29.2 (section 1, Fig. 4 ), K2/21, K2/22, K2/23, K2/24, K2/25, K2/26, 3/10, 3/12, 3/12.2, 3/12.5 (section 3, Fig. 3), 5/10, 5/10B, 5a/1, 5a/1.2 (sections 96-5, 96-5a), Medvezhya and Emyaksin formations, Anabar Uplift, Siberian Platform, Russia. Correlated with the upper Fortunian Stage.

Description.—Conch gently curved and slightly longitudinally helically twisted, with subcircular to broadly oval cross-section. Outer surface covered with widely spaced, weakly to strongly developed transverse folds with flattened tops and numerous superimposed frequent growth lines ( Fig. 50A 1, A 2). Inner surface of the conch (replicated on internal mould) covered with transverse folds. Rare septa concave toward the aperture ( Figs. 50F , 53C 1, apical parts). Early growth stage (as replicated on internal moulds) with regularly spaced transverse folds separated by relatively broader interspaces ( Fig. 50G ).

Remarks.—The gently helically twisted transversely oval conchs are a diagnostic feature of the monospecific genus Khetatheca , according to Missarzhevsky (1989). The holotype came from locality M424 along the Kotuj River. Sysoev (1959b: 89) attributed these beds to the Atdabanian Stage, but Missarzhevsky (1989: 160) regarded them as sub-Tommotian or lower Tommotian. According to the chemostratigraphy of the Medvezhya and Kugda-Yuryakh formations (Fig. 1), these beds belong to lower part of Cambrian Stage 2. A single specimen of Khetatheca cotuiensis from sample M424/10 is from the type locality ( Fig. 53C ).

Stratigraphic and geographic range.— Upper Fortunian and lower part of Cambrian Stage 2 of the Siberian Platform and probably western Mongolia (Missarzhevsky 1989).

Figures

Fig. 50. Orthothecid hyoliths from early Cambrian Medvezhya Formation, western Anabar Uplift, Siberia, Russia; samples K2/21 (A), 3/12 (E), 3/12.2 B, G, H) and K2/25 (C, D, F) from section 3. A–C, F–H. Khetatheca cotuiensis (Sysoev, 1959b), calcium phosphatic internal moulds, SMNH X5841– 5843 (A–C, respectively), SMNH X5846–5848 (F–H, respectively). A1, lateral view; A2, internal mould with fragment of phosphatized wall enlarged; A3, oblique transversal view of apical end; A4, inner surface of phosphatized fibrous wall enlarged; B, C, lateral views of smaller specimens; F, G, H2, lateral; H1, oblique lateral views of larger specimens. D, E. Khetatheca cotuiensis? (Sysoev, 1959b) with wide oval cross-section and wider spaces between transverse folds than in B and C, calcium phosphatic internal moulds; SMNH X5844, 5845, respectively. D, E1, lateral views; E2, oblique transversal view of apertural end. Scale bar 25 μm (A4), 500 μm (A2, B–F), 1 mm (A1, A3, G, H1).

Fig. 53. Orthothecid hyoliths Turcutheca sp. (A, B, D–G) and Khetatheca cotuiensis (Sysoev, 1959b) (C), from early Cambrian Emyaksin (A, B, F, H, I ), Medvezhya (C, D), and basal Kugda-Yuryakh (E, G) formations, Anabar Uplift, Siberia, Russia; samples 1/22 (D), section 1; K2/60 (E, G), section 3; 5 a/34.5 (F, H, I) and 5a/34.75 (A, B), section 96-5a; M424/10 (C), section M424. A, B, D–G. SMNH X5864, 5865 (A, B, respectively), SMNH X5867– 5870 (D–G, respectively). A, B, D, E, lateral views of calcium phosphatic internal moulds with transverse septae; F1, lateral view of phosphatic coating of tube fragment with two septae; F2, oblique transverse view of septum near aperture of fragment; G, transverse view of oval septum from apical side of internal mould. C. SMNH X5866, calcium phosphatic internal mould. C1, oblique lateral view of blunt apical end with septum; C2, lateral view of mould with two transverse septae at apical end. H, I. SMNH X5871, 5872, respectively, calcium phosphatic inner coatings of the hyolith conch wall and septae. Transverse views of rounded triangular septum from the apical (H) and apertural (I) sides. Scale bar 500 μm (A, B, I), 1 mm (C–H).

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.