Selindeochrea tripartita

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 : 414-417

Selindeochrea tripartita (Missarzhevsky in Rozanov et al., 1969)

Fig. 71 .

Material.—Several hundred internal moulds, including figured SMNH X5988–5991, and one internal with external mould, SMNH X5987, from samples 3/12, 3/12.2, K2/25, K2/26 (section 3, Fig. 3), 5a/6 and 5a/9 (section 96-5a, Fig. 2 ), Medvezhya and Emyaksin formations. Correlated with the upper Fortunian and lowermost Cambrian Stage 2.

Description.—Slowly expanding, slightly curved tubes with three radially extended lobes separated by deep grooves. Externally, each lobe carries a prominent median longitudinal keel ( Fig. 71A ). Each keel may exceed the internal diameter of the tube and regularly undulates with folds oriented ca. 45° towards the aperture. The lobes become more prominent aperturally. Along the middle part of each lobe (on internal moulds) there is a narrow depression ( Fig. 71B, D , white arrow). Within the grooves, the growth lines are gently curved towards the aperture.

Remarks.—The presence of prominent longitudinal keels is diagnostic for Selindeochrea and distinguishes it from Anabarites .

Stratigraphic and geographic range.— Upper Fortunian Stage and lower part of Cambrian Stage 2; Siberia (Rozanov et al. 1969:158, pl. 8:11, 16, 20; Matthews and Missarzhevsky 1975: pl. 2: 3; Val’kov 1982: 75, pl. 12: 1‒6; Missarzhevsky 1989: pl. 13: 8; Khomentovsky and Karlova 1989: 57, pl. 4: 6; Khomentovsky et al. 1990: pl. 11: 4; Pel’man et al. 1990: pl. 3: 8; Kouchinsky et al. 2009: fig. 20), probably western Mongolia ( Esakova and Zhegallo 1996: 92, 93, pl. 3: 5‒8) and probably Avalonia (Landing 1988: 690, fig. 9, fig. 13, in part; Landing et al. 1989: 757, fig. 5, figs. 13, 14, in part). Selindeochrea missarzhevskyi (Vasil’eva, 1986)

Fig. 72.

Material.— 30 specimens, including figured SMNH X5993– 5996 , from sample 5a/34.75 (section 96-5a, Fig. 2 ), Emyaksin Formation , and seven internal moulds and one external mould, including figured SMNH X5992 , 5997 , from samples 1/28.5 and 1/29.2 (section 1, Fig. 4 ), from the Medvezhya Formation , Anabar Uplift, Siberian Platform, Russia .

Correlated with the uppermost Fortunian and lower part of Cambrian Stage 2.

Description.—Tubes somewhat irregularly curved and longitudinally folded in three wide laterally extended lobes that have median keels. Outer surface with undulating growth lines. Lumen (internal mould) with three longitudinal lobes, each with rhomboidal or domal transverse profile. Lobes separated from each other by deep narrow grooves that shallow apically. In this direction, the lobes also acquire a rounded shape. Cross-section rounded at the apex.

Remarks.—The new material assigned to Selindocrea missarzhevskyi includes internal moulds very similar to those of other Selindeochrea fossils, i.e., with casts of prominent longitudinal keels and undulatory, transverse growth lines. The presence of longitudinal keels was also noted by Vasil’eva (1986). Selindeochrea missarzhevskyi is different from most other fossils attributed to Selindeochrea herein in having wider and straight lobes of the internal molds that are not longitudinally twisted. It most closely resembles S. tripartita , in particular apically, but S. missarzhevskyi differs in having more acute terminations of the lobes (as shown by internal moulds) and a greater apical angle of the thecae.

Stratigraphic and geographic range.— Cambrian Stage 2 (lower part) of the Siberian Platform.

Figures

Fig. 71. Anabaritid Selindeochrea tripartita (Missarzhevsky in Rozanov et al., 1969), from early Cambrian Medvezhya Formation, western Anabar Uplift, Siberia, Russia; samples 3/12 (A), K2/26 (B), 3/12.2 (C, E), K2/25 (D), section 3. A. SMNH X5987, calcium phosphatic internal mould with external mould of longitudinal keels; A1, transverse apical view; A2, view of apertural end; A3, lateral view. B–D. SMNH X5988–5990, respectively. B, C1, D, lateral; C2, transversal views of calcium phosphatic internal moulds. Scale bar 500 μm.

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. 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.