Pelagiella spp.

Kouchinsky, Artem, Alexander, Ruaridh, Bengtson, Stefan, Bowyer, Fred, Clausen, Sébastien, Holmer, Lars E., Kolesnikov, Kirill A., Korovnikov, Igor V., Pavlov, Vladimir, Skovsted, Christian B., Ushatinskaya, Galina, Wood, Rachel & Zhuravlev, Andrey Y., 2022, Early-middle Cambrian stratigraphy and faunas from northern Siberia, Acta Palaeontologica Polonica 67 (2), pp. 341-464 : 384-386

Pelagiella spp.

Fig. 30B, C, E–H .

Material.—Several tens of calcium phosphatic internal moulds, including figured SMNH Mo 194740–194745, from samples 19/12.75, 19/29, 19/40, 20/1B, 20/6B, Erkeket Formation, Khorbusuonka River, Botoman–Toyonian stages (correlated with the Cambrian Stage 4) and several calcium phosphatic internal moulds from sample 21/57.75, Tyuser Formation, Delgadella anabara – Nevadella Zone, Atdabanian stage (correlated with the Cambrian Stage 3). Several calcium phosphatic internal moulds from samples 22/50, 22/55.5, and 22/67, Sekten Formation, Lena River, uppermost Toyonian and Amgan stages (correlated with the Cambrian Stage 4 and lowermost Wuliuan Stage). Siberia, Russia.

Remarks.—Smooth rapidly expanding internal moulds consisting of up to one whorl. Peripheral surface of the whorl rounded, passing into convex spiral and umbilical surfaces. Cross-section of the whorl rounded triangular to ovaliform. The moulds probably belong to several species. They are similar to internal moulds described from the Emyaksin Formation ( Kouchinsky et al. 2015a: figs. 19B, C, 20). In the latter forms, the outer shell surface bears transversal ribs also described from Costipelagiella nevadense Skovsted, 2006b . A phosphatised wall fragment found in sample 22/50 has a cancellate ornament identical to that described from Cambretina mareki Horný, 1964 (see Kouchinsky et al. 2011) and consists of rounded 30–40 μm wide polygons. It is not however clear if the fragment belongs to the same species as internal moulds and if this type of external surface ornamentation is unique to Cambretina mareki .

Class and order uncertain

Family Khairkhaniidae Missarzhevsky, 1989

Genus Nomgoliella Missarzhevsky, 1981

Type species: Nomgoliella sinistrovolubilis Missarzhevsky, 1981 ; Beds with Anabarella plana, Cambrian Stage 2; Khasagt-Khajrkhan-Uul Ridge, Salaany-Gol Rivulet, Western Mongolia.

Nomgoliella sinistrovolubilis Missarzhevsky, 1981 Fig. 30J .

Material.—Single internal mould, SMNH Mo194747, from sample 15/23, Chuskuna Formation, upper Kessyusa Group, Khorbusuonka River. Siberia, Russia. Lower part of Cambrian Stage 2.

Description.—Sinistrally coiled low-spired internal mould, ca. 2 mm in diameter, consisting of a single rapidly expanding whorl with broad oval cross-section. Initial part in contact with the expanded whorl. Dorsal side of the internal mould representing ca. 1/2 revolution of the whorl is somewhat flattened.

Remarks.—Dorsally flattened internal mould of a single specimen described herein can be interpreted as preservational difference from the Mongolian forms. Barskovia Golubev, 1976 , and Nekolenia Vasiljeva, 1998 (probably synonymous after Parkhaev 2002: 34) are different in being loosely coiled and having almost circular cross-section of the whorl, although B. hemisymmetrica (Kouchinsky et al. 2017: fig. 23) may show broadly oval outlines.

Stratigraphic and geographic range.— Cambrian Stage 2 of the Siberian Platform and western Mongolia.

Figures

Fig. 30. Pelagiellid and khairkhaniid molluscs from the lower Cambrian Erkeket Formation, Khorbusuonka River (A, B, D, E, G, H), Sekten Formation, lower reaches of the Lena River (C, F, I) formations and Kessyusa Group, Khorbusuonka River (J), Siberia, Russia; samples 19/12.75 (A, D, G, H), 19/29 (B), 19/40 (E), 22/50 (C, F, I), and 15/23 (J). A, D. Pelagiella sp. 1, internal moulds, SMNH Mo194738 and 194739, respectively. B, C, E–H. Pelagiella spp., internal moulds, SMNH Mo194740–194745, respectively. E1, apertural view; E2, adapical view. I. Phosphatised wall of Pelagiella sp. cf. Cambretina mareki Horný, 1964, SMNH Mo194746. J. Nomgoliella sinistrovolubilis Missarzhevsky, 1981, internal mould, SMNH Mo194747. J1, apertural view; J2, adapical view. Scale bar: 1 mm (J) and 500 µm (A–I).