Pelagiellidae Knight, 1956

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 : 383

Family Pelagiellidae Knight, 1956

Remarks.—Based on observation of chetae in Pelagiella from the Kinzers Formation ( Thomas et al. 2020), Landing et al. (2021a) concluded that pelagiellids, except for Pelagiella atlantoides , probably belong to the order Sabellida Levinsen, 1883 , of the class Polychaeta Grube, 1850. Sabellids appeared, however, by the end of the Permian, while almost all Palaeozoic tubeworms are probable lophophorates judging by their tube microstructure ( Ippolitov et al. 2014). Pelagiella has a more typical molluscan shell microstructure ( Li et al. 2017), morphology of the early shell and protoconch (e.g., Kouchinsky et al. 2011: figs. 8–10), and invariability in the degree of coiling ( Vinn and Mutvei 2009; Ippolitov et al. 2014), as would not be expected in annelid tubeworms. Moreover, chitin synthesis is maintained by the same genetic machinery in molluscs as that in annelids and brachiopods (e.g., Zakrzewski et al. 2014; Schiemann et al. 2017). Thus, the original interpretation of pelagiellids as molluscs is in better agreement with the entire complex of features being observed ( Thomas et al. 2020).

Genus Pelagiella Matthew, 1895

Type species: Cyrtolites atlantoides Matthew, 1894 ; Lower Cambrian ; New Brunswick, Canada .

Pelagiella sp. 1

Fig. 30A, D .

Material.—Two calcium phosphatic internal moulds, SMNH Mo194738 and 194739, from sample 19/12.75. Erkeket Formation, Khorbusuonka River, Siberia, Russia. Lower Botoman stage (correlated with the Cambrian Stage 4).

Description.—Shell represented by smooth very rapidly expanding internal moulds consisting of up to 3/4 whorl without preserved aperture. Peripheral surface of the whorl rounded and strongly extended distally, separated from the spiral and umbilical surfaces by an angle. Spiral surface concave, umbilical surface convex. Cross-section of the whorl extended drop-shaped.

Remarks.— Pelagiella sp. 1 described herein is very similar to Pelagiella sp. 1 from the upper Emyaksin Formation ( Kouchinsky et al. 2015a: fig. 19A) and is different from Pelagiella spp. below in a strongly extended peripheral part of the whorl and depressed spiral side. External shell sculpture not preserved. Internal moulds are most similar to Pelagiella madianensis (Zhou and Xiao, 1984) from Cambrian Series 2 of Gondwana. Ornamentation of P. madianensis consists of granules arranged in spiral rows Parkhaev in Gravestock et al. 2001; Li et al. 2017, 2019), whereas the ornamentation of probably the same species, Pelagiella sp. 1 (see Kouchinsky et al. 2015a: fig. 18) consists of commarginal ribs.

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).