Horsegullia sp.

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 : 377-379

Horsegullia sp.

Fig. 24L, O, P .

Material.—Several tens of phosphatic internal moulds, including figured SMNH Mo194709–194711, from sample 20/1B, Erkeket Formation, Khorbusuonka River, Siberia, Russia. Lower Botoman stage (correlated with the lower Cambrian Stage 4).

Description.—Smooth internal moulds coiled through 1/4 of a whorl and moderately laterally compressed. Initial part indistinct. Apex bluntly rounded and gently hooked, projecting beyond the apertural margin. The aperture has a drop-like outline, with length/width ratio ca. 1.8, with rounded margins tapering towards the subapical side of the shell. The parietal train on the subapical side is low, narrow and short.

Remarks.—The form is different from Horsegullia horsegulliensis by a less extended but more rounded blunt apex as well as by a more elongated aperture and a less distinct parietal train. The latter species is described from Cambrian stages 3–4 of Gondwana ( Australia and North China; Gravestock et al. 2001; Li et al. 2021).

Family Stenothecidae Runnegar and Jell, 1980

Genus Mellopegma Runnegar and Jell, 1976

Type species: Mellopegma georginense Runnegar and Jell, 1976 ; Drumian Stage; Gowers Formation, Georgina Basin , South Australia .

Mellopegma sp.

Fig. 24C .

Material.—Single internal mould, SMNH Mo194713, from sample 20/6B, Erkeket Formation, Khorbusuonka River, Siberia, Russia. Toyonian stage (correlated with the Cambrian Stage 4).

Remarks.—Laterally compressed internal mould with faint transverse folds and deep subapical sinus. The apex is slightly shifted dorsally (supra-apically) and abut with a bulbous initial part. Different from the most similar Mellopegma indecorum (Missarzhevsky in Rozanov et al., 1969) in having a prominent bulbous initial part and lacking subapical curvature.

Genus Stenotheca Salter in Hicks, 1872

Type species: Stenotheca cornucopia Salter in Hicks, 1872. See remarks on the type species by Vendrasco et al. (2011b).

Stenotheca cf. drepanoida (He and Pei in He et al., 1984)

Fig. 24D .

Material.—Single specimen, SMNH Mo194714 represented by a silicified shell from sample 20/6B, Erkeket Formation, Khorbusuonka River, Siberia, Russia. Toyonian stage (correlated with the Cambrian Stage 4).

Remarks.—Shell strongly laterally compressed and coiled through 1/4 of a whorl, fragmentarily preserved without apex. The fossil is similar to Stenotheca drepanoida (He and Pei in He et al. 1984) known from the upper part of Cambrian Stage 3–lower Stage 4 of South China, South Australia, and Siberia ( Vendrasco et al. 2011a).

Figures

Fig. 24. Molluscs from the lower Cambrian Erkeket Formation, Khorbusuonka River (A–F, J–N) and Sekten Formation, lower reaches of the Lena River (G–I), Siberia, Russia; samples 11/2B (E, F, J), 11/16.1 (K, M, N), 19/12.75 (A), 19/11.75 (B), 20/1B (L, O, P), 20/6B (C, D), and 22/50 (G–I). A, B. Davidonia anabarica (Parkhaev, 2005), internal moulds, SMNH Mo194701 and 194702, respectively. C. Mellopegma sp., internal mould, SMNH Mo194713. D. Stenotheca cf. drepanoida (He and Pei in He et al., 1984), silicified shell, SMNH Mo194714. D1, lateral view; D2, oblique apertural view. →