Michniakia minuta Missarzhevsky

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 : 401-403

Michniakia minuta Missarzhevsky in Rozanov and Missarzhevsky, 1966

Fig. 42 .

1966 Michniakia minuta sp. nov.; Missarzhevsky in Rozanov and Missarzhevsky 1966: 106–107, pl. 10: 5, 6.

1986 Michniakia minuta Missarzhevsky in Rozanov and Missarzhevsky, 1966; Ermak and Pelman 1986: 189–190, pl. 26: 1, 2.

2004 Michniakia minuta Missarzhevsky in Rozanov and Missarzhevsky, 1966; Gubanov et al. 2004a: 212, fig. 6: 14–18, 33.

Material.—Several tens of calcium phosphatic internal moulds with fragments of phosphatised shells, including SMNH X11353–11357, from sample 20/1B, Erkeket Formation, Khorbusuonka River, Siberia, Russia. Lower Botoman stage (correlated with the lower Cambrian Stage 4).

Description.—Planispiral slowly expanding shell, up to 1 mm in diameter, coiled to 3/4 revolution, with blunt flattened apex and circular cross-section, ca. 350 µm at the aperture. Shell and internal moulds form a loop bringing the apex into contact with the later growth stage at ca. 3/4 of a whorl. A single conch is slightly anisostrophically (dextrally) coiled ( Fig. 42A 4).

The wall consists of at least two layers. The outer layer exposed on the shell surface is longitudinally striated perpendicularly to faint growth lines near the apex ( Fig. 42C 1 ). Near the aperture, striation is tangentially oriented and aligned with circular to ovaliform pores, up to 10 μm in diameter ( Fig. 42A 1 ). The inner layer has an apparently smooth surface and forms the apical septum ( Fig. 42D, E 1 ). The aperture with a gentle sinus in its outer (ventral) margin ( Fig. 42E 3). A single specimen ( Fig. 42B ) is preserved with a broadly oval, nearly circular operculum sealing the aperture. The operculum and conch are bilaterally symmetrical along the same (dorso-ventral) plane. The ventral margin of operculum is gently incurved to fit the ventral sinus of the aperture. The dorsal margin of operculum is almost straight and aligned with the dorsal edge of the aperture. The operculum has a slight radial plication in its dorsal part. The distal part of the operculum is flattened and overlaps the apertural margin. The initial part of the operculum is situated in its centre.

Remarks.—The septum-like structure suggests decollating growth mode assumed for Cupitheca — type conchs of hyoliths (see below). The fibrous striation with aligned pores in the wall, flattened apical part with a rounded septum, aperture closed by operculum suggest a hyolithan affinity of Michniakia previously reported as a mollusc. Based on hyolith-type microstructures, it has been suggested that hyolith morphologies included planispirally coiled forms (Feng et al. 2001). Other coiled mollusc-like shells such as isostrophic Khairkhania , anisostrophic Barskovia , Nomgoliella , and Philoxenella demonstrate features compatible with the same affinity (Kouchinsky et al. 2017). Planispiral, evolute shells of Ardrossania paveyi Runnegar in Bengtson et al., 1990, from the Mernmerna Formation of South Australia have shells up to 4 revolutions and 1.5 mm in diameter, 300–400 µm in cross-section diameter at the aperture (Gravestock et al. 2001). It has been suggested to be similar to gastropod-like molluscs and hyoliths (Bengtson et al. 1990). Parkhaev in Gravestock et al. (2001) attributed Ardrossania to the family Khairkhaniidae Missarzhevsky, 1989 .

Stratigraphic and geographic range.— Cambrian Series 2, Siberia ( Russia) and Spain.

Family Protowenellidae Peel, 2021b

Genus Protowenella Runnegar and Jell, 1976

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

Figures

Fig. 42. Hyolith Michniakia minuta Missarzhevsky in Rozanov and Missarzhevsky, 1966, from the lower Cambrian Erkeket Formation, Olenyok Uplift, Siberia, Russia; sample 20/1B. A–E. Internal moulds with partially phosphatised shell and phophatic coatings, SMNH X X11353–11357, respectively. A. Phosphatised shell with longitudinal and tangential fibrosity and pores in the wall. A3, lateral view showing ventral sinus of the aperture (arrow); →

Fig. 1. Locality maps. A. Map of the Siberian Platform. B. Khorbusuonka River. Localities referred to in the main text: 96-1, 96-2, 96-3, Malaya Kuonamka (MK) and 96-6, 96-7, 96-8, Bol’shaya Kuonamka (BK) rivers, eastern flank of the Anabar Uplift; 3, 11, 15, 19, 20, Khorbusuonka River, Olenyok Uplift; 21, 22, Lena River lower reaches, Ulakhan-Ald’yarkhay Brook; Le, Lena River middle reaches, Tiktirikteekh and Bydyangaya Brooks; Al, Aldan River, between sections “Dvortsy” and Ulakhan- Sulugur. Margin of the Siberian Platform is shown by a dashed line in A.