Microcornus cf. eximius Duan, 1984

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 : 409-411

Microcornus cf. eximius Duan, 1984

Fig. 52A, B, D, F–H .

Material.—Several tens of calcium phosphatic internal moulds, including SMNH X11215–11220, from samples 21/33, 21/46.1, 21/51, 21/52, 21/54, Tyuser Formation, Dokidocyathus lenaicus –Tumuliolynthus primigenius Zone, Tommotian stage– Delgadella anabara Zone, Atdabanian stage (upper Stage 2–Stage 3), 22/50, Sekten Formation, Lena River, upper Toyonian stage (correlated with the Cambrian Stage 4) and from 20/1B, 19/12.75, 19/25.5, 19/33, 19/55, Erkeket Formation, Khorbusuonka River, Botoman– Amgan stages (correlated with the Cambrian Stage 4–lower Wuliuan Stage). Siberia, Russia.

Description.—Internal moulds representing straight moderately expanding conchs. In dorsal view, the apical angle is ca. 30°. Cross-section rounded triangular. Aperture orthogonal with a long semi-circular ventral ligula, dorsal and a pair of lateral sinuses. Dorsal side arched, with flattened flanks, passes with angulation into a flattened ventral side. Aperture with slightly protruding ventral side. Larger internal moulds with a flared aperture suggesting presence of lateral sinuses in the conch. Initial part bulbous separated from the main part by a shallow constriction, flattened dorso-ventrally, slightly curved dorsally and carrying a pointed tip.

Remarks.— Microcornus eximius Duan, 1984 , is most similar but its conch has a well-developed median sinus, which is not evident on the internal moulds herein and weakly expressed in the available phosphatised conchs ( Fig. 52A, B ). Flared lateral parts at the aperture of internal moulds suggesting presence of lateral sinuses in the conch. The form is similar to Hyolithida indet. 3 ( Kouchinsky et al. 2011: fig. 11G–I), from sample 8/32, Drumian Stage of the Anabar Uplift. It is distinguished from Microcornus sp. ( Fig. 52C ) by a larger apical angle. The latter form is in its turn similar to Microcornus sp. described by Kouchinsky et al. (2011: fig. 12L–Q) preserved as silicified conchs from sample 8/18.5, from lower Wuliuan Stage beds of the Kuonamka Formation. Such a morphology can also be attributed to Parkula bounites , reported herein from opercula. M. eximius is also reported from Series 2 of South China (Duan 1984), North China ( Pan et al. 2019), Australia (Bengtson et al. 1990), Antarctica ( Wrona 2003), and Laurentia ( Malinky and Skovsted 2004).

Figures

Fig. 52. Hyolith conchs from the lower Cambrian Erkeket Formation, Khorbusuonka River (C, D, F–H) and Tyuser (A, B) and Sekten (E) formations, lower reaches of the Lena River, Siberia, Russia; samples 19/12.75 (D, F, G), 19/25.5 (H), 19/33 (C), 21/51 (A, B), and 22/50 (E). A, B, D, F–H. Microcornus cf. eximius Duan, 1984, phosphatised conchs (A, B) and calcium phosphatic internal moulds (D, F, G), SMNH X11215–11220, respectively. C, E. Parkula bounites? Bengtson in Bengtson et al., 1990, phosphatised conchs, SMNH X11221–11222, respectively. B1, C2, D2, F3, oblique apertural, B2, C1, D1, F1, G1, dorsal; and D3, F2, G2, lateral views; E, enlargement showing inner surface of ventral side of a fragmentary conch. Scale bar: 500 µm (A–D, F–H) and 250 µm (E).