Microcornus, Mambetov, 1972

Kouchinsky, Artem, Bengtson, Stefan, Clausen, Sébastien & Vendrasco, Michael J., 2015, An early Cambrian fauna of skeletal fossils from the Emyaksin Formation, northern Siberia, Acta Palaeontologica Polonica 60 (2), pp. 421-512 : 453-454

Microcornus ? sp.

Fig. 25C .

Material.—Several tens of calcium phosphatic internal moulds, including SMNH X 4655, from sample 7/36.7; several additional moulds from samples 7/27.5, 7/31.5, 3/59, and 3/65; Emyaksin Formation, Bol’shaya Kuonamka and Malaya Kuonamka rivers; correlated with the interval of Repinaella – Judomia zones, Atdabanian Stage.

Description.—Short conchs, gently curved dorso-ventrally, expand fast from a relatively thick bulbous protoconch. The dorsal side is arched without angulations and passes into a convex, but more flattened ventral side. In dorsal view, the apical angle is ca. 30°. The bulbous juvenile conch with a small tip is

1

2

somewhat dorso-ventrally flattened and shifted dorsally ( Fig. 25C 1 ), separated from the main part by a constriction.

Remarks.—This material is generally similar to Microcornus Mambetov, 1972 , described from Stages 3–5 of the Siberian Platform, Kazakhstan, Mongolia, Australia, Avalonia (Britain), and South China ( Bengtson et al. 1990), but preservation as smooth internal moulds with a straight aperture precludes more detailed assignement.

Genus Linevitus Sysoev, 1958

Type species: Hyolithus obscurus Holm, 1893 ; Lejopyge laevigata Zone , middle Cambrian; Sweden.

Remarks.—Placement of the following Siberian species under Linevitus Sysoev, 1958 is questioned by Berg-Madsen and Malinky (1999), because the type material is represented by much larger fossils. Moreover, since the genus is not established on a firm morphological ground, the name Linevitus appears to be justified only for the type species ( Berg-Madsen and Malinky 1999).

Figures

Fig. 25. Hyolith conchs from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/36.7. A. Hyolithida sp. indet. 1, SMNH X 4656; lateral (A1), dorsal (A2), and apertural (A3) views. B. “Lenatheca groenlandica (Poulsen, 1932)”?, initial part, SMNH X 4640; apertural (B1), dorsal (B2), and lateral (B3) views. C. Microcornus? sp., SMNH X 4655; lateral (C1), apertural (C2), and dorsal (C3) views. D. Loculitheca? sp., SMNH X 4634; D1, oblique apical view, septum; D2, plan view of the conch. E–G. Conotheca cf. C. mammilata Missarzhevsky in Rozanov et al., 1969. E. SMNH X 4625; lateral view. F. SMNH X 4626; dorsal view. G. SMNH X 4627; lateral view. Scale bar 600 μm.

Fig. 1. Map showing location of sections 1, 3, Malaya Kuonamka River; 6, 7, Bol’shaya Kuonamka River.