Amphigeisina danica ( Poulsen, 1966 )

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 : 477

Amphigeisina danica ( Poulsen, 1966)

Fig. 53E .

Material.—One figured sclerite SMNH X 4763 and two additional specimens from sample 1/9; carbonate concretions of the basal Kuonamka Formation, Malaya Kuonamka River; Bergeroniellus expansus Zone , upper Botoman Stage.

Description.—Slender, gently curved, simple bilaterally symmetrical calcium phosphatic sclerites.A relatively narrow concave side extends from the tip towards the base. The opposite, broader, strongly convex side of the sclerite is open in the slightly flared basal part. The internal cavity extends to the tip. The wall is composed of two layers with amorphous microstructure. The surface of the convex side is smooth, whereas faint transverse folds are present on the concave side. The two sides meet to produce two longitudinal carinae flanked by two thin flanges formed by the outer layer. The flanges reduce gradually towards the tip, where they merge with the carinae.

Stratigraphic and geographic range.—The occurrence of Amphigeisina danica reported herein from the lower part of Cambrian Stage 4 (upper Botoman Stage) is the earliest known from the Siberian Platform, where it also occurs in the Toyonian Stage representing the upper portion of Stage 4 (as Protohertzina yudomica , after Demidenko 2006) and in the middle Cambrian Stage 5, in the Kuonamka Formation of the Malaya Kuonamka and Bol’shaya Kuonamka rivers ( Kouchinsky et al. 2011). The species is also known from the upper part of Series 2 of Kazakhstan ( Gridina 1991), Avalonia, Britain ( Hinz 1987), and Laurentia ( Landing 1974; John S. Peel, personal communication 2010) and from Series 3 of Baltica ( Poulsen 1966; Bengtson 1976) and South and North China ( Qian et al. 2004; Steiner et al. 2007).

Figures

Fig. 53. Protoconodonts from lower Cambrian basal Kuonamka Formation, Malaya Kuonamka River and Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia. A–D. Gapparodus bisulcatus (Müller, 1959), sample 1/9. A. SMNH X 4764; A 1, view on the longitudinally concave side and basal opening of sclerite; A 2, lateral view. B. SMNH X 4765; B 1, lateral view; B 2, view on the longitudinally concave side. C. SMNH X 4766; C 1, lateral view; C , oblique view on the longitudinally concave side and basal opening of sclerite; C , view on the longitudinally concave side. D. SMNH X 4767; D 1, lateral view; D 2, close-up of basal part of D 1. E. Amphigeisina danica (V. Poulsen, 1966), SMNH X 4763; sample 1/9; E 1, view on the basal →