Edrioasteroidea Billings, 1858

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 : 494

Class Edrioasteroidea Billings, 1858 Order and family uncertain

Edrioasteroid thecal plates

Fig. 64A–F .

Material.—Five disarticulated plates, SMNH Ec 31511– 31515, from sample 7/70, two plates from sample K1-3B, and one, SMNH Ec 31516, from sample 7/64. Emyaksin Formation, Bol’shaya Kuonamka and Malaya Kuonamka rivers; uppermost Judomia Zone , upper Atdabanian Stage (sample 7/64) and Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Plates triangular or trapezoidal, nearly symmetrical in plan view and curved claw-like in the lateral view. They consist of a concave and an opposite convex triangular or trapezoidal surface, perpendicular to a slightly concave and smooth face surrounded by a rounded rim ( Fig. 64B 3 , C 1 , D, arrowed). The two sides converge towards an embayed rounded edge. The embayments are produced by four to six more or less distinct ridges radiating from the face on the concave surface of the plate. These ridges are divided into two sets and are situated almost bilaterally symmetrically on the plate. The median pair of ridges runs almost parallel, whereas the lateral ones diverge. The opposite surface lacks ridges ( Fig. 64B 2 ).

Remarks.—Similar plates were illustrated by Hinz (1987: pl. 15; 11), from the Lower Comley Limestone Callavia Zone of Avalonian Britain (correlated with the upper Atdabanian– lower Botoman Stages ( Brasier 1989b; Brasier et al. 1992), by Landing and Bartowski (1996: 755, fig. 8) and Skovsted (2006: figs. 7.7, 7.8), from the Bonnia-Olenellus Zone of Laurentia correlated with Cambrian Stage 4. Two apparently similar plates were also described by Rozhnov et al. (1992: pl. 6: 8, 9) from the uppermost Atdabanian Stage of southeastern Siberian Platform. The plates were tentatively regarded by Landing and Bartowski (1996) as edrioasteroid ambulacral plates, based on comparision with the middle Cambrian Totiglobus Bell and Sprinkle, 1978 and with the late early Cambrian echinoderm plates illustrated by Sprinkle (1973: pl. 25) that were later identified as edrioasteroid plates by Sprinkle (see Landing and Bartowski 1996: 755). Plates described by Hinz (1987), Landing and Bartowski (1996), and herein are different, however, from the plates of the mouth frame of Totiglobus , which carry sets of more regular deeper vertical parallel ridges and notches oriented along the plate peristomal margin. They are more similar to the plates from the basal attachment disc of Totiglobus , which bear parallel to radiating ridges along their inner and outer surfaces ( Bell and Sprinkle 1978; Clausen and Peel 2012). Together with the ambulacral plate (see below), the edrioasteroid plates described herein are among the oldest occurrences of edrioasteroids, demonstrating that the latter were already widespread in Cambrian Stage 4 ( Clausen and Peel 2012; Zamora et al. 2013).

Figures

Fig. 64. Edrioasteroid thecal-plates from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia. A. SMNH Ec 31511, sample 7/70; view of triangular-surface of a thecal-plate with 6 ridges radiating from concave base (at the top). B. SMNH Ec 31512, sample 7/70; B1, view of triangular surface with 4 ridges radiating from the concave base (at the top); B2, view of the opposite, smooth triangular surface; B3, lateral view (peripheral rim of concave base arrowed) of the claw-like plate. C. SMNH Ec 31513, sample 7/70; C1, oblique view of concave base (peripheral rim arrowed) and triangular surface bearing radiating ridges; C2, lateral view. D. SMNH Ec 31514, sample 7/70; view of triangular, ridged surface and perpendicular concave base (rim arrowed). E. SMNH Ec 31515, sample 7/70; view of trapezoidal surface with faint ridges and perpendicular, rimmed concave-base (to the top). F. SMNH Ec 31516, sample 7/64; view of faintly ridged surface and concave base (at the top). G. Edrioasteroid ambulacral plate, SMNH Ec 31517, sample 7/70; G1, upper view (lateral sutural pore/gap arrowed); G2, oblique view (lateral, abradial? prong arrowed). Scale bar: 600 μm, except G, 300 μm.

Fig. 3. Helcionellid mollusc Yochelcionella cf. Y. greenlandica Atkins and Peel, 2004 internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/36.7. A. SMNH Mo 160417; lateral (A 1, A5), sub-apical (A 2), upper (A 3), and supra-apical (A 4) views; A 6, polygonal texture on the surface of snorkel mould. B. SMNH Mo 160418; lateral (B 1), sub-apical (B 2), and upper (B 3) views; B 4, enlargement of B 2, with a shallow groove between the snorkel and aperture; B 5, apical part of the mould with tubercles; B 6, close-up of B 5. C. SMNH Mo 167648; lateral (C 1), sub-apical (C ), and upper (C ) views. D. SMNH Mo 167649; lateral view. Scale bar 600 μm, except A , 60 μm; B , 150 μm; B , 300 μm; B , 120 μm.

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

Fig. 2. Derivation of fossiliferous samples (arrows with numbers) and chemostratigraphic correlation of sections 1 and 3 (Malaya Kuonamka River) with sections 6 and 7 (Bol’shaya Kuonamka River). Correlation with carbon isotope features II–VII from stratotypes of south-eastern Siberian Platform (Brasier et al. 1994) according to Kouchinsky et al. (2001). Note that the zero level in all sections corresponds to the water level above which the sections were exposed in 1996. Description of fauna from the lower part of the Emyaksin Formation in complementary outcrops at the Bol’shaya Kuonamka River (below that zero level) will be published elsewhere. Fauna from the overlying Kuonamka Formation is published by Gubanov et al. (2004) and Kouchinsky et al. (2011).