Dibrachicystidae

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 : 427-428

Family Dibrachicystidae ? Zamora and Smith, 2012 Proximal stem plates

Fig. 63A, B .

Material.—Two phosphatised plates, SMNH Ec36873 and 36874, from sample 19/10.25, Erkeket Formation, Khorbusuonka River, Siberia, Russia. Lower Botoman stage (correlated with the lower Cambrian Stage 4).

Description.—Plates with triangular chevron-like outline, ca. 0.3 and 0.6 mm wide, with arcuate adaxial surface (bordering the stem lumen) bordered by a rim. The abaxial, triangular part of the plate is relatively flat, slightly curved, and made of coarse galleried to fasciculated stereom.

Remarks.—The proximal stem plates described herein are similar to those described from the Drumian Stage of Siberia ( Kouchinsky et al. 2011: fig. 38A–F) and identified by Zamora et al. (2013: fig.13.5 f, g) as proximal stem plates of dibrachicystids ( Zamora and Smith 2012). Dibrachicistidae is a basal group of the Rhombifera, the earliest undoubted representatives of which are reported from Wuliuan Stage of Gondwana ( Zamora 2010; Zamora and Smith 2012; Zamora et al. 2013). Articulated specimens of dibrachicystids show a tripartite stem with multiple holomeric columnals ( Zamora and Smith 2012), similar to those ( Fig. 62G, H ) found together with the plates described herein.

Class Edrioasteroidea Billings, 1858

Order and family uncertain

Ambulacral flooring plate

Fig. 63I .

Material.—Single phosphatised ossicle, SMNH Ec36875, from sample 19/11.75, Erkeket Formation, Khorbusuonka River, Siberia, Russia. Lower Botoman stage (correlated with the lower Cambrian Stage 4).

Description.—Plate is hourglass-shaped in plane view, ca. 1.4 mm long, ca. 1.0 mm wide, and ca. 0.7 mm thick. Its external (upper) side is subdivided into wider abradial facet (right parts of Fig. 63I ) and more flattened adradial surface (see Kouchinsky et al. 2011: fig. 36) connected by a short median area. The latter carries a circular shallow pit ( Fig. 63I 1 : arrow), ca. 120 µm in diameter, a presumable insertion of a ligament for a cover-plate. Two semi-elliptical lateral gaps in median area represent sutural pores between adjacent ambulacral plates.

Remarks.—The plate is of the same age as the one described by Kouchinsky et al. (2015a) from the Anabar Uplift in Siberia (see also Kouchinsky et al. 2011; Clausen and Peel 2012; Zamora et al. 2013; Peel et al. 2016).

Figures

Fig. 63. Echinoderm plates from the lower Cambrian, Erkeket Formation, Khorbusuonka River, Siberia, Russia; samples 19/10.25 (A, B), 19/11.75 (E, G–I), 19/12.75 (C, F), 19/25.5 (J, L), 20/2B (K), and 20/6B (D, M). A, B. Proximal stem plates of Rhombifera, SMNH Ec36873 and 36874, respectively. C–G. Edrioasteroid thecal plates (from basal attachment disc?), SMNH Ec36876–36880, respectively. H. Plate from edrioasteroid mouth-frame, SMNH Ec36881. I. Ambulacral flooring plate of Edrioasteroidea, SMNH Ec36875. I1, oblique view, a shallow pit for presumably insertion of a ligament for a cover-plate is arrowed; I2, upper view. J–L. Ctenidial plates (Echinodermata, Ctenocystoidea), SMNH Ec36882–36884. K1, plain view on a trapezoidal dorsal blade; K2, plain view of the plate showing dorsal blade and hook-shaped process; K3, plate in lateral view. M. Echinodermata, thecal frame-plate of unknown affinity, SMNH Ec36885. Scale bar: 1 mm (D, F, I, M) and 500 µm (A–C, E, G, H, J–L).

Fig. 62. Blastozoan plates from the lower Cambrian Erkeket Formation, Khorbusuonka River, Siberia, Russia; samples 20/2B (A, B), 20/5B (I, J, K, U), 19/12.75 (C, F), 19/11.75 (D, E), 19/10.25 (G, H), 19/107 (L, S, T), 19/46.5 (M, N, P, Q), 19/65.5 (O), and 19/2B (R). A, B. Brachiolar, biserial plates, SMNH Ec36852 and 36853.A1, articulation surface and aboral canal, arrowed; A2, aboral surface; A3, oral surface. C–F. Brachial, uniserial plates, SMNH Ec36854–36857, respectively. G–R. Stem holomeric columnals, SMNH Ec36858–36869, respectively. K1, lateral view; K2, oblique view showing articulation facet; M1, lateral view of a stem fragment consisting of three articulated ossicles; M2, oblique view showing articulation facet; N1, fragmentary stem ossicle in oblique lateral view; N2, view on the articulation facet;O1, elongated stem ossicle, oblique lateral view showing articulation facet; O2, lateral view; R1, discoidal stem ossicle, view on the articulation facet; R2, oblique lateral view. S–T. Distal fragments of holomeric stem including columnals articulated with a holdfast, SMNH Ec36870–36871, respectively.U. Holdfast?, SMNH Ec36872. Scale bar: 500 µm (E–J, K1, M–O) and 250 µm (A–D, K2, L, P–U).

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.