Blastozoa Sprinkle, 1973

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 : 425

Subphylum Blastozoa Sprinkle, 1973

Class, order, and family uncertain Brachiolar plates

Fig. 62A, B .

Material.—Several thousand phosphatised ossicles, including figured SMNH Ec36852 and 36853, from samples 19/5.5, 19/10.25, 19/11.75, 19/12.75, 19/25.5, 19/26.75, 19/29, 19/31.75, 19/33, 19/40, 19/43.25, 19/46.5, 20/1B, 20/2B, 20/3B, 20/4B, 20/5B, 20/6B, Erkeket Formation, Botoman– Toyonian stages (correlated with the Cambrian Stage 4) and sample 19/70.5, Yunkyulyabit-Yuryakh Formation, lower Mayan stage (Drumian Stage), Khorbusuonka River, and samples 22/0, top of the Tyuser Formation, 22/50 and 22/67, Sekten Formation, Botoman–Toyonian stages (Cambrian Stage 4), lower reaches of the Lena River. Siberia, Russia.

Description.—The brachiolar plates are preserved as isolated elements crescent-shaped in cross-section ( Fig. 62B ) or articulated hoof-shaped pairs ( Fig. 62A ) delimiting a slightly concave oral and convex aboral surfaces. Disarticulated (isolated) brachiolars are up to 0.5 mm long, with a length-to-width ratio of 0.5–2, and may slightly taper distally. Smaller brachiolars tend to be more elongated than the bigger ones. Articulation in a pair of opposed plates takes place along two flattened articulation surfaces (longitudinal ridges) delimiting a large, oval aboral canal ( Fig. 62A 1 : arrowed). The oral surface is variably concave, and consist of a median furrow flanked by two poorly developed but continuous ridges delimiting two variably developed lateral depressions. The external surface of the plates (latera) are smooth, and straight. Distal and proximal articulation surfaces are parallel and consist of a peripheral flat wall delimiting crescent shaped depressions around the aboral canal. Oral articulation between two opposite plates is straight and submedian (along the median oral furrow), while aboral articulation is either longitudinal or oblique. Articulated elements are aligned and opposite.

Remarks.—Brachiolar plates represent biserial feeding appendages of blastozoans. They are here considered as eocrinoids sensu lato, although some may represent the distal arm of Rhombiferans (see below). The plates are grossly similar to Brachiolar type B described by Kouchinsky et al. (2015a: figs. 62A–H, 63A–D) from the Judomia Zone, Atdabanian stage (correlated with the Cambrian Stage 3) and Calodiscus –Erbiella Zone, lower Botoman stage (correlated with the lower part of Cambrian Stage 4).

Figures

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.