Ctenocystoidea Robison and Sprinkle, 1969

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 : 429

Class Ctenocystoidea Robison and Sprinkle, 1969 Order Ctenocystoida Ubaghs and Robison, 1988 Family Ctenocystidae Sprinkle and Robison, 1978 Ctenidial plates

Fig. 63J–L .

Material.—Three secondarily phosphatised plates, all figured, SMNH Ec36882–36884, from samples 20/2B and 19/25.5 Erkeket Formation, Khorbusuonka River, Siberia, Russia. Uppermost Botoman–basal Toyonian stages (correlated with the Cambrian Stage 4).

Description.—Anterolateral plates of the lower-surface (ventral) marginal-frame (ctenidial plate, from about 500 to 750 µm in length) composed of an anterior hook-shaped process and a trapezoidal to squared dorsal blade (about 450 µm in maximum width). The gutter-like anterior process is lanceolate and arcuate in lateral view with a concavo-convex, U-shaped transverse section. The lateral walls of the posterior process decrease laterally and posteriorly at the junction with flat to slightly concave posterior blade, the latter being bordered with posteriorly decreasing walls.

Remarks.—These ctenidium plates are the oldest ctenoid evidences reported so far. Similar hook-shaped ctenidial plates are reported in most ctenocystoid species (Zamora et al. 2012). Ctenidium plates of Ctenocystis ( Robison and Sprinkle 1969) have similar gutter-like, hook-shape anterior blade with the gutter oriented outwards ( Rahman and Clausen 2009). The ctenocystoids are interpreted as basalmost, bilateral to slightly asymmetrical echinoderms ( Rahman and Clausen 2009; Zamora et al. 2012), although they occur relatively late in the early echinoderm record (Wuliuan Stage). This discovery from the Cambrian Stage 4 of the Khorbusuonka River thus significantly reduces the incongruence between the phylogenetic position of ctenocystoids and their first appearance data. Along with other remains reported herein, they place the Siberian Platform as one of the diversification centres of basal echinoderms (SOM 5).

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).