Archaeocyatha Bornemann, 1884

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 : 449

Class Archaeocyatha Bornemann, 1884

Calcareous, originally consisting of high-magnesium calcite cups of archaeocyaths are prone to dissolution either during diagenesis or rock treatment. Internal moulds of canals and cavities in the cups or calcium phosphatic veneers replicating skeletal structures allow, however, recognition of the original morphology of archaeocyaths in three-dimensional details. Phosphatised fragments of archaeocyaths are preserved as a part of small shelly fossil assemblages, found in the Tommotian stage of Siberia (herein), Cambrian Stage 3 Salaany-Gol Formation of the Mongolian Zavkhan Terrane (Debrenne et al. 1990; Pruss et al. 2019), Shuijingtuo Formation of the Three Gorges area of the Yangtze Platform ( Zhang et al. 2016), and Puerto Blanco Formation of Mexico (Laurentia, Devaere et al. 2019: fig. 23), Cambrian Stage 4 Browns Pond Formation of the Eastern Laurentian Taconic allochthon ( Landing and Bartowski 1996; AZ own observation in Ed Landing’s collection 2008), Bastion Formation of North-East Greenland ( Skovsted 2006a) and glacial erratics of Antarctica ( Wrona 2004). Archaeocyaths cups from Cambrian Stage 4–Wuliuan Stage, Henson Gletscher Formation were interpreted as an octocoral by Peel (2017a: fig. 4) or as problematic fossils from Series 2 of Australia by Laurie (1986: fig. 10). Archaeocyaths, preserved in calcium phosphate, resemble in some cases superficially other groups and can be confused with ecdysozoan moults (i.e., Microdictyon ), ossicles of echinoderms preserved in calcite and fragmented along the cleavage planes in monocrystals ( Fig. 64 ) and phosphatised fragments of hyolithid helens ( Kouchinsky et al. 2015a: fig. 32 and Fig. 55 herein).

Figures

Fig. 64. Thecal plates of Echinodermata from the lower Cambrian Erkeket Formation of Khorbusuonka River (B, C, E–O) and the middle Cambrian Sekten Formation, lower reaches of the Lena River (A, D), Siberia, Russia; samples 19/11.75 (O), 19/12.75 (J), 20/2B (I, K–M), 20/5B (B, C, F, G, N), 20/6B (E, H), and 22/67 (A, D). A–O. Thecal plates, SMNH Ec36886–36900, respectively. Scale bar 500 µm.

Fig. 55. Hyolithid helens from the lower Cambrian Erkeket Formation, Khorbusuonka River (A, B, E, F) and Tyuser (D) and Sekten (C) formations, lower reaches of the Lena River, Siberia, Russia; samples 19/10.75 (A, B, E), 20/1B (F), 21/21 (D), and 22/50 (C). A–F. Phosphatized hyolithid appendages (helens), SMNH X11230–11235, respectively.A2, enlargement of A1 showing proximal end; B1, upper surface; B2, oblique view on the upper and frontal (left side in B1) surfaces; B3, view on the rear margin of the helen (right side in B1); B4, close-up of proximal portion; E1, oblique lateral view; E2, F1, plain view; F2, F3, view on the margin (left side in F1). Scale bar: 500 µm (C, D), 250 µm (A1, B1–B3, E, F), 80 µm (A2, B4).