Chancelloria spp.

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 : 394

Chancelloria spp.

Fig. 36 .

Material.—Thousands of disarticulated sclerites from many samples (SOM 3), including figured specimens, SMNH X11094–11105, from samples 11/2B, 11/16.1, 19/11.75, 21/21, 21/51, and 21/54; ranging in age from the Tommotian stage correlated with the upper part of Cambrian Stage 2) to the Toyonian stage (Cambrian Stage 4), in the Khorbusuonka River and Ulakhan-Ald’yarkhay sections, Siberia, Russia.

Remarks.—Articulated sclerites from the lower Tommotian stage D. regularis Zone , from the basal Erkeket Formation and from basal part of the Middle Member of the Tyuser Formation are represented by rare 7+1, 8+1, and 9+1 sclerites and a single 11+1 sclerite (SOM 3). The majority of sclerites from Cambrian Stage 3–lower Stage 4 have nine lateral rays with one central ray (9+1) and five rays without a central ray (5+0). Sclerites composed of six, seven or ten lateral rays and a central ray (6+1, 7+1, and 10+1) are relatively rare. Chancelloria spp. sclerites have lateral rays gently curved away from the plane of the basal facet central ray incompletely preserved; Fig. 36A–D ) and there is no firm evidence to support the presence of a particular number of species. Chancelloria sclerites have phosphatised walls composed of longitudinally oriented along the surface aciculate laths ( Fig. 36P–S ), similar to those described from chancelloriids previously ( Kouchinsky 2000; Kouchinsky et al. 2015a: fig. 34A).

A single sclerite ( Fig. 36E ) with three rays bent away from the basal facet (3+0 type) may represent a species of Allonnia Doré and Reid, 1965 (Bengtson et al. 1990), which has not been previously reported from the Tommotian strata of the Siberian Platform. Single sclerites with three rays also attributable to Allonnia sp. occur in samples from the upper part of Cambrian Stage 3 (21/54) and lower Mayan Stage (19/5B) ( Fig. 36N, O ). Diameter of the basal facet in each ray of Allonnia sp. (?) from the Tommotian stage is about half of that in younger forms.

Several tens of articulated sclerites from samples 19/11.75 and 19/12.75 have four or five rays with one of the rays bent outwards. Those can be attributed to Archiasterella Sdzuy, 1969 . Archiasterella - type sclerites occur in a relatively few samples from Cambrian Stage 3 compared to Chancelloria spp. and some of them may correspond to the 4+0 sclerites known from Chancelloria . In samples 19/11.75 and 19/12.75, the 7+1 and 9+1- type sclerites of Chancelloria spp. are found in association with the Archiasterella sclerites.

Figures

Fig. 36. Sclerites of chancelloriids from the lower Cambrian Erkeket Formation, Khorbusuonka River (A, K, N, P–S) and Tyuser Formation, lower reaches of the Lena River (B–J, L, M, O), Siberia, Russia; samples 11/16.1 (A), 19/5B (N), 19/11.75 (P–S), 19/12.75 (K), 21/21 (B–H), 21/51 (I, J), 21/54 (L, M, O).A–D, P, R, S. Chancelloria spp. with a central ray, SMNH X11094–11100, respectively. F–H, M, Q. Chancelloria spp. without a central ray, SMNH X11101–11105, respectively. E, N, O. Allonnia sp., SMNH X11106–11108, respectively. I–L. Archiasterella sp., SMNH X11109–11112, respectively. Scale bar: 500 µm (A–O, Q) and 250 µm (P, R, S).