Chancelloria spp.

Kouchinsky, Artem, Bengtson, Stefan, Clausen, Sébastien & Vendrasco, Michael J., 2015, An early Cambrian fauna of skeletal fossils from the Emyaksin Formation, northern Siberia, Acta Palaeontologica Polonica 60 (2), pp. 421-512 : 456-457

Chancelloria spp.

Fig. 34 .

Material.—Thousands of disarticulated sclerites from many samples ranging in age from the lower Dokidocyathus regularis Zone of the Tommotian Stage to the Bergeroniellus expansus Zone of the upper Botoman Stage, Malaya Kuonamka and Bol’shaya Kuonamka rivers (SOM 2). In addition, over twenty articulated or partially articulated sclerites of 6+1 and 7+ 1 types from samples 3/42, 7/17.8, 7/36.7, 7/70, and K1-3B; single forms 3+0 from sample 3/42, 4+0 from samples 7/70 and K1-3B, 5+0 from samples 7/34.7 and K1-3B, and 9+0 from sample 7/70. Figured specimens are SMNH X 3416, 4669–4681. After the chemical dissolution of the rock, disarticulated sclerites mostly represent internal moulds composed partly of phosphate, and partly of finegrained limestone matrix. The moulds are rarely glauconised in section 3, but more often glauconitic in other sections. Articulated and some disarticulated sclerites from samples 7/36.7, 7/70, and K1-3B preserve phosphatised walls.

Remarks.—The majority of sclerites have six to seven lateral rays with one central ray (6–7+1), with lateral rays gently curved away from the plane of the basal facet and sometimes laterally, while the slender central ray is almost perpendicular to the base of the rosette ( Fig. 34B, G, H ). Forms composed of three, four, or five lateral rays (3–5+0) are relatively rare ( Fig. 34F, K–M ). Several species of Chancelloria may be present, although 6–7+ 1 types of sclerites are similar to those of Chancelloria eros Walcott, 1920 , the lectotype of which has common 6–7+1 and rare 4–5+0 sclerites in the scleritome ( Bengtson et al. 1990). Sclerites of 3+0 type may derive from Allonnia sp. ( Bengtson et al. 1990). Phosphatised internal moulds of much larger disarticulated sclerites co-occur with smaller sclerites in sample 7/70. A single disarticulated sclerite from sample 6/66.2 demonstrates a composition of aciculate laths ( Fig. 34A ; see also Kouchinsky 2000a). This microstructure may reflect the original structure of the mineralised wall, and it has also been observed in middle Cambrian chancelloriid sclerites of 4+0 and 6–7+ 1 types from Greenland (AK and John Peel, personal observations 2012).

Figures

Fig. 34. Chancelloriids Chancelloria spp. from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River and Malaya Kuonamka River, Anabar Uplift, Siberia. A. SMNH X 3416, sample 6/66.2, internal mould of a disarticulated ray with aciculate remains of presumably original microstructure of the wall; lateral (A 1 ) and basal (A 2 ) views. B–D. Articulated sclerites of 6+1 type. B. SMNH X 4672, sample 7/70; lateral (B 1 ) and external (B 2 ) views. C. SMNH X 4673, sample 7/70; basal view. D. SMNH X 4674, sample K1-3B; basal view. E, J, N. Internal moulds of basal parts of larger disarticulated sclerites; sample 7/70. E. SMNH X 4669; basal view. J. SMNH X 4670; oblique basal view. N. SMNH X 4671; external view. F. Articulated sclerites of 4+0 type, SMNH X 4681, sample 7/70; external view. G–I. Articulated sclerites of 7+1 type. G. SMNH X 4675, sample 7/70; lateral view. H. SMNH X 4676, sample K1-3B; external view. I. SMNH X 4677, sample K1-3B; basal view. K–M. Articulated sclerites of 5+0 type. K. SMNH X 4678, sample 7 /34.7; lateral view. L. SMNH X 4679, sample K1-3B; lateral view. M. SMNH X 4680, sample K1-3B; external view. Scale bar 600 μm, except A, 300 μm.