Rhombocorniculum cancellatum ( Cobbold, 1921 )

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 : 467-471

Rhombocorniculum cancellatum ( Cobbold, 1921)

Figs. 46–48.

Material.—About two hundred sclerites are available from the Emyaksin Formation of the Malaya Kuonamka and Bol’shaya Kuonamka rivers; correlated with the Judomia Zone, Atdabanian Stage. Several tens of sclerites from sample 3/62.5 and a single one from sample 3/65; over twenty five sclerites, including six figured specimens SMNH X 4742–4747 from sample 6/66.2; eighteen sclerites, including SMNH X 4748– 4753, from sample 7/55, twelve sclerites, including SMNH X 4754 and 4755, from sample 7/55.8, twenty sclerites, including SMNH X 4756–4760, from sample 7/60, ten sclerites, including SMNH X 4761 and 4762, from sample 7/62.

Description.—Sclerites demonstrate a range of variation between slender narrow and broad flattened ones, commonly associated with transitional forms (as in the type strata; see Hinz 1987).

The slender sclerites are elongated and narrow (up to 1.5 mm long and 0.2 mm wide). They can be curved up to 90° and twisted up to 90° clock- or anticlockwise (R-, dextral, and L-, sinistral, forms, respectively). One of the lateral margins of the sclerites is thicker than the other one, which can be blade-like, thus producing an asymmetric drop-like cross-section which is better expressed in the basal part of the sclerites.The sclerite tip is straight and acute. A central simple lumen produces a relatively narrow rounded or oval opening at the basal end of the sclerite. The proximal part,

Fig. 46. Problematicum Rhombocorniculum cancellatum ( Cobbold, 1921) from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Ana- → bar Uplift, Siberia; sample 6/66.2. A, E, F. Slender sclerites. A. SMNH X 4742; A

1

, close-up of the proximal end of A

2

. E. SMNH X 4743; lateral view.

F. SMNH X 4744; F

1

and F

2

, lateral views. D. Sclerite of a transitional morphology, SMNH 4745; D

1

and D

2

, lateral views; D

3

, proximal end of sclerite;

D 4, D 5, close-up showing longitudinal striation on the external surface of sclerite. B, C. Broad sclerites. B. SMNH X 4746; B 1, close-up of the proximal end with openings of tubules of B

2

. C. SMNH X 4747; C

2

, proximal end with central lumen of specimen in C

1

. Scale bar 300 μm, except A

1

, 60 μm; B

1

,

15 μm; C

2

, 120 μm; D

3

, D

4

, 30 μm; D

5

, 3 μm.

more laterally ( Figs. 47B 1 , C 2 , 48F 2 ). Similarly to R. insolutum (see above), the wall is composed of narrow tubules, 0.1–0.2 μm in diameter (Figs. 46B 1, D 3 –D 5, 47B 1, C 2, 48F 2).

The broader sclerites are found in association with the slen- der ones, but are relatively less common. The broader sclerites share the same general morphology, being flattened, curved up to 90°, and twisted longitudinally up to one revolution, with L- and R-forms also recognised among them ( Hinz 1987; Li et al. 2003). The sclerites are narrower and blade-like towards one of the lateral sides. One of the broad sides is slightly concave, whereas the opposite one is convex (e.g., Fig. 47B 1 ). The sculpture is similar to the one described above for slender sclerites. The sclerites have a tapering apical tip and extended basal end with an asymmetrically placed rounded opening of the central lumen.The wall contains tubules, whereas the outer surface exhibits fine longitudinal striation, just as in the slen- der forms.For comparisons see remarks to R. insolutum below.

Stratigraphic and geographic range.—Distributed worldwide in the Cambrian Stage 3, upper Atdabanian–lower Botoman equivalent strata. In the material from the Malaya and Bol’shaya Kuonamka rivers, R. cancellatum occurs within the Judomia Zone of the Atdabanian Stage. R. cancellatum occur in Series 2 of Kazakhstan ( Missarzhevsky and Mambetov 1981), South and North China ( Li et al. 2003; Steiner et al. 2007), Australia ( Gravestock et al. 2001), Avalonia ( Landing et al. 1980; Hinz 1987), and the Mediterranean region ( Elicki 2005, 2007).

Figures

Fig. 47. Problematicum Rhombocorniculum cancellatum (Cobbold, 1921) from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia. A, C, F. Slender sclerites, sample 7/55. A. SMNH X 4748; A2, proximal end of sclerite in A1. C. SMNH X 4749; C2, proximal end of sclerite in C1. F. SMNH X 4750; lateral view. B, D, E. Sclerites of transitional morphology, sample 7/55. B. SMNH X 4751; B2, proximal end of sclerite in B1. D. SMNH X 4752; lateral view. E. SMNH X 4753; lateral view. G, H. Sclerites, sample 7/55.8. G. Broad sclerite, SMNH X 4754; G1, view on the broader side; G2, lateral view. H. Basal part of sclerite, SMNH X 4755. Scale bar: 300 μm, except A2, B1, C2, 60 μm; H, 120 μm. usually somewhat narrowing towards its base, can bear one to that described for R. insolutum (see above). In many cases, or several stacked rounded structures (Fig. 48A), similar to however, the tubercles are nearly perpendicular to the wall growth increments. These rounded structures can either be (Figs. 46A1, A2, 47A2, C2, H). The lumen is surrounded by smooth or carry a smoothened sculpture. The exterior surface relatively thick walls. The wall contributes to at least half of of the sclerite is covered with a cancellate sculpture, similar the sclerite width between the broader sides and significantly

Fig. 1. Map showing location of sections 1, 3, Malaya Kuonamka River; 6, 7, Bol’shaya Kuonamka River.

Fig. 2. Derivation of fossiliferous samples (arrows with numbers) and chemostratigraphic correlation of sections 1 and 3 (Malaya Kuonamka River) with sections 6 and 7 (Bol’shaya Kuonamka River). Correlation with carbon isotope features II–VII from stratotypes of south-eastern Siberian Platform (Brasier et al. 1994) according to Kouchinsky et al. (2001). Note that the zero level in all sections corresponds to the water level above which the sections were exposed in 1996. Description of fauna from the lower part of the Emyaksin Formation in complementary outcrops at the Bol’shaya Kuonamka River (below that zero level) will be published elsewhere. Fauna from the overlying Kuonamka Formation is published by Gubanov et al. (2004) and Kouchinsky et al. (2011).

Fig. 48. Problematicum Rhombocorniculum cancellatum (Cobbold, 1921) from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia. A–C, E, F. Slender sclerites, sample 7/60. A. SMNH X 4756; A1, distal end of sclerite; A2, proximal end of sclerite; A3, lateral view. B. SMNH X 4757; lateral view. C. SMNH X 4758; lateral view. E. SMNH X 4759; lateral view. F. SMNH X 4760; F1, lateral view; F2, close-up of basal end showing microstructure. D, G. Sclerites, sample 7/62. D. Slender sclerite, SMNH X 4761; lateral view. G. Broad sclerite, SMNH X 4762; G1 and G2, lateral views. Scale bar 300 μm, except F2, 60 μm.