Sulugurella sp.

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 : 447-449

Sulugurella sp.

Fig. 72I–K .

Material.—Several dolomitised spicules and fragments from samples 19/4.25, 19/5.5, 20/1B, including figured SMNH Sp11438–11440, Erkeket Formation, Khorbusuonka River, Siberia, Russia. Lower Botoman stage (correlated with the lower part of Cambrian Stage 4).

Description.—Incomplete pentact spicules with four rays situated in the same plane at an angle 90° between each other. The paratangential rays are 50–100 µm in diameter and somewhat thinner than a perpendicular axial ray.

Remarks.—The surface of spicules is strongly corroded and displays crystal faces of secondary dolomite replacing original high-magnesium calcite. Calcareous pentactines as well as hexactines and tetractines with the overall spicule morphology typical of hexactinellids are relatively common in lower Cambrian strata and described under the generic name Calcihexactina Sdzuy, 1969 (e.g., Brock and Cooper 1993; Peel 2019). In the original description of the type species, Calcihexactina franconia Sdzuy, 1969 , from the Wuliuan Stage of the Franconian Forest, Germany, Sdzuy (1969) indicated axial canals that is absent from calcarean spicules. Thus, the calcareous composition in Sulugurella - type spicules is secondary and they can be attributed to hexactinellids. On the other hand, they can be originally calcitic and represent remains of some stem sponges possessing hexactine spicules composed partly or wholly of calcite (Botting and Muir 2018).

Inflated hexactines

Fig. 77 .

Material.—Several tens of hexactines, including figured SMNH Sp11500–11518, from samples 19/10.25, 19/11.75, 19/12.75, Erkeket Formation, Khorbusuonka River, lower Botoman Stage (correlated with the lower part of Cambrian Stage 4), sample 11/16.05, basal Erkeket Formation and sample 21/21, Tyuser Formation, Lena River, Dokidocyathus regularis Zone, Tommotian Stage (correlated with the upper part of Cambrian Stage 2). Siberia, Russia.

Description.—Phosphatised spicules, 0.4–1.0 mm in size, composed of six tapering rays with smooth surface. The opposite rays are situated along three axes crossed at nearly right angles with each other. Presence of axial canals is not evident owing to preservation. Some of the spicules have relatively slim rays and regular hexact morphology ( Fig. 77A, G, I ). In other spicules ( Fig. 77C–F, J–S ), the central part is swelled and can be almost spherical ( Fig. 77P ), 300–500 µm in diameter. Strongly inflated spicules with conical rays lacking axial canals occur ( Fig. 77D ). In some of these forms, rays are underdeveloped or eroded and form short knobs.

Remarks.—Inflated hexactines with conical rays are described as Cjulanciella asimmetrica Fedorov in Fedorov and Pereladov, 1987 from the Ovatoryctocara –Schistocephalus Zone, Amgan stage (correlated with the uppermost Stage 4– lowermost Wuliuan Stage), Kuonamka Formation, Siberia. Cjulanciella asimmetrica demonstrates a high variety of siliceous spicules in their assemblages. The type species syntypes include diactines, tetractines and pentactines (Fedorov and Pereladov 1987). The oldest allied spicules are detected in the uppermost Atdabanian (correlated with the Cambrian Stage 3) Emyaksin Formation in the south-western Anabar Uplift of the Siberian Platform ( Shabanov et al. 1987). Similar spicules were reported from the Cambrian Stage 3 of the Altay Sayan Foldbelt in Siberia ( Sugai et al. 2004) and Wuliuan–Drumian stages of the Siberian Platform ( Kouchinsky et al. 2011: fig. 40E–G), lower Cambrian of Laurentia ( Skovsted and Holmer 2006: fig. 5G, H), middle–upper Cambrian of Australia (Bengtson 1986: fig. 9E; Mehl 1998: pl. 1: 1), South China (Ding et al. 1992; Dong and Knoll 1996: fig. 7.1; Steiner et al. 2007), and upper Cambrian of Argentina (Heredia et al. 1987). Mehl (1996, 1998) considered such spicules as a part of a single dermal sponge skeleton built in conjunction with Thoracospongia - type follipinules but at present in situ associations have been found only in the Wuliuan Henson Gletscher Formation of North Greenland ( Peel 2017 b, 2019).

Phosphatised hexactines with simple rays co-occur with hexa- and tetraradiated spicules of heteractinids in our material ( Fig. 77 ) and some of them may derive from the same scleritomes. Hexactines vary in size 2–3 times and some of them have a swelled axial zone. Presence of such spicule types in the heteractinid assemblages from the lower Botoman stage of northern Siberian Platform is noted by Kouchinsky et al. (2015a). Such spicules also occur in the Dokidocyathus regularis Zone, Tommotian stage (correlated with the upper part of Cambrian Stage 2), wherefrom hexaradiate heteractinid spicules are unknown. Although such hexactins with conical and often swelled rays may represent siliceous hexactinellid spicules diagenetically replaced by carbonate and phosphate minerals, axial cavities may have originated from dissolution of carbonate under phosphatic coatings during chemical extraction. The origin of such spicules is thus not clear, but some of them ( Fig. 74U, W, X, Y ) are very similar to tetraradiates with accessory distal rays described above from heteractinids. It cannot therefore be excluded that first heteractinids with tetraradiate spicules appeared in the upper part of Cambrian Stage 2 .

Figures

Fig. 72. Calcareous (dolomitised) spicules from the lower Cambrian Erkeket Formations, Khorbusuonka River, Siberia, Russia; samples 19/4.25 (H, I), 19/5.5 (A, J), 19/8.5 (D), 19/10.25 (C), 19/11.75 (E, G), 19/25.5 (B, F), and 20/1B (K). A–H. Dodecaactinella sp., SMNH Sp11430–11437, respectively. I–K. Sulugurella sp., SMNH Sp11438–11440, respectively. Scale bar 250 µm.

Fig. 77. Inflated hexactines from the lower Cambrian Erkeket Formation, Khorbusuonka River (A–N) and Tyuser Formation, lower reaches of the Lena River (O–S), Siberia, Russia; samples 19/10.25 (C, D, H, K), 19/11.75 (A, B, E, F, J, L, M), 19/12.75 (G, I), 11/16.05 (N), and 21/21 (O–S). A, B, G–I. Hexactines with smooth tapering rays, SMNH Sp11500–11504, respectively. C–F, J–S. Strongly inflated hexactines with smooth tapering rays, SMNH Sp11505–11518, respectively. Scale bar 500 µm.

Fig. 74. Heteractinid spicules from the lower Cambrian Erkeket Formation, Khorbusuonka River (A–E, G, I, K–Q, V, Y) and Tyuser Formation, lower reaches of the Lena River (F, H, J, U, W, X, Z), Siberia, Russia; samples 11/16.05 (Y), 19/10.25 (E, I, K, L, O–Q), 19/11.75 (A–D, R–T, V), 19/12.75 (G, M, N), 21/21 (U, W, X), and 22/0 (F, H, J, Z). A–H, J, L–P, S. Polyactinal spicules with an axial ray, SMNH Sp11460–11474, respectively. U, W–Y. Inflated polyactinal spicules from the Tommotian Stage, SMNH Sp11475–11478, respectively. I, K, Q, R, T, V, Z. Microcoryne cf. cephalata Bengtson in Bengtson et al., 1990, SMNH Sp11479–11485, respectively.Scale bar: 250 µm (A–J, N, O, Q, R, V,Y, Z) and 500 µm (K–M, P, S, T, U, W, X).