Dodecaactinella 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 : 441-444

Dodecaactinella sp.

Fig. 72A–H .

Material.—Over a hundred of dolomitised spicules and fragments from samples 19/1.5, 19/3.25, 19/4.25, 19/5.5, 19/8.5, 19/10.25, 19/11.75, 19/25.5, 19/31.75, including figured SMNH Sp 11430–11437, Erkeket Formation, Khorbusuonka River, Siberia, Russia. Botoman and Toyonian stages (correlated with the Cambrian Stage 4).

Description.—Equiangular symmetrical triactine-type spicules, typically ca. 0.5 mm in size, up to 1 mm in the available material. The main rays diverge at 120° and branch dichotomously ( Fig. 72B, D, E, G ) or trichotomously ( Fig. 72A, C ) in the same plane at ca. 50 µm from the center of the spicule to its periphery. All rays 20–80 µm in diameter, do not taper and are smooth.

Remarks.—Such acid-extracted spicules from different localities ( Mostler 1985, 1996; Shabanov et al. 1987; Bengtson et al. 1990; Dong and Knoll 1996; Peel 2019) display faces of corroded crystals of calcite or dolomite constituting their recrystallised, originally high-magnesium calcite rays. Such a composition of spicules as well as their distinct triactine symmetry fits to those of the Calcarea (e.g., Jones and Jenkins 1970; Uriz 2006).

Reif (1968) assigned dodecaactinellid spicules to hexactinellids, but Rigby and Toomey (1978) correctly attributed them to calcareans. Dodecaactinella -like triactines are globally distributed in the Cambrian strata beginning from the Cambrian Stage 2 ( Reif 1968; Mostler 1985; Shabanov et al. 1987; Bengtson et al. 1990; Wood et al. 1993; Elicki 1994; Kruse et al. 1995; Dong and Knoll 1996; Mehl 1998; Sugai et al. 2004; Wrona 2004; Ivantsov et al. 2005a; Skovsted and Peel 2010; Peel 2019). Besides Dodecaactinella Reif, 1968 , such spicules are known as Phobetractinia Reif, 1968 , Sardospongia Mostler, 1985 , Polyactinella Mostler, 1985 , Bengtsonella Mostler, 1996 (preoccupied name replaced by Mostlerhella Özdikmen, 2009 ), and Pseudosardospongia Fedorov in Pegel et al., 2016. They differ by the presence or absence of the third- and fourth-order branching and additional rays, probably resulting from incomplete fragmentation of fused pharetronid-type skeletons. The latter are known from Bottonaecyathus Vologdin, 1940 , and Gravestockia Reitner, 1992 , and may therefore belong to different species and genera. While relatively large branching Bottonaecyathus participated in early Cambrian reef building in Tuva and Mongolian microcontinents, small Gravestockia was a cryptobiont in metazoan reefs and microbialites of Australia ( Zhuravlev 2001).

Class incertae sedis

Order Heteractinida Hinde, 1887

Hexaradiate spicules

Fig. 73 .

Material.—Several hundred phosphatised spicules, including figured SMNH Sp11441–11459, from samples 19/5.5, 19/8.5, 19/10.25, 19/11.75, 19/12.75, 19/29, Erkeket Formation, Khorbusuonka River, 22/0 and 22/50, from the uppermost Tyuser and Sekten formations, lower reaches of the Lena River. Siberia, Russia; lower Botoman and Toyonian stages correlated with the Cambrian Stage 4).

Description.—Spicules with six paratangential rays regularly radiating from a central disk at ca. 60º between each other in the plan view and with a slight angle in the lateral view with respect to the plane of the central disk. One side of the central disk is slightly concave and smooth. The opposite, convex side can be smooth ( Fig. 73B ), or bear a short axial ray with a robust base slightly inclined towards the central disk ( Fig. 73F 1 , H, I, J, O: arrow). In other forms, the convex side of the spicule carries 10–15 accessory rays radiating from the central disk ( Fig. 73G, K–S ). The accessory rays can be short and blunt or more prominent and pointed.

Remarks.—In a number of publications on Cambrian spicules, such hexaradiates are ascribed to the heteractinid sponge Eiffelia Walcott, 1920 (e.g., Bengtson et al. 1990; Skovsted 2006a). The type material of Eiffelia is represented by complete skeletons consisting of tetraradiate spicules and flat hexaradiate spicules with long lateral rays and central disks ( Walcott 1920; Botting and Butterfield 2005). Rays perpendicular to the plane of the other four rays were observed in some of the tetraradiates, whereas all hexaradiate spicules had no evidence of perpendicular rays (Botting and Butterfield 2005:1555).Bengtson et al. (1990), who described very similar hexaradiate spicules from upper Cambrian Stage 3–lower Stage 4 of Australia as Eiffelia araniformis (Missarzhevsky in Mambetov and Missarzhevsky, 1981), synonymised several species of Lenastella Missarzhevsky in Missarzhevsky and Mambetov, 1981, from coeval deposits of Kazhakhstan with E. araniformis , as well as Actinoites Duan, 1984 , and Niphadus Duan, 1984 , from South China. Mehl (1998) described such hexaradiates from the Drumian Stage of Australia. Spicules from Kazakhstan and Siberia have more massive bases of their rays and larger central disks compared to slender rays and relatively smaller central disks in Australian forms. Nodes on the convex side of central disk observed in the specimens from Australia are not revealed in the material from Kazakhstan and Siberia. Hexaradiate spicules of E. araniformis (Missarzhevsky in Missarzhevsky and Mambetov, 1981) do not have accessory rays but occasionally a short single distal ray.

Peel (2016, 2019) described spicules of Eiffelia floriformis Peel, 2019 , from Cambrian Stage 4 Henson Gletscher Formation, Peary Land, North Greenland and suggested that six- and four-rayed spicules of undetermined heteractinids from the lower Botoman stage Emyaksin Formation of the Anabar Uplift in Siberia ( Kouchinsky et al. 2015a: fig. 72A, C–E, G, H) can be attributed to the same species. Hexaradiates and co-occurring tetraradiate forms with accessory rays (see below) described herein are, however, different in having rays radiating at a slight angle to the central disk, longer accessory rays, and long axial rays in tetraradiates.

In some chemically extracted specimens, a star-like hollow depression is present ( Fig. 73C–E ). Probably, the hollow space within the central discs of hexaradiates and axial parts of their rays appeared owing to dissolution of a primary labile carbonate mineral, such as aragonite, and incomplete phosphatisation of the core. This suggestion is also corroborated by broken off and subsequently rounded and phosphatised tips of rays. A fibrous microstructure of these elements is indicative of their probable primary aragonite mineralization in the form of acicular radiating needles ( Fig. 73C, D ). Such a microstructure and composition are not typical of any extant sponges but occur in radiocyaths, which two-walled globular and pyriform rigid skeletons are built of giant spicule-like elements, meroms, consisting of distal and proximal star-like nesasters connected by long shafts ( Kruse et al. 2015).

Spicules of the heteractinid Eiffelia Walcott, 1920 , and the protomonaxonid sponge Lenica Gorjansky, 1977 , were interpreted as bimineralic, namely calcitic with a silica core (Botting and Butterfield 2005; Bengtson and Vinther 2006; Bengtson and Collins 2015; Nadhira et al. 2019; Drozdov et al. 2022). Such a complex mineralogy could potentially lead to a selective dissolution and phosphatisation, but appears to be a less plausible explanation of preservational features observed herein.

Tetraradiate spicules with accessory rays

Fig. 74A–H, J, L–P, S .

Material.—Several hundred phosphatised spicules, including figured SMNH Sp11460–11474, from samples 19/10.25, 19/11.75, 19/12.75, Erkeket Formation, Khorbusuonka River and sample 22/0, uppermost Tyuser Formation, Lena River. Siberia, Russia; lower Botoman stage (correlated with the Cambrian Stage 4).

Description.—Spicules with four rays radiating from central disk, respectively, at ca. 90º between each other in the plan view, with a slight angle in the lateral view. A single long ray emerges perpendicular to the central disk. On the opposite side of the spicule, 10–15 accessory rays radiate from the central disk. Those can be knobby or more prominent and pointed. A single central prominent and pointed ray can be present among them.

Remarks.—In general such skeletal elements resemble basic heteractinid sponge spicules possessing a longer and more massive proximal ray, which is directed inwards the sponge wall ( Pickett 2002). Fused spicules and mostly broken rays indicate that these elements have formed relatively rigid and moderately thick-walled skeletons, which are typical of a number of heteractinids ( Pickett 2002).

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. 73. Heteractinid spicules from the lower Cambrian Erkeket Formations, Khorbusuonka River, Siberia, Russia; samples 19/10.25 (A–F, I), 19/11.75 (H, J, P, R), and 19/12.75 (G, K–O, Q, S). A–S. Hexaradiate spicules, SMNH Sp11441–11459, respectively. C–E, S, spicules with dissolved internal parts of the rays reminiscent of axial canals. F1, spicule in lateral view; F2, concave side of the spicule with radiating six paratangential rays. H, I, strongly eroded spicules. G, K–S, hexaradiate spicules with accessory rays.Arrow in O points to fused spicules.Arrow in S points to dissolved inner portion of the ray reminiscent of axial canal. Scale bar: 250 µm (C–G, J, M–O, Q, S) and 500 µm (A, B, H, I, K, L, P, R).

Fig. 1. Locality maps. A. Map of the Siberian Platform. B. Khorbusuonka River. Localities referred to in the main text: 96-1, 96-2, 96-3, Malaya Kuonamka (MK) and 96-6, 96-7, 96-8, Bol’shaya Kuonamka (BK) rivers, eastern flank of the Anabar Uplift; 3, 11, 15, 19, 20, Khorbusuonka River, Olenyok Uplift; 21, 22, Lena River lower reaches, Ulakhan-Ald’yarkhay Brook; Le, Lena River middle reaches, Tiktirikteekh and Bydyangaya Brooks; Al, Aldan River, between sections “Dvortsy” and Ulakhan- Sulugur. Margin of the Siberian Platform is shown by a dashed line in A.

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).