Type species: Yochelcionella cyrano Runnegar and Pojeta, 1974; Redlichia chinensis Zone, Ordian Stage; Mootwingee Range, New South Wales, Australia.

Remarks.—Yochelcionella is erroneously placed under Family Helcionellidae Wenz, 1938 in Gubanov et al. (2004) and Kouchinsky et al. (2011).

Material.—Eight calcium phosphatic internal moulds, including four figured specimens SMNH Mo 160417, 160418, 167648, and 167649 from sample 7/36.7. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Shell slightly cyrtoconic, laterally somewhat compressed, with a sub-apical tube (snorkel) and overhanging apex inclined towards snorkel. In lateral view, the supra-apical margin is convex near the apex, but straightens towards the aperture. Three to four transverse folds run around the inner shell surface (as reflected by internal moulds), more prominent laterally. The aperture is oval, with a length/width ratio of ca. 1.5. The diameter of bulbous apex varies from 0.2 to 0.3 mm. Snorkel, drop-like or oval in cross-section (Fig. 3A₂-C₂), narrows distally without apparent flaring and projects upwards at ∼45° with respect to the apertural plane.

Remarks.—The fossils herein were described as Yochelcionella sp. by Kouchinsky (2000a: 130, fig. 9). Comparison with shells and internal moulds of the same size belonging to twenty known species of Yochelcionella shows that the fossils described herein are most similar to the holotype of Y. greenlandica Atkins and Peel, 2004 from Stage 4 of Laurentia (Atkins and Peel 2008: fig. 8 A–D), but the latter has a slightly more pronounced overhang of the apex and a relatively slim snorkel.

Type species: Helcionella jacutica Missarzhevsky in Rozanov and Missarzhevsky, 1966; Dokidocyathus regularis Zone, Tommotian Stage; Tiktirikteekh, near Churan village, middle Lena River, Siberia.

Material.—Five calcium phosphatic internal moulds, including four figured specimens SMNH Mo 167650–167653 from sample 7/36.7. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Low cyrtoconic, laterally somewhat compressed shell. Apertural margin suboval in outline, narrowing towards straight subapical margin, with length/width ratio 1.8 to 2.0. Apex bulbous and somewhat compressed laterally, strongly inclined and projecting beyond the apertural margin. Sub-apical side concave and relatively short. Supra-apical side long and evenly convex. The inner surface of the shell (as reflected by internal moulds) bears concentric folds.

Remarks.—The fossils have diagnostic features of Bemella, but their assignment is hampered by incomplete preservation as smooth internal moulds.

Type species: Parailsanella acris Zhegallo in Voronova et al., 1987; Fallotaspis Zone, Montezuman Stage; Mackenzie Mountains, Northwest Territories of Canada.

Remarks.—Parailsanella includes bilaterally symmetrical, laterally compressed, slightly cyrtoconic univalves. Parailsanella is smaller in general and more laterally compressed than Ilsanella Missarzhevsky, 1981. Ilsanella is usually represented by larger (>1 cm) forms, with straight apertural margin, whereas in mm-sized Parailsanella (sensu Parkhaev in Gravestock et al. 2001) there is a sinus or train, which is not, however, reported from the type species, P. acris (Voronova et al. 1987: 45, pl. 20: 1). Lateral compression is not as conspicuous as in stenothecids (see below). Parailsanella is most similar to Mackinnonia Runnegar in Bengtson et al., 1990 (see below), but the latter can be distinguished by a lower shell, strongly hooked apex and more prominent train with a pegma-like structure. Comparison with generally similar Anabaroconus Gubanov, Kouchinsky, Peel, and Bengtson, 2004 (see also Kouchinsky et al. 2011) is hampered by its different mode of preservation as silicified shells. The otherwise similar Capitoconus Skovsted, 2004 was originally distinguished from Parailsanella only by a larger and more elongated protoconch. The forms described below fit the diagnosis of Parailsanella, although without a sinus or train preserved.

Material.—Thirty-nine calcium phosphatic internal moulds, including four figured specimens SMNH Mo 160415, 160416, 167655, and 167656 from sample 7/36.7. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Slightly cyrtoconic, laterally somewhat compressed shells. Supra-apical surface evenly convex; sub-apical surface concave. Aperture broadly oval, with length/ width ratio 1.7 to 2.0. Low and broad comarginal folds, faint on small (juvenile) specimens, are situated on the inner shell surface (as reflected by internal moulds). The apex is ∼0.3 mm in lateral view, bulbous and pinched at its base.

Remarks.—The surface of internal moulds, excluding apical area, is commonly covered with pits ∼5 µm in diameter and 10–20 µm apart (Fig. 6A). In one case, a juvenile mould preserves a shallow polygonal texture with centres of polygons 10–20 µm apart (Fig. 6B). Parailsanella sp. 1 is very similar to Capitoconus inclinatus Skovsted, 2004 from Stage 4 of Greenland, but the latter form has a relatively larger globose protoconch and laterally more compressed shell. The fossils described herein are discussed as Ilsanella sp. by Kouchinsky (2000a: 130, fig. 8).

Material.—Single calcium phosphatic internal mould SMNH Mo 167658 from sample 7/70. Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Slightly cyrtoconic and laterally compressed shell. Aperture is narrow oval, with length/width ratio of 3. The apex is bulbous, ∼0.3 mm in lateral view. The inner shell surface (as reflected by internal moulds) is covered with comarginal folds.

Remarks.—The form is similar to Capitoconus artus Skovsted, 2004 from Stage 4 of Greenland. The latter is preserved, however, with remains of phosphatised shell, with prominent sharp transverse ribs and a more distinct apex (Skovsted 2004: fig. 5). Parailsanella sp. 2 is more compressed laterally and has more distinct narrower comarginal folds than Parailsanella sp. 1 herein.

Type species: Figurina figurina Parkhaev in Gravestock et al., 2001; Abadiella huoi Zone, lower Cambrian; Horse Gully section, Yorke Peninsula, Australia.

Material.—Two phosphatised internal moulds, SMNH Mo167659 and 167660 from sample 7/70. Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Low, slightly cyrtoconic, laterally somewhat compressed shell, 0.8–0.9 mm in width and height and 1.7 mm long, with rounded apex displaced towards the apertural margin and indistinct protoconch. The inner shell surface (as reflected by internal moulds) has faint transversal folds. Subapical surface is very short. Aperture oval, a little flared, with subapical train.

Remarks.—Internal mould is covered with shallow pits, 10 µm in diameter (Fig. 8A₄). The fossils have diagnostic features of Figurina Parkhaev in Gravestock et al. 2001. Compared to the type species, F. figurina Parkhaev in Gravestock et al. 2001 (Series 2, Australia), F. capitata Parkhaev in Gravestock et al. 2001 (Series 2, Australia), F. groenlandica Skovsted, 2004 (Stage 4, Laurentia), and F. nana (Zhou and Xiao, 1984) (Series 2, North China and Australia), the fossils are most similar to F. nana (see Gravestock et al. 2001: pl. 35: 7), but one of the specimens herein (Fig. 8B) appears wider. Figurina cf. F. nana are also similar to internal moulds of juveniles of Mackinnonia anabarica (see below, Figs. 9C, 11C), but are twice as big and have wider and less curved apex.

Type species: Mackinnonia davidi Runnegar in Bengtson et al., 1990; Abadiella huoi Zone, lower Cambrian; Horse Gully section, Yorke Peninsula, Australia.

Material.—Twenty two calcium phosphatic internal moulds, including six figured specimens SMNH Mo 160419, 160420, 167662–167665 from sample 7/70 and fifteen specimens from sample K1-3B. Uppermost Emyaksin Formation, Bol'shaya Kuonamka and Malaya Kuonamka rivers; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Cyrtoconic, laterally compressed shell, with length/width ratio 1.8 to 2.5. The apex is displaced to near the apertural margin and hooked. The aperture is drop-like in dorsal profile, narrowing towards the subapical margin. The subapical margin of the aperture curves upwards and forms a train. The protoconch is not distinct on the internal mould. The inner shell surface (as reflected by internal moulds) bears concentric folds, which are most prominent near the apertural margin of larger specimens and tend to disappear in the subapical region. Smaller (juvenile?) shell has only one fold on the inner surface, near the apertural margin and continuous with the pegma-like structure (Figs. 9C, 11C).

Remarks.—The surface of the moulds is covered with 10–40 µm wide, blunt tubercles and bears reticulate microornamentation best seen on the convex parts (folds) of the mould. The tubercles tend to be situated in comarginal rows, similar to what occurs in Mellopegma (Vendrasco et al. 2011b). Where concentric folds are developed, the tubercles are concentrated along their surface. Reticulation is formed by shallow concave polygons, ca. 20 µm wide. On the surface of concentric folds, the polygons become deeper and more variable in size, with a diameter typically ranging from 2–10 µm (Fig. 10G). The microornamentation flattens out in the subapical area. In addition to the polygonal texture, some specimens of Mackinnonia show imprints of a laminar shell microstructure in the concave bands between folds on the internal mould (Fig. 10C, G). As demonstrated by Runnegar (in Bengtson et al. 1990: fig. 159g), the troughs on internal moulds of Mackinnonia correspond to areas where the shell was thicker, and where the inner layer of shell microstructure would be preserved, whereas the convex folds correspond to thin regions of shell where the outer layer of shell microstructure could be preserved. Thus, Mackinnonia appears to have had the common molluscan shell microstructure pattern of an outer prismatic shell layer (represented by polygons) and an inner laminar shell layer. This pattern characterises modern monoplacophorans, as well as Mellopegma and other stenothecids (Vendrasco et al. 2011b).

Parkhaev (in Gravestock et al. 2001) reassigned Mellopegma rostratum Zhou and Xiao, 1984 to Mackinnonia Runnegar in Bengtson et al., 1990. According to Parkhaev (in Gravestock et al. 2001) and Parkhaev (2005), M. obliqua Landing and Bartowski, 1996 from Stage 4 of Laurentia, as well as Bemella costa Zhou and Xiao, 1984 and B. anhuiensis Zhou and Xiao, 1984 from North China, should be regarded as junior synonyms of M. rostrata (Zhou and Xiao, 1984). However, the illustrations of M. rostrata (as well as B. costa and B. anhuiensis) are not detailed, and it appears that individuals of this species tend to lack the more frequent and prominent internal ridging that characterises M. davidi. M. rostrata is also curved more than M. davidi, and for these reasons we view the synonymy as questionable. The ornamentation of the internal mould of Mackinnonia anabarica from the lower Botoman Stage of Siberia with tubercles would be the only reliable difference from otherwise similar M. davidi Runnegar in Bengtson et al., 1990 from Cambrian Series 2 of South Australia (Bengtson et al. 1990; Gravestock et al. 2001) and M. rostrata from Stage 4 of South and North China (Zhou and Xiao 1984; Feng et al. 1994) and lower Stage 5 of the Mediterranean region (Spain; Wotte 2006). It is, however, hard to confirm or deny the lack of tubercles in the original M. rostrata illustrated by Xiao and Zhou (1984) from China. M. rostrata and M. anabarica appear to share the same higher degree of coiling than that illustrated in M. davidi by Bengtson et al. (1990: fig. 159), whereas M. davidi has more frequent and prominent internal ridging.

The fossils described herein were discussed as Mackinnonia sp. by Kouchinsky (2000a: 131, fig. 10). Mackinnonia anabarica Parkhaev, 2005 and Leptostega hyperborea Parkhaev, 2005 (see below) were originally described from sample B-247, which is from an unknown locality, according to Parkhaev (2005). However, the same sample available in the material of Vladimir V. Missarzhevsky and collected by Larisa G. Voronova derives from near the mouth of the Ulakhan-Tyulen Brook, Bol'shaya Kuonamka River (as written on a label). Thus, it is equivalent to section 6 herein, which should be considered the type locality for those two species.

Stratigraphic and geographic range.—Lower Botoman Stage, Siberian Platform.

Type species: Leptostega irregularis Geyer, 1986; Acadoparadoxides mureroensis Zone, Leonian Stage; Porma, Cantabrian Mountains, Spain.

Material.—Four incomplete calcium phosphatic internal moulds, including three figured specimens SMNH Mo 167667–167669 from sample 7/70. Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Laterally compressed shell nearly orthoconic shell, with sub-central or slightly inclined apex (Fig. 12A₁). The length/width ratio at the apertural region is ca. 2.7. The inner shell surface (as reflected by internal moulds) carries strong and broad concentric folds, prominent on the narrow sides of the moulds. The aperture narrows slightly towards the sub-apical side

Remarks.—The moulds were collected from the type beds of Leptostega hyperborea Parkhaev, 2005 (see Remarks for Mackinnonia anabarica). L. hyperborea is reported from the Botoman and, probably, Amgan Stages (herein and Kouchinsky et al. 2011, respectively), thus, occurs within Stage 4—lower Drumian Stage of Siberia. Leptostega Geyer, 1986 was first described from the basal middle Cambrian of Spain (Geyer 1986; see Kouchinsky et al. 2011: 133 for comparison of L. irregularis Geyer, 1986 with L. hyperborea), Acadoparadoxides mureroensis Zone, lower Leonian Stage. The lower part of the zone can be correlated with the upper Toyonian Stage of Siberia (Geyer 1998, 2005; Geyer and Landing 2004) and the Series 2/Series 3 transition.

Stratigraphic and geographic range.—Lower Botoman Stage and, probably, Amgan Stage, Siberian Platform.

Type species: Pararaconus staitorum Runnegar in Bengtson et al., 1990; Abadiella huoi Zone, lower Cambrian; Horse Gully section, Yorke Peninsula, Australia.

Material.—A single phosphatic internal mould SMNH Mo 160421 from sample 7/70. Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Remarks.—The fossil represents a slightly laterally compressed (length/width ratio of the aperture ca. 1.4) orthoconic internal mould with a slightly displaced apex. The surface of the mould is smooth and covered with polygons, 10–20 µm wide (Fig. 13C, D, F; see also description of surface textures in Kouchinsky 2000a). The distinguishing characters of both known species of Pararaconus, P. staitorum Runnegar in Bengtson et al., 1990 (type species) and P. paradoxus Parkhaev in Gravestock et al. 2001 from the Abadiella huoi Zone of Australia relate to the presence/absence (respectively) of lateral depressions (buttresses on internal moulds) near the aperture (Parkhaev in Gravestock et al. 2001). The ventral margin of the fossil herein is jagged and the original aperture margin appears to be broken off. Thus without knowing these character states we cannot determine with certainty to which species of Pararaconus this fossil belongs. The fossil is discussed as Obtusoconus sp. by Kouchinsky (2000a: 131, fig. 11).

Type species: Mellopegma georginense Runnegar and Jell, 1976; Ptychagnostus atavus-Peronopsis opimus Zone, Floran Stage; eastern Georgina Basin, western Queensland, Australia.”

Material.—Seven calcium phosphatic internal moulds, including three SMNH Mo 160410, 167612, and 167671 from sample 7/70 and one internal mould SMNH Mo 167672 from sample B-247 (collected by Larisa G. Voronova and found in the collection of Vladimir V. Missarzhevsky). Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Remarks.—Cyrtoconic, laterally highly compressed shell. The apex projects within the apertural margin. The aperture is elongated oval in outline (length/width ratio 3.0–3.7) and has a slight lateral flaring and subapical train. The surface of the internal mould bears faint concentric folds and is covered with evenly spaced blunt tubercles (Fig. 14D). The fossil is discussed as “Unnamed form 3” by Kouchinsky (2000a: fig. 5M–O). For discussion and comparison with other species see Vendrasco et al. (2011b: 18).

Stratigraphic and geographic range.—Lower Botoman Stage, Siberian Platform (Vendrasco et al. 2011b and herein) and Eastern Transbaikalia (Parkhaev 2004).

Type species: Enigmaconus parvus Mackinnon, 1985; Ptychagnostus cassis Zone, Boomerangian Stage; Cobb Valley, Nelson Province, South Island, New Zealand.

Remarks.—Enigmaconus Mackinnon, 1985 is represented by a similar pegma-bearing, but slightly cyrtoconic whith a more extended apex species E. parvus Mackinnon, 1985. Pegma-like structures in the sub-apical part of the shell are also known from several forms of molluscs attributed to different genera. The pegma was first described in ribeirioid rostroconchs (Pojeta and Runnegar 1976). It represents a hard internal plate that connected the left and right sides of the ribeirioid rostroconch shell and is visible as a groove on internal moulds. In some lower-middle Cambrian helcionellids, a similar structure is produced by a sub-apical invagination or internal thickening of the wall. When Enigmaconus and Eurekapegma were described from the Middle Cambian of New Zealand (Mackinnon 1985), the pegma was known only from the cosmopolitan lower Cambrian mollusc Watsonella (= Heraultipegma). At that time, Watsonella was considered a rostroconch (Pojeta and Runnegar 1976), and Mackinnon (1985) speculated that Enigmaconus may have been ancestral to rostroconchs, based on the shared characteristic of the pegma. Since that time, other pegma-bearing species have been reported from the lower Cambrian of France (Kerber 1988) and Mongolia (Esakova and Zhegallo 1996), middle Cambrian of Greenland (Peel 1994) and Australia (Vendrasco et al. 2010). Runnegar (1996) suggested that because of the widespread nature of pegma-like structures in Cambrian molluscs, they likely evolved convergently in different lineages.

Etymology: From pyramid, referring to the shape of the internal moulds.

Type material: Holotype: SMNH Mo 167676, internal mould (Fig. 15). Paratypes: SMNH Mo 160422, 167677 and 167678, internal moulds (Fig. 16).

Type locality: 70°43′N, 112°52.5′E (section 7 herein, or section A-54 in Val'kov 1975), Bol'shaya Kuonamka River, eastern flanks of the Anabar Uplift, northern Siberia.

Type horizon: Uppermost Emyaksin Formation (sample 7/70), Calodiscus-Erbiella Zone, lower Botoman Stage.

Material.—Twenty-eight calcium phosphatic internal moulds, including four figured SMNH Mo 160422, 167676–167678 from the type horizon (sample 7/70) and one specimen from sample K1-3B.

Diagnosis.—Bilaterally symmetrical, nearly orthoconic shells with a large centrally placed apex and wide oval aperture with a broad sub-apical sinus. A narrow cleft (cast of pegma-like structure) separates a flattened smooth sub-apical portion from the rest of the mould. The surface of the internal moulds is covered with somewhat inclined and scaly polygons.

Description.—Orthoconic shell, with a broad centrally placed apex and wide oval aperture. A deep narrow pegma-like protrusion (reflected by a cleft on the internal mould) projects ventrally (Fig. 16A₃) and separates a flattened portion from the rest of the shell. Aperture with a broad sub-apical sinus. Almost the entire surface of the internal mould is covered with flattened convex polygons, ca. 20 µm wide, that gradually become smooth towards the top of the mould.

Remarks.—The fossil is discussed as Enigmaconus sp. by Kouchinsky (2000a: 133, fig. 12).

Stratigraphic and geographic range.—Known only from its type locality and horizon.

Type species: Cyrtolites atlantoides Matthew, 1894; lower Cambrian, New Brunswik, Canada.

Remarks.—Herein, Pelagiella is tentatively merged with Cambretina Horný, 1964 and Costipelagiella Horný, 1964 (see also Kouchinsky et al. 2011: 136).

Material.—Nine calcium phosphatic internal and external moulds, including figured SMNH Mo 167679–167685 from sample 7/70 and six moulds from sample K1-3B. Uppermost Emyaksin Formation, Bol'shaya Kuonamka and Malaya Kuonamka rivers; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Very rapidly expanding shell consisting of up to ¾ whorl. Aperture wide, asymmetrical, with pinched and strongly drawn-out peripheral side (Figs. 17A₃, 19A₂). The peripheral surface of the whorl is rounded and distally extended, separated from the spiral and umbilical surfaces by an angle.The spiral surface of the whorl is depressed and concave, whereas the umbilical surface is convex (Figs. 17A₁, 19A₁). Ornamentation of the shell exterior consists of fine radial striations, 1–3 µm wide, and transversal, narrow ribs. Distance between ribs up to ∼50 µm (Fig. 18). The ornamentation of ribs fades out towards the periphery of the whorl (Fig. 18B). A sinusoidal course of ribbing follows the relief of the umbilical and upper spiral surfaces.

Remarks.—The fossils described here are generally most similar to Pelagiella madianensis (Zhou and Xiao, 1984) illustrated in Gravestock et al. (2001: pl. 46), but the ornamentation of P. madianensis consists of granules arranged in spiral rows (Parkhaev in Gravestock et al. 2001). The ornamentation of Pelagiella sp. 1 herein consists of comarginal ribs similarly spaced as in the middle Cambrian Pelagiella sp. cf. Costipelagiella zazvorkai from the overlying Kuonamka Formation (Kouchinsky et al. 2011), but they do not appear to extend to the periferal portion of the whorl. Pelagiella sp. 1 differ from Pelagiella sp. 2 described below in having a strongly extended peripheral part of the aperture and whorl, a more depressed spiral side and more rapidly expanding whorl. Unlike other species of Pelagiella, Pelagiella sp. 1 has a strongly extended peripheral part of the aperture.

Material.—One calcium phosphatic internal mould, SMNH Mo 160429 and one fragmentary external mould, SMNH Mo 167688 of the same specimen, from sample K1-3B. Two internal moulds SMNH Mo 167686–167687 from sample 7/70. Uppermost Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Rapidly expanding shell consisting of ¾ whorl. Aperture is asymmetrical oval (Figs. 19B₂, C₃, 20A₂). The peripheral surface of the whorl is rounded and passes without angulation into convex spiral and umbilical surfaces of the whorl. Ornamentation of the shell exterior (Fig. 20B) consists of fine radial striations, 1–3 µm wide, and transversal narrow ribs that appear on the surface from ca. ½ revolution. Distance between ribs increases with growth and towards periphery of the whorl, up to ∼25 µm.

Remarks.—The fossils are discussed as Pelagiella cf. subangulata by Kouchinsky (2000a: 135, fig. 14). On one of the internal moulds, a low polygonal texture is developed unilaterally as a radially extended area transitional between the peripheral and spiral surfaces of the whorl (Fig. 20A₄, A₅; Kouchinsky 2000a: fig. 14A, arrow E). The texture consists of polygons, up to 50 µm in width, with tubercles, 5–10 µm in diameter, at their centres (Fig. 20A₄, A₅).

Among other pelagiellids, the forms described herein are distinguished by the ornamentation of the shell exterior with comarginal ribs thought to be diagnostic of Costipelagiella Horný, 1964. The ribs are twice as densely spaced as in the middle Cambrian C. zazvorkai Horný, 1964 and Pelagiella sp. cf. Costipelagiella zazvorkai from Siberia (Kouchinsky et al. 2011), although the shell exterior was apparently similarly smooth over the initial half revolution. C. nevadense Skovsted, 2006 from Laurentia and Costipelagiella sp. cf. C. zazvorkai from the middle Cambrian of New Zealand (MacKinnon 1985) are more laterally compressed, but have similarly spaced ribs, which do not extend, however, to the peripheral portion of the whorl in the former form.

Type species: Fordilla troyensis Barrande, 1881; lower Cambrian, New York State, USA.

Remarks.—Elicki and Gürsu (2009) noted a high degree of intraspecific and intrageneric variation in the very similar Cambrian bivalves Fordilla Barrande, 1881 and Pojetaia Jell, 1980. In addition to the extensive variation within lower-level taxa, many of the proposed distinguishing characteristics such as muscle scar placement and dentition are not preserved in many fossils. Thus the most useful diagnostic character between these two genera appear to be shell shape, with the umbo typically more anteriorly placed in Fordilla (Jell 1980; Runnegar and Bentley 1983). Elicki and Gürsu (2009) argued that Pojetaia and Fordilla can be reliably distinguished by total size (Fordilla is larger) and the ratio of anterior length to total length. In the fossil described herein (Fig. 21A), the umbo is distinctly shifted towards the anterior margin in a way more reminiscent of Fordilla troyensis, the type species of that genus. The smaller specimen (Fig. 21B) may be a poorly preserved juvenile, but its more centrally placed umbo, ration of anterior length to total length (ca. 45%) and total length (ca. 1 mm) are similar to Pojetaia Jell, 1980 (see Elicki and Gürsu 2009).

Material.—One calcium phosphatic internal mould, SMNH Mo 167690 (Fig. 21A) and, probably, SMNH Mo 167692 (Fig. 21C, larger fragment), from sample 7/70. Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Shell ca. 2.5 mm in length (Fig. 21A), equivalved, with valves suboval in lateral outline and anteriorly located umbones (Fig. 21A₁). Dentition is represented by casts of a single weakly impressed tooth and corresponding socket on each valve (Fig. 21A₂, A₃). Ligament area elongated, straight, opisthodetic, occupying more than half of the dorsal margin. In lateral view, the angle between dorsal and posterior margins is ∼135° (Fig. 21A₁). The ration of anterior length (horizontal distance between anterior margin and the center of the umbo) to total length is ca. 36% (see Elicki and Gürsu 2009).

Remarks.—The fossil in Fig. 21A is most similar to F. sibirica in general morphology, but the definitive assignment is hampered by the lack of muscular imprints preserved. Krasilova (1977) described F. sibirica based on twenty internal moulds from the Atdabanian of Siberia. Runnegar and Pojeta (1992) listed F. sibirica as a junior synonym of F. troyensis, but Elicki and Gürsu (2009) argued that these species can be differentiated by features such as a straight dorsal margin, smaller size, less concavity, and a different pattern of shell microstructure. We concur with Elicki and Gürsu. The fossil here differs from F. troyensis in having a straighter dorsal margin and lacking a dorsoventral expansion in the posterior region of the valves. F. germanica Elicki, 1994 has a more elongated shape.

Imprints of shell microstructure are visible at the posterior margin (Fig. 21A₄). As with Pojetaia the inner shell microstructure near the margin of Fordilla consists of laminae whose growth direction parallels the closest region of the valve margin (Fig. 21A₄). Elsewhere the shell microstructure is less well organised (Runnegar and Pojeta 1992; Vendrasco et al. 2011a). This pattern has been described in Pojetaia runnegari, Fordilla troyensis, and now Fordilla cf. F. sibirica. The occurrence of this pattern of microstructure in Fordilla cf. F. sibirica supports the arguments that it is a diagnostic character of the Family Fordillidae and that this was the primitive type of shell microstructure in bivalves, which may have been a precursor to nacre (Vendrasco et al. 2011a).

Type species: Majatheca tumefacta Missarzhevsky in Rozanov et al., 1969; upper Tommotian Stage, lower Cambrian; Tiktirikteekh, near Churan village, middle Lena River, Siberian Platform.

Material.—Twenty phosphatised internal moulds and conchs from sample 7/25.5 (correlated with the Profallotaspis jakutensis Zone), sample 7/27.5 (correlated with the Repinaella Zone), samples 7/28.5, 7/29, 7/31.5, and 7/34 (correlated with the Delgadella anabara Zone); twenty-three phosphatised conchs, including SMNH X 4600–4604, from sample 7/36.7, correlated with the Delgadella anabara Zone, Atdabanian Stage; Emyaksin Formation, Bol'shaya Kuonamka River. Seven phosphatised conchs, including SMNH X 4605 and 4606, from sample 7/70 and several additional conchs from sample K1-3B; several phosphatised opercula, including SMNH X 4607 and 4608, from samples 1/7.8 and K1-3B (respectively); Uppermost Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Conch gently curved dorsally at the initial part, with broadly oval aperture. Ventral and dorsal margins of the aperture protrude slightly. Apical angle is ca. 25°. The initial part is rounded and separated from the main conch by a septum. The wall consists of two layers of fibrous bundles. In the outer one, the bundles are inclined longitudinally towards the conch apex and contain pore canals. The bundles of the inner layer run transversely around the conch.

Operculum outline is oval. On the inner surface of operculum, a well-developed circular ridge delimits a rounded triangular or rounded trapezoidal depression in the middle. The ridge and medial depression carry radial folds and expose radially oriented fibres in places (Kouchinsky 2000b: fig. 7A, D). The marginal part of the operculum is about onethird of the opercular radius in width and is formed of stacked growth laminae penetrated by pores (Fig. 24; see description of these porous opercula in Kouchinsky 2000b).

Remarks.—The fossils are described by Kouchinsky (2000b: 69, fig. 3) as Allathecid? hyolith, microstructural type A, forms 1 and 2. Missarzhevsky (in Rozanov et al. 1969: 119) noted that the wall of Majatheca tumefacta is built of acicular units (“crystals”) perpendicular to the surface. Opercula, illustrated as Allatheca sp. in Rozanov et al. (1969: pl. 11: 4, 8) and attributed to M. tumefacta in Sokolov and Zhuravleva (1983: pl. 22: 7), as well as Allatheca sp. 2 (Meshkova 1974: pl. 11: 12) are very similar to those in Fig. 24A herein. Conchs and opercula herein attributed to M. tumefacta co-occur in sample K1-3B. Sample K1-3B from the Malaya Kuonamka section contains a variety of often well-preserved hyolith opercula, in contrast to the coeval sample 7/70 from Bol'shaya Kuonamka that contains only poorly preserved opercula but well-preserved hyolith conchs. The general disparities in the numbers of hyolith opercula versus conchs from the same level is probably explained by local differences in hydrodynamic sorting (see Malinky and Skovsted 2004).

Stratigraphic and geographic range.—Upper Tommotian-lower Botoman Stages, Siberian Platform.

Type species: Allatheca corrugata Missarzhevsky in Rozanov et al., 1969; Dokidocyathus regularis Zone, Tommotian Stage, lower Cambrian; Tiktirikteekh, near Churan village, middle Lena River, Siberian Platform.

Material.—Eleven phosphatised conchs and internal moulds, including SMNH X 4609–4612, from sample 7/36.7. Emyaksin Formation, Bol'shaya Kuonamka; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Slightly dorso-ventrally curved conch, 1–2 mm long, with convex dorsal and flattened ventral sides. Dorso-lateral part is semi-circular in transverse section, and passes via rounded margins into a slightly convex ventral part. In dorsal view, the apical angle is ca. 20°. The aperture has an apparently straight dorsal margin. The conch is ornamented dorso-laterally with distinct narrow transverse ribs. The ventral surface is smooth.

Remarks.—The cross-section and ornamentation of the conchs are typical of Allatheca corrugata Missarzhevsky in Rozanov et al., 1969 (tentatively synonymised herein with A. anabarica Val'kov, 1975). The illustrated individuals are therefore interpreted as probable juveniles of typically larger, several cm-sized Allatheca corrugata, which are reported from the middle part of Emyaksin Formation (Val'kov 1975, 1987).

Type species: Conotheca mammilata Missarzhevsky in Rozanov et al., 1969; upper Tommotian Stage, lower Cambrian; Tiktirikteekh, near Churan village, middle Lena River, Siberian Platform.

Material.—Six specimens from sample 3/42; three specimens, including SMNH X 4620, from sample 3/65; five, including SMNH X 4621 and 4622, from sample 7/27.5; two from sample 7/28.5; five, including SMNH X 4623, from sample 7/29; SMNH X 4663 from sample 7/30, eight from sample 7/31.5, including SMNH X 4624; one from sample 7/33; one from sample 7/34; nine phosphatised conchs with internal moulds, including SMNH X 4625–4627, from sample 7/36.7; several specimens from sample 7/55; one specimen, SMNH X 4628, from sample 7/70, five from sample K1-3B and five from sample 1/7.8. Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; correlated with the Repinaella Zone, Atdabanian Stage—Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Slightly dorso-ventrally curved conch, with a straight aperture and circular cross-section. The surface is covered with straight transverse growth lines. Apical angle is up to 15°. Juvenile conch is blunt and bulbous. Wall microstructure consists of inner layer of transverse fibres running around the conch, and the outer layer composed of longitudinally oriented fibres (Fig. 27A, B).

Remarks.—Conotheca mammilata Missarzhevsky in Rozanov et al., 1969 is known from the Tommotian and Atdabanian Stages of Siberia. In the type material, the apical angle ranges from 8–20° and the presence of both robust and gracile forms was indicated in the original description of C. mammilata by Missarzhevsky (in Rozanov et al. 1969: 113). Specimens with better-preserved phosphatised walls show microstructure similar to that of some other hyoliths described herein (Fig. 27A–D; see also Kouchinsky 2000b).

Material.—One phosphatised conch, SMNH X 4629, from sample 7/55; Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Judomia Zone, upper Atdabanian Stage.

Description.—Conch helically curved about a half revolution clockwise towards the direction of growth. The aperture is straight and rounded, slightly oval. The surface of conch is covered with straight, faint, transverse lines. The apical angle is ca. 20°. Wall microstructure is similar to that of other hyoliths (see also Kouchinsky et al. 2009: fig. 45).

Stratigraphic and geographic range.—Lower Cambrian, Atdabanian Stage, Siberian Platform.

Material.—Several phosphatised opercula with cardinal processes, including SMNH X 4630–4632, from sample K1-3B; uppermost Emyaksin Formation, Malaya Kuonamka; Calodiscus-Erbiella Zone, lower Botoman Stage. One operculum, SMNH X 4633, from sample 7/31.5; Emyaksin Formation, Bol'shaya Kuonamka; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Remarks.—Circular opercula from sample K1-3B with two prominent cardinal processes, nearly perpendicular to the surface of the operculum (Fig. 28A–C). The processes diverge at ca. 30°. The inner surface has a smooth distal ridge at the margin, low rounded polygonal pattern in the middle and faint radial striation laterally. The margin thins out gradually. Conchs with circular cross-section are not found in sample K1-3B, but occur in the coeval sample 7/70 from Bol'shaya Kuonamka.

The operculum from sample 7/31.5 has weakly developed cardinal processes and radially oriented lateral folds of the circular distal ridge (Fig. 28D). The ridge slopes abruptly towards the margin of the operculum. Radial folds developed laterally on the distal ridge are reminiscent of those described as tubular clavicles (Malinky and Skovsted 2004) in Conotheca australiensis Bengtson in Bengtson et al., 1990, of which the interior surface morphology is otherwise different. Several straight fragments of conchs with circular cross-section (Fig. 27H) co-occur with the operculum in sample 7/31.5.

Type species: Circotheca anulata Sysoev, 1959; lower Cambrian; Kotuj River, left bank, 6 km downstream of the mouth of Kotujkan River, Siberian Platform.

Material.—Several tens of fragments of phosphatised internal moulds, including SMNH X 4634, from sample 7/36.7. Emyaksin Formation, Bol'shaya Kuonamka; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Remarks.—The moulds are straight, 1–2 mm long, slightly diverging (<5°) and covered with faint thin transversal growth lines. Oval septae with straight margins are perpendicular to the wall and concave towards the conch aperture. The fossils are most similar morphologically to Loculitheca Sysoev, 1968. Turcutheca Missarzhevsky in Rozanov et al., 1969 and Ovalitheca Sysoev, 1968 are also similar but have laterally curved conchs; no such curvature is recognised in the fragments available.

Type species: Quadrotheca clinisepta Sysoev, 1960; upper Atdabanian Stage, lower Cambrian; mouth of the Sinyaya River, middle reaches of the Lena River, Siberian Platform.

Material.—Nineteen phosphatised conchs and internal moulds, including SMNH X 4641 and 4642, from sample 7/70 and one additional internal mould, SMNH X 4643, from sample B-247. Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Straight conch, rectangular in cross-section and covered with faint straight transversal growth lines. The four sides of the conch are slightly depressed and separated from each other by a longitudinal keel. The apical angle is <5°. A smooth septum, concave towards the apex, is preserved in one of the fragments (Fig. 26G₂).

Remarks.—In one of the internal moulds, the apertural end is a little flared (Fig. 26F). Phosphatised casts of pore channels perpendicular to the wall are preserved between inner and outer phosphatic crust delimiting the now-dissolved wall (Fig. 26G, H).

Stratigraphic and geographic range.—Atdabanian and lower Botoman Stages, Siberian Platform.

Type species: Hyolithus (Orthotheca) bayonet var. groenlandicus Poulsen, 1932; lower Cambrian, northeast Greenland.

Remarks.—According to Malinky and Skovsted (2004), the genus name Lenatheca is inadequately founded by Missarzhevsky (in Rozanov et al., 1969) on poorely preserved type material of Hyolithus (Orthotheca) bayonet var. groelandicus Poulsen, 1932. Investigation by Malinky and Skovsted (2004) revealed difference between Poulsen's type material and available Siberian specimens from Missarzhevsky's collection. Val'kov (1975: 77) attributed, however, such forms found by him in the uppermost Emyaksin Formation to Lenatheca groenlandica (Poulsen, 1932). Taking into account inappropriate designation to the genus and species, the fossils are described herein under “Lenatheca groenlandica (Poulsen, 1932)” to refer to their definition by Val'kov (1975).

Material.—Twenty phosphatised internal moulds, including SMNH X 4635–4639, from sample 7/70 and probably six more, including SMNH X 4640, from sample 7/36.7. Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage and Delgadella anabara Zone, Atdabanian Stage (respectively).

Description.—Conch straight (Fig. 26A) or slightly curved laterally (Fig. 26C). In dorsal view the apical angle is ca. 30°. Cross-section drop-shaped (Fig. 26C₂) or rounded triangular at the juvenile conch (Fig. 26A₁), acquiring ventral invagination at later stages (Fig. 26B₁, C₃, E). Arched dorsal side with median longitudinal ridge on top and inflated flanks. Antero-lateral edges bluntly rounded. Ventral side contains a broad median depression with a flattened bottom. Septa are oval in outline, with two radial folds diverging at ca. 40° on the dorsal portion (Fig. 26D, E).

Material.—Twenty-five phosphatised conchs, including SMNH X 4613–4617, from sample 7/70, fourteen from sample K1-3B and three from sample 1/7.8; uppermost Emyaksin Formation, Bol'shaya Kuonamka and Malaya Kuonamka rivers; Calodiscus-Erbiella Zone, lower Botoman Stage. Several conch fragments, including SMNH X 4618 and 4619, from samples 7/33 and 7/31.9; Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Almost straight or slightly dorso-ventrally curved conchs, up to several mm long. The dorso-lateral part is semi-circular in transverse section, passing via rounded margins into a flattened ventral part. In dorsal view, the apical angle is ca. 15°. The conch wall includes two horizontal layers of fibres (microstructure is described in detail by Kouchinsky 2000b). Fibres in the inner layer run transversely on the dorsal side and branch towards the aperture laterally (Fig. 23F, H). The fibres may branch and anastomose intensively and are less regularly arranged on the ventral side of the conch, although their typical orientation there is transverse. The outer layer consists of longitudinally oriented fibres on the dorsal side (Fig. 23I).

Remarks.—The hyoliths are discussed as allathecid(?) hyolith, microstructural type B, forms 1 and 2 by Kouchinsky (2000b: 70, figs. 4, 5).

Material.—Ten internal moulds, including SMNH X 4644 and 4645, from sample 7/70 and eight from sample K1-3B; uppermost Emyaksin Formation, Bol'shaya Kuonamka and Malaya Kuonamka rivers (respectively); Calodiscus-Erbiella Zone, lower Botoman Stage. Several internal moulds from sample 7/54; Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Judomia Zone, upper Atdabanian Stage.

Description.—Conchs, 2–4 mm long, slightly curved dorsoventrally, with apical angle ca. 10°. Initial part gradually narrows towards the apex (Fig. 26J). Cross-section rounded triangular (Fig. 26I₃, I₄). Apertural part expands in a funnel-like fashion. Surface of internal mould is smooth, covered with faint discontinuous transversal striae.

Type species: Triplicatella disdoma Conway Morris in Bengtson et al., 1990; Abadiella huoi Zone, lower Cambrian; Curramulka, Yorke Peninsula, Australia.

Ethymology: From Latin papilio, butterfly; for the butterfly-like appearance of the operculum.

Type material: Holotype: SMNH X 4646 (Fig. 29A). Paratype: SMNH X 4647 (Fig. 29B).

Type locality: 70°08.5′N, 114°00′E, Malaya Kuonamka River, left bank at the village Zhilinda, eastern flanks of the Anabar Uplift, northern Siberian Platform.

Type horizon: Uppermost Emyaksin Formation, Calodiscus-Erbiella Zone, lower Botoman Stage, lower Cambrian.

Material.—Several phosphatised opercula, including holotype SMNH X 4646, from sample K1-3B and paratype SMNH X 4647, from sample 7/70; uppermost Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers (respectively); Calodiscus-Erbiella Zone, lower Botoman Stage.

Diagnosis.—Species of Triplicatella with the operculum rounded trapezoidal in plan view and with two prominent folds on the dorsal margin.

Description.—Operculum rounded trapezoidal in plan view, 0.8–1 mm wide, with flattened outer surface. Two prominent and laterally flattened blade-like radial folds of the dorsal surface diverge at ca. 80°, but their bases are perpendicular to the dorsal margin of the operculum (Fig. 29A₁, B₁). Invagination of the dorsal margin produces three distinct folds on the outer surface. The folds continue ventrally as two low ridges, which level off towards the margin of the operculum. The ventral part of the operculum has two broad and shallow folds.

Remarks.—Triplicatella Conway Morris in Bengtson et al., 1990 includes three described species, T. disdoma Conway Morris in Bengtson et al., 1990 (type species), T. sinuosa Skovsted, Peel, and Atkins, 2004, and T. peltata Skovsted, Peel, and Atkins, 2004. There are also four forms in open nomenclature (Skovsted et al. 2004). T. papilio sp. nov. is different from other known forms of Triplicatella in having only two inner prominent folds of the dorsal margin.

Stratigraphic and geographic range.—Known only from its type locality and horizon.

Material.—Single phosphatised operculum, SMNH X 4648, from sample B-247 and a single fragment from sample 7/70; uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Remarks.—The fossil is very similar to Triplicatella sinuosa from Cambrian Stage 4 of Laurentia (Skovsted et al. 2004) in having three strongly developed inner folds of the dorsal margin. Two weak ventral folds are recognizable. The lateral margins of the operculum are not completely preserved, but the operculum was presumably elliptical in outline

Stratigraphic and geographic range.—Bonnia-Olenellus Zone, Dyeran Stage of Greenland and lower Botoman Stage of the Siberian Platform.

Material.—Single phosphatised operculum, SMNH X 4649, from sample 7/70; uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—The operculum has a rounded trapezoidal shape in plan view, ca. 1 mm wide, convex, with a single and prominent fold on the dorsal margin. In lateral view, the crest of the fold reaches the apex (Fig. 29D₄). The internal surface is smooth, with a deep median depression in the sub-central part. Two short and rounded converging lateral ridges are situated near the dorsal margin.

Remarks.—Specimens of Triplicatella peltata Skovsted, Peel, and Atkins, 2004 from the Cambrian Stage 4 of Laurentia are similar in outline, number of folds and structure of the inner surface, although the dorsal fold is broader and more prominent than in the fossil herein.

Material.—Single phosphatised operculum SMNH X 4650, from sample K1-3B; uppermost Emyaksin Formation, Malaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—The operculum has a circular outline, but is partly broken in the middle. Two massive, laterally flattened cardinal processes are directed at ca. 90° from the inner surface of the operculum. Ridges directed ventrally represent the continuation of the processes. The distal side of the ridges dip steeply towards the margin of the operculum.

Material.—Three phosphatised opercula, including SMNH X 4651 and 4652, from sample K1-3B; uppermost Emyaksin Formation, Malaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—The operculum has a circular outline. Prominent cardinal processes diverge at ca. 40°. They are higher than the clavicles, which extend in the ventral direction from the cardinal processes. The processes are broken off in one of the specimens (Fig. 30F). The distal ridge dips steeply towards the margin of the operculum, and more gently towards the interior.

Material.—Several phosphatised opercula, including SMNH X 4653 and 4654, from sample K1-3B; uppermost Emyaksin Formation, Malaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—The operculum have a rounded trapezoidal outline. Cardinal processes are short, whereas the blade-like clavicles are higher. In a larger specimen (Fig. 30C), the clavicles include a series of radial folds. The distal ridge facing the margin dips steeply, whereas more gently towards the interior.

Type species: Microcornus parvulus Mambetov, 1972; Rhombocorniculum cancellatum Zone, lower Cambrian; Geres Member of the Shabakty Group, Ushbas River, Lesser Karatau, Kazakhstan.

Material.—Several tens of calcium phosphatic internal moulds, including SMNH X 4655, from sample 7/36.7; several additional moulds from samples 7/27.5, 7/31.5, 3/59, and 3/65; Emyaksin Formation, Bol'shaya Kuonamka and Malaya Kuonamka rivers; correlated with the interval of Repinaella-Judomia zones, Atdabanian Stage.

Description.—Short conchs, gently curved dorso-ventrally, expand fast from a relatively thick bulbous protoconch. The dorsal side is arched without angulations and passes into a convex, but more flattened ventral side. In dorsal view, the apical angle is ca. 30°. The bulbous juvenile conch with a small tip is somewhat dorso-ventrally flattened and shifted dorsally (Fig. 25C₁), separated from the main part by a constriction.

Remarks.—This material is generally similar to Microcornus Mambetov, 1972, described from Stages 3–5 of the Siberian Platform, Kazakhstan, Mongolia, Australia, Avalonia (Britain), and South China (Bengtson et al. 1990), but preservation as smooth internal moulds with a straight aperture precludes more detailed assignement.

Type species: Hyolithus obscurus Holm, 1893; Lejopyge laevigata Zone, middle Cambrian; Sweden.

Remarks.—Placement of the following Siberian species under Linevitus Sysoev, 1958 is questioned by Berg-Madsen and Malinky (1999), because the type material is represented by much larger fossils. Moreover, since the genus is not established on a firm morphological ground, the name Linevitus appears to be justified only for the type species (Berg-Madsen and Malinky 1999).

Material.—Single specimen from sample 7/30, three specimens, including SMNH X 4657, from sample 7/31.5 and four specimens from sample 7/31.9; correlated with the Delgadella anabara Zone, Atdabanian Stage. One specimen, SMNH X 4658, from sample 7/70; Calodiscus-Erbiella Zone, lower Botoman Stage. Emyaksin Formation, Bol'shaya Kuonamka River.

Description.—Conch straight sided, with a sub-triangular cross-section. Dorsal surface rounded in the middle, with flanks dipping towards rounded lateral longitudinal ridges, grading into a slightly convex ventral surface. In dorsal view, the apical angle is ca. 30°. The conch is covered with transverse growth lines slightly curved towards the apex on dorsum. On venter, the growth lines are strongly curved towards the aperture and parallel with the edge of a long semi-circular ligula. Growth lines produce small lateral sinuses. Apical end not preserved.

Remarks.—This is very similar to the holotype of Linevitus minutus Missarzhevsky illustrated by Rozanov and Missarzhevsky (1966: 108, pl. 11: 7, 8; John Malinky, personnal communication in 2008), from the Tommotian Stage of Siberia. The curvature towards dorsum seen in the species is not apparent in the fragments available. The fossils are also similar to Hyolithida indet. 3 (Kouchinsky et al. 2011: fig. 11G–I).

Type species: Parkula bounites Bengtson in Bengtson et al., 1990; Abadiella huoi Zone, lower Cambrian; Kulpara, Yorke Peninsula, Australia.

Material.—Two phosphatised opercula, SMNH X 4660 and 4661, from sample K1-3B. Uppermost Emyaksin Formation, Malaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—The operculum has a crescent-shaped dorsal lobe and a strongly convex semi-circular ventral lobe. The dorsal margin is inclined at ca. 90° with respect to the ventral lobe. The dorsal lobe bears blade-like clavicles situated along the margin between two lobes. Cardinal processes diverge at ca. 90°.

Remarks.—The operculum is most similar to Parkula esmeraldina Skovsted, 2006, which is also different from P. bounites Bengtson in Bengtson et al., 1990 in having more diverged, longer and more dorsally situated cardinal processes. No conchs attributable to Parkula have been found associated with the opercula in our material. Parkula spp. are known from Cambrian Series 2 of Australia (Bengtson et al. 1990; Gravestock et al. 2001), Antarctica (Wrona 2003), and Laurentia (Malinky and Skovsted 2004; Skovsted 2006; Skovsted and Peel 2007).

Material.—Over twenty internal moulds, including SMNH X 4656, from sample 7/36.7 and several additional moulds from samples 7/54 and 7/55. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara and Judomia zones, Atdabanian Stage.

Description.—Conch slightly dorso-ventrally curved, with convex, arched dorsal and slightly convex ventral sides and lateral longitudinal ridge between them. Cross-section is rounded triangular. The aperture has a slight dorsal sinus and a protruding prominent ventral lip. Shallow dorsal sinus just inside aperture. Apical angle ca. 20°.

Material.—Two pyritised conchs, including SMNH X 4659, from sample 1/9. The last specimen represents an almost complete but unsatisfactorily preserved conch with operculum attached to the aperture. Carbonate concretions of the basal Kuonamka Formation, Malaya Kuonamka River; Bergeroniellus expansus Zone, upper Botoman Stage.

Description.—The conch is gently curved dorso-ventrally at its initial part and has a convex, arched dorsal surface with 8–9 low longitudinal folds and a flattened ventral surface. In dorsal view, the apical angle is ca. 15°. The juvenile conch is elongated, tapered, slightly curved dorsally with respect to the main conch.

Material.—Single phosphatised fragment SMNH X 4665, from sample 7/70. Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Discussion.—The fragment of an elongated blade-like fossil. The external surface (seen in the upper part of the images in Fig. 32) is ornamented with continuous transverse terraces. The fossil contains longitudinally oriented fibres reminiscent to those in hyoliths described herein. The fossil can be tentatively interpreted as a fragment of a hyolithid helen, oriented in Fig. 32 with its more proximal portion to the right.

Remarks.—Halkieriids are loosely placed within the lophotrochozoan phylogeny, ranging from stem-group Lophotrochozoa/ Mollusca (Conway Morris and Caron 2007) to subclass Diplacophora Vinther and Nielsen, 2005 of the class Polyplacophora De Blainville, 1816 (Parkhaev and Demidenko 2010: 955).

Type species: Halkieria obliqua Poulsen, 1967; lower Cambrian, Bornholm.

Material.—Single phosphatised sclerites from samples 7/25.6 (SMNH X 4666), 7/27.5, and 7/36.7 (SMNH X 4667). Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the interval of Profallotaspis jakutensis-Delgadella anabara zones, Atdabanian Stage.

Description.—Asymmetrical, elongated, blade-like and curved sclerites with transversely convex lower and upper sides. The base of sclerites has a rounded triangular, broad (Fig. 33A₁) or extended narrower (Fig. 33B₂) aperture surrounded by undulating walls with auricules. Wall microstructure consists of aciculate elements assembled in broad laths, arranged in transversal rows (Fig. 33A₃; see also Bengtson et al. 1990; Porter 2008).

Type species: Chancelloria eros Walcott, 1920; middle Cambrian; Burgess Shale, British Columbia, Canada.

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).

Type species: Camenella garbowskae Missarzhevsky in Rozanov and Missarzhevsky, 1966; Dokidocyathus regularis Zone, Tommotian Stage, lower Cambrian; Chekurovka village, lower Lena River, Siberia.

Remarks.—See Bengtson (1986a) and Skovsted et al. (2009) for emended diagnosis of the genus, synonymics and discussion of the nominal species included in Camenella. The sellate sclerites, Camenella garbowskae Missarzhevsky in Rozanov and Missarzhevsky, 1966 and mitral sclerites, C. kozlowskii Missarzhevsky in Rozanov and Missarzhevsky, 1966 are unified herein within one species, C. garbowskae Missarzhevsky in Rozanov and Missarzhevsky, 1966. As in other species of Camenella described with a bimembrate scleritome model (see Skovsted et al. 2009), sinistral and dextral symmetry variants (D- and L-forms sensu Bengtson 1970) of both types of sclerites co-occur. The sclerites are of the same size range and share similar surface ornamentation with radial and comarginal folds.

Holotype: Sellate sclerite GIN 3470/73 (Rozanov and Missarzhevsky 1966: pl. 11: 4).

Type locality: Near Chekurovka village, lower reaches of Lena River, northern Siberia.

Type horizon: Lower Tyuser Formation, Dokidocyathus regularis Zone, Tommotian Stage.

Material.—Over one hundred calcium phosphatic sellate and mitral sclerites, including SMNH X 4682–4699, from samples 3/0, 3/1.5, 3/5, 3/21.5, 6/0.3, 6/1.8, 6/2, 6/3.9, K6-4B, 6/4.9, K6-5.8B, 6/6.6, 6/6.8, 6/9.1, 6/14, 6/18, 7/16, and 7/19. Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; Tommotian Stage. Identifiable in the collection are 46 sellates (45%) and 57 mitrals (55%), of which 59% of sellates and 51% of mitrals are of the dextral variant.

Emended diagnosis.—Species of Camenella with sellate and mitral sclerites having well-developed co-marginal ribs. Sellate sclerites asymmetrical, with distinct sella separating larger and smaller lobes. Larger lobe with several radial ridges. Apex slightly coiled, up to 1/2 whorls. Duplicature in large sclerites adpressed to the inner surface laterally and near the apex. Mitral sclerites pyramidal, with well developed obplicate and accrescent sides. Plicate side with usually four pronounced radial ridges separated by deep folds. Obplicate side with several faint radial ridges and striation. Accrescent and decrescent sides without radial ridges.

Description.—Sellate sclerites (Figs. 35, 36) are asymmetrical, up to 5 mm long and 3.5 mm wide (after Missarzhevsky in Rozanov and Missarzhevsky 1966) in plan view, with a sella dividing the sclerite into larger and smaller lobes (e.g., Figs. 35A₁, B₁, 36B₁, C₁). The sella occupies 1/4 to 1/2 of the sclerite width. It represents a deep longitudinal depression with each side marked by a ridge. With the apical angle ∼30°, the ridges diverge greater in smaller specimens. The ridge situated on the larger lobe may slightly overhang the floor of the sella (e.g., Figs. 35A₆, 36B₃). With respect to the smaller one, the larger lobe is higher, approximately twice as long and more than four times wider. The larger lobe carries several (up to 15 in available specimens) variously expressed radial ridges and striations. The smaller lobe is ornamented with longitudinal striations. Co-marginal folds are usually well expressed on the entire dorsal surface of sclerites. The ventral surface of the sclerite generally reproduces the relief of the dorsal surface, but is only weakly ornamented with smooth longitudinal and co-marginal folds. A duplicature strongly adpressed to the opposing wall occupies apical and lateral parts of larger specimens (Fig. 35A₄). The apex is coiled through less than half a whorl.

Pyramidal mitral sclerites (Figs. 37, 38) represent asymmetrical and slightly helically twisted cones, up to 3 mm long (measured from apex to the apertural margin) and up to 2.5 mm wide (after Missarzhevsky in Rozanov and Missarzhevsky 1966). The aperture is plicated subquadrate (in smaller specimens) or subrectangular in outline (compare e.g., Figs. 37E, 38D with Figs. 37D₁, 38C₁). The apex is slightly curved over the decrescent side of the sclerites (see explanation of morphological terms in comments to Fig. 37A). The plicate and obplicate sides bear several radial ridges, better developed on the plicate side. The ridges increase relief from the apex towards the aperture. The plicate side usually carries four well-defined radial ridges, two of which mark the boundaries with the accrescent and decrescent sides, respectively. The ridges are separated from each other by deep, narrow troughs (e.g., Fig. 37A). By contrast, the accrescent and decrescent sides lack strong radial ridges and are covered with longitudinal striation (Fig. 37A). Continuous co-marginal folds cover the entire surface of sclerite. They curve towards the apex on the accrescent and decrescent sides. The internal surface is covered with smooth radial and co-marginal folds reflecting those on the exterior surface of sclerites (Fig. 37E). The tapering apex is circular in cross-section and exhibit a perforation, 20–100 µm in diameter (e.g., Fig. 37B₂).

Remarks.—Camenella reticulosa Conway Morris in Bengtson et al., 1990 is distinguished by very strong co-marginal ribs with superimposed reticulation, strong coiling and wider sella of the sellate sclerites, as well as pronounced radial ribs and intermittent folds on both plicate and obplicate sides of the mitral sclerites. The sellate sclerites in C. parilobata differ in having lobes of similar size. The mitral sclerites of Camenella garbowskae are distinguished from those of C. baltica and C. parilobata by more expressed helical twist.

Vladimir V. Missarzhevsky (Rozanov and Missarzhevsky 1966: 96) first noted the similarity between Camenella garbowskae Missarzhevsky in Rozanov and Missarzhevsky, 1966 and Camena kozlowskii Missarzhevsky in Rozanov and Missarzhevsky, 1966 and suggested that they might represent disarticulated skeletal parts of the same organism. Further developments of terminology and taxonomy of tommotiids proposed by Bengtson (1970, 1977, 1986a) are followed here.

Less common, small sellate sclerites (both sinistral and dextral symmetry variants) with a flared margin may well also belong to the C. garbowskae scleritome (see Camenella cf. C. garbowskae in Fig. 39). Rare simplified, laterally compressed mitral sclerites, named “planiform” by Bengtson (1986a) after Camenella plana (Missarzhevsky in Rozanov et al., 1969), are described from Camenella baltica and C. parilobata scleritomes. Several planiform mitrals, attributed herein to Camenella plana (see Fig. 40), co-occur with more common pyramidal mitral sclerites of C. garbowskae. The presence of planiform mitrals in the assemblage supports the suggestion by Bengtson (1986a) that such sclerites may have formed an integral part of the Camenella scleritome, but a more abundant collection of sclerites is needed to confirm this. Planiform mitrals occur in both sinistral and dextral symmetry variants. They are strongly flattened in accrescent-decrescent direction and curved with slight to considerable helical twist towards the descrescent side. Their plicate side carries two to three prominent radial ridges and intervening deep folds. Camenella cf. C. plana is similar to sclerites of C. garbowskae in ornamentation. The former, however, are more variable than the pyramidal mitrals of C. garbowskae and may include a different species.

The wall of Camenella sclerites consists of calcium phosphatic growth laminae. As demonstrated by the inner surface of several well-preserved mitrals of Camenella sp. from sample 6/3.9 (Fig. 41), the alternating laminae are composed of more or less integrated flattened tablet-like aggregates of calcium phosphate. The tablets are not observed in other Camenella sclerites available in the collection. The mitrals of Camenella sp. are curved and more flattened in accrescent-decrescent direction than pyramidal mitrals of C. garbowskae, although less flattened than mitrals of C. plana. The ornamentation of the outer surface is also different. Nodes at intersections of radial and co-marginal ridges on all but decrescent sides produce prominent outgrowths in the apical direction. Camenella sp. occurs in dextral and sinistral symmetry variants.

Camenella admiranda (Missarzhevsky in Rozanov and Missarzhevsky, 1966) is represented by a single apical fragment of a large mitral sclerite with regularly spaced low radial ridges and distinct comarginal folds (Fig. 42). Sellate sclerites of C. admiranda are not known.

Stratigraphic and geographic range.—Tommotian Stage, Siberia.

Material.—Sellate sclerites: one (dextral), SMNH X 4711, from sample 6/18 and two sclerites (dextral and sinistral), SMNH X 4712 and 4713, from sample 7/16; correlated with the Dokidocyathus lenaicus Zone, Tommotian Stage. One (sinistral), SMNH X 4710, from sample 7/31.9; correlated with Delgadella anabara Zone, Atdabanian Stage. Emyaksin Formation, Bol'shaya Kuonamka River.

Description.—The sellate sclerites are asymmetrical, up to 2 mm long and 1.6 mm wide in plan view, with a sella dividing the sclerite into larger and smaller lobes. The sella occupies up to ⅓ of the sclerite width between a ridge situated on the larger lobe and a strong angulation of the margin of the smaller lobe. The ridge slightly overhangs the floor of the sella (Fig. 43D₁). The larger lobe carries five very prominent radial ridges, and the entire dorsal surface of the sclerite is ornamented with distinct comarginal folds. The larger lobe is much higher than the smaller one, longer, and up to 10 times wider in plan view (Fig. 43A₁, A₂). The apex is coiled through up to ¾ whorl (Fig. 43C₄, D₂).

Remarks.—The sclerites differ from Camenella garbowskae in having less numerous and stronger radial ribs and intervening deep folds on the larger lobe, and in a more coiled apex.

Camenella reticulosa Conway Morris in Bengtson et al., 1990 is distinguished by a superimposed reticulation of co-marginal ribs, even stronger coiling and wider sella of the sellate sclerites. The sellate sclerites in C. parilobata differ in having lateral lobes nearly equal in width. Missarzhevsky (in Rozanov et al. 1969: 169) noted morphological variability of Camenella and described forms with prominent acute radial ridges from the lower reaches of Lena River as C. garbowskae (Rozanov and Missarzhevsky 1966: pl. 13: 11–12).

Type species: Lapworthella nigra Cobbold, 1921; Callavia Zone, lower Cambrian; Comley, Shropshire, Britain.

Material.—Single sclerites from samples 3/53.5, 3/55.5, 6/41.7 (correlated with the Delgadella anabara Zone, Atdabanian Stage), and ⅕ (Calodiscus-Erbiella Zone, lower Botoman Stage), but more common in samples 7/60, 7/62, and 7/64 (correlated with the upper Judomia Zone, upper Atdabanian Stage); SMNH X 4716–4721. Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers.

Description.—Pyramidal, asymmetrically coiled sclerites (broken off fragments up to 1 mm long, but can be somewhat longer), with oval or subrectangular slightly undulating aperture (up to 1 mm wide), commonly elongated transversely to direction of coiling. Apex represents a simple smooth cone with circular cross-section. Apical angle varies between ca. 15° and 45°. External ornamentation consists of distinct transversal ridges bearing short, apically oriented tubercles (denticles). The density of denticles is 35–40 per mm.

Remarks.—L. dentata Missarzhevsky in Rozanov et al., 1969 from Stage 3 of Siberia is most similar to L. schodackensis (Lochman, 1956) from Laurentia (Landing, 1984) and L. cornu (Wiman, 1903) from Baltica and Avalonia, from Cambrian Series 2, Stage 4. Each of these species is represented by diverse morphologies with variable apical angle, apertural cross-section (circular to compressed oval or rectangular), axial curvature and spacing of growth ridges (Bengtson 1980; Landing 1984). Missarzhevsky (in Rozanov et al. 1969: 164) indicated denticles as the only difference between L. dentata and L. schodackensis. The denticles are present, however, in L. schodackensis (35–144 per mm, after Landing 1984). Landing (1984) also showed that this feature may be obscured by diagenesis and/or preparation in some specimens. Specimens of L. cornu (Wiman, 1903) from Sweden typically carry 10–20 denticles per mm, whereas L. dentata has an intermediate number of denticles, 25–45 per mm (Bengtson 1980). Landing (1984) suggested that L. schodackensis should be synonymised with L. dentata and L. cornu. This synonymisation would be acceptable if only the presence or absence of denticles is considered, with L. cornu (Wiman 1903) the oldest available name. Otherwise, they are different in the relative density of denticulation. Poorly preserved single specimens of Lapworthella cf. L. tortuosa Missarzhevsky in Rozanov and Missarzhevsky, 1966 and Lapworthella sp. are also available in our material (Fig. 44A, B).

Stratigraphic and geographic range.—Lower Cambrian, Atdabanian and lower Botoman Stages, Siberian Platform.

Material.—Two conical sclerites, SMNH X 4722–4723, from sample 6/3.9. Emyaksin Formation, Bol'shaya Kuonamka; correlated with the lower Dokidocyathus regularis Zone, Tommotian Stage.

Description.—Two calcium phosphatic cones are of uncertain affinity. The larger cone (Fig. 45A) has a truncated apex, multilayered wall, smooth eroded external surface and presumed growth increments preserved inside. In the smaller cone (Fig. 45B), the outer surface has a longitudinal striation that passes into a cancellate and tuberculated ornament at the flared basal part of scerite (Fig. 45B₄).

Type species: Helenia cancellata Cobbold, 1921; Callavia Zone, lower Cambrian; Comley, Shropshire, Britain.

Emended diagnosis.—Calcium phosphatic elongated sclerites with a cancellate external ornament, thick wall and a central lumen. The wall is composed of thin parallel hollow tubules arranged obliquely to the surface.

Remarks.—Two species are recognised here, R. cancellatum (Cobbold, 1921) and R. insolutum Missarzhevsky in Missarzhevsky and Mambetov, 1981. Other described species of Rhombocorniculum are synonymised herein with R. cancellatum, following the synonymy list in Esakova and Zhegallo (1996: 102), because they fall within the range of variation for the latter species. For that reason, morphological distinction of R. ungulatum Missarzhevsky (1989: 214, pl. 25: 11), originally described as R. cancellatum by Missarzhevsky (1977: pl. 1: 3–6; see also remarks discussing variability on p. 13 therein), is considered not sufficient to recognise a separate species (but see Esakova and Zhegallo 1996: 103).

Rhombocorniculum is similar in external ornamentation to some bradoriid spines from the same beds as the type material and described under the name Rushtonites Hinz, 1987, which is probably a junior synonym of Mongolitubulus Missarzhevsky, 1977 (Skovsted and Peel 2001; Topper et al. 2007; see also description of Mongolitubulus spines below).

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, usually somewhat narrowing towards its base, can bear one or several stacked rounded structures (Fig. 48A), similar to growth increments. These rounded structures can either be smooth or carry a smoothened sculpture. The exterior surface of the sclerite is covered with a cancellate sculpture, similar to that described for R. insolutum (see above). In many cases, however, the tubercles are nearly perpendicular to the wall (Figs. 46A₁, A₂, 47A₂, C₂, H). The lumen is surrounded by relatively thick walls. The wall contributes to at least half of the sclerite width between the broader sides and significantly more laterally (Figs. 47B₁, C₂, 48F₂). Similarly to R. insolutum (see above), the wall is composed of narrow tubules, 0.1–0.2 µm in diameter (Figs. 46B₁, D₃–D₅, 47B₁, C₂, 48F₂).

The broader sclerites are found in association with the slender 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. 47B₁). 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 slender 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).

Type material: Holotype: GIN 4296/31 from sample 897/1 (Fig. 49D); paratypes: SMNH X 4724–4726 from sample 897/3 (Fig. 49A–C).

Type locality: Right bank of the Yudoma River, upstream of the mouth of Tyallaakh Rivulet, southeastern Siberian Platform

Type horizon: Exposed top of the Pestrotsvet Formation, early Atdabanian age.

Material.—Among 62 sclerites available from the Emyaksin Formation of Malaya Kuonamka and Bol'shaya Kuonamka, only three (ca. 5%) are clearly originally leaf-like, broad morphotypes, although another two to three appear transitional in morphology (see below). The following specimens from the Anabar Uplift are described in stratigraphically ascending order.

Four slender morphotypes, including two proximally compressed (probably diagenetically) sclerites SMNH X 4728–4730 (Fig. 50B–D) and a bifurcated apical part of another relatively broader sclerite or a twinned sclerite SMNH X 4727 (Fig. 50A) from sample 7/19; one fragment of a slender sclerite from sample 3/27.5; both samples from the same level correlated herein with the upper Dokidocyathus lenaicus Zone, upper Tommotian Stage.

One slender morphotype from sample 3/34 and one from sample 7/25.5; one broad sclerite SMNH X 4740 (Fig. 51I), four slender ones SMNH X 4731–4734 (Fig. 51A–D) and six additional slender morphotypes from sample 7/25.6; six slender sclerites from sample K7/22.6B; the samples derive from beds correlated with the Profallotaspis jakutensis Zone, lowermost Atdabanian Stage.

Three slender sclerites SMNH X 4735–4737 (Fig. 51F, G₁, H₁), one broad sclerite SMNH X 4738 (Fig. 51J) and one sclerite of transitional morphology SMNH X 4739 (Fig. 51E), as well as sixteen other slender sclerites from sample 7/27.5; seven slender sclerites from sample 7/28.5 and two from sample 7/29; the three samples derive from strata ranging through the Repinaella Zone, Atdabanian Stage.

Fifteen slender morphotypes from sample 3/40, one figured broad sclerite SMNH X 4741 (Fig. 52) and one slender from sample 3/42, five slender sclerites from sample 6/39.1; correlated with the lower Delgadella anabara Zone, Atdabanian Stage.

Additionally, the holotype from sample 897/3 (Fig. 49D) and three slender sclerites SMNH X 4724–4726 from sample 897/1 (Fig. 49A–C) collected by Stanislav N. Serebryakov and Mikhail A. Semikhatov and included among paratypes by Vladimir V. Missarzhevsky (in Missarzhevsky and Mambetov 1981: 69) are figured herein for comparison. They derive from the exposed top of the Pestrotsvet Formation, cropping out at the right bank of the Yudoma River, upstream of the mouth of Tyallaakh Rivulet, southeastern Siberian Platform, and like the holotype have reportedly an early Atdabanian age.

Description.—Two morphotypes of calcium phosphatic sclerites are herein included within the species, slender sticklike and broad leaf-like ones.

The slender morphotype (representing the type material) comprises elongated narrow (up to 2.5 mm long and 0.2 mm wide) conical sclerites. They are gently curved in one plane, rounded, nearly oval in cross section. The tip of the sclerite is straight and acute. The proximal part usually narrows somewhat towards the base and can bear several circular terraces or a few circular narrow and shallow depressions (clitella). These circular structures usually carry a more smooth sculpture, compared to the ornamentation of the other exterior surface of the sclerite covered with a cancellate sculpture. The sculpture is formed of tessellated tubercles oriented obliquely to the surface towards the apex while rows of tubercles form a rhombic pattern oblique to the longitudinal axis. The sclerites have a central simple longitudinally oriented lumen rounded or oval in cross-section, often open at the proximal end of sclerite. The lumen is surrounded by relatively thick walls. The wall contributes to at least one quarter of the sclerite width. The proximal end of the sclerites can be closed, with no lumen visible. Although some diagenetic compression of the sclerites is possible, the forms with an extended oval cross-section are interpreted herein as transitional in morphology between the broad and typical slender morphotypes (Fig. 51E). The distinction is not that clear, however, and therefore these transitional forms are generally attributed to the slender morphotype.

The broad sclerites associated with the slender ones are rare, and so are forms of transitional morphology, more closely resembling the slender sclerites (see the foregoing section Material). The broad sclerites share the same general morphology, being flattened, not twisted longitudinally, but the apical part is shifted (to the right in the specimens available), having concave and convex opposite broad sides. The sclerites have an apical tip and extended lenticular basal end. The margin of the lumen is not well preserved at the basal part but is probably elongated towards the lateral sides of the sclerite (Fig. 51I₂, 52C). The sculpture is very similar to what is usually observed on the surface of the slender sclerites. The wall contains tubules, whereas the outer surface exhibits fine longitudinal striation, same as in slender forms (Fig. 52E, F). One sclerite, probably of the broad morphotype, shows bifurcation of its apical part (Fig. 50A). The sclerite is unusual among those attributed to R. insolutum and interpreted as aberrant. Other specimens available from that sample (Fig. 50B–D) are similar to slender forms, but proximally compressed.

The wall of all sclerites is essentially composed of narrow tubules, 0.1–0.2 µm in diameter (Figs. 52D, 51G, H). The tubules run parallel to each other and the surface of the wall but tend to dip slightly towards the apex of the sclerite (Fig. 49B₂). The tubules situated near the outer surface apparently have a smaller diameter. The outer surface of the wall exhibits a fine longitudinal striation. Individual striae, ca. 0.1 µm in diameter (Fig. 51G₃), represent the outermost casts of tubules. They follow the wall surface, run parallel to each other and do not appear to bifurcate or anastomose.

Remarks.—The type material described by Missarzhevsky and Mambetov (1981) includes only slender sclerites (see Fig. 49), but those co-occur with broad forms in the Kuonamka collection. Since broad morphotypes and transitional forms are also known from R. cancellatum (see above), we conclude that, by analogy with the latter, the broad sclerites described herein can be attributed to R. insolutum. In contrast, broad sclerites of R. cancellatum are longitudinally twisted. The slender sclerites of R. cancellatum are also twisted longitudinally and clearly asymmetrical transversally, with one of their lateral sides being blade-like, more flat than the other one. The central lumen is also different in general, being relatively smaller in diameter and more isodiametrically rounded in cross-section.

It has already been reported that Rhombocorniculum sclerites grew continuously by basal-internal accretion (Landing et al. 1980; Landing 1995) and contain canals oriented oblique to the wall surface (Li et al. 2003). Co-occurrence of dextral and sinistral sclerites of R. cancellatum suggests a bilaterally symmetrical animal, whereas the slender and broad elements probably represented distinct sclerite types within a scleritome. Although superficially similar to some bradoriid spines, microstructural data suggest that Rhombocorniculum is difficult to associate with arthropods (Skovsted et al. 2006). Judging from the similar diameter of the tubules in the wall and the fine longitudinal stria on its outer surface, it seems that they are formed by the same organic threads, interpreted herein as microvilli. These microvilli likely extended from chaetoblast-like secreting cells at the base of the sclerites. Such an interpretation of the microstructure in combination with the bristle-like appearance of Rombocorniculum sclerites suggests that they may represent chaetae of the type occurring in annelids. In modern polychaetes, microvilli produce longitudinally oriented tubules of a similar diameter by means of deposition by a chaetoblast of sclerotised chitin at their bases and their subsequent retraction (Hausam and Bartolomaeus 2001). Peripheral microvilli may be responsible for the formation of the surface sculpture of the sclerite. The earliest known annelids are non-mineralised fossils from the early Cambrian Sirius Passet fauna of North Greenland (Conway Morris and Peel 2008), but their first hard (diagenetically mineralised) remains are scolecodonts from the upper Cambrian (Eriksson et al. 2004). Scolecodonts have a similar network of pores produced by systems of tubules 0.1–0.25 µm in diameter and exposed on the eroded surfaces of denticles (Szaniawski and Gaździcki 1978).

Stratigraphic and geographic range.—Rhombocorniculum insolutum was first described from the lower Atdabanian of the Siberian Platform (Missarzhevsky and Mambetov 1981). It was also reported from the upper portion of Bed 14 of the Pestrotsvet Formation, Isit section, middle Lena River, from the middle Dokidocyathus lenaicus Zone of the Tommotian Stage (Sokolov and Zhuravleva 1983; Rozanov and Sokolov 1984; Brasier 1989a; Rozanov and Zhuravlev 1992; Shabanov et al. 2008). The latter occurrence is coeval with the lowermost ones reported herein from the eastern Anabar Uplift (see section Material above). R. insolutum has not been recovered herein from beds containing R. cancellatum above the δ13C trough between peaks IV and V (Fig. 2; Brasier et al. 1994; Kouchinsky et al. 2001). The stratigraphical distribution of R. insolutum in the Malaya and Bol'shaya Kuonamka sections covers the entire area of excursion IV, ranging from the upper Dokidocyathus lenaicus Zone of the Tommotian through the lower Delgadella anabara Zone of the Atdabanian Stage (Fig. 2). R. insolutum is also reported from the Camenella baltica Zone of Avalonia (SE Newfoundland, Brasier 1984, 1986, 1989a; Landing 1988; Landing et al. 1998; and Britain, Brasier 1986) and Schmidtiellus mickwitzi Zone of Baltica (Bornholm, Poulsen 1967; Brasier 1989a).

Type species: Hertzina? danica Poulsen, 1966; lower middle Cambrian; Bornholm.

Remarks.— By contrast with other species of “protoconodonts” herein (see also Kouchinsky et al. 2011), Amphigeisina danica does not exhibit longitudinal fibrous microstructure of the type available in grasping spines of chaetognaths (e.g., Vannier et al. 2007).

Material.—One figured sclerite SMNH X 4763 and two additional specimens from sample 1/9; carbonate concretions of the basal Kuonamka Formation, Malaya Kuonamka River; Bergeroniellus expansus Zone, upper Botoman Stage.

Description.—Slender, gently curved, simple bilaterally symmetrical calcium phosphatic sclerites. A relatively narrow concave side extends from the tip towards the base. The opposite, broader, strongly convex side of the sclerite is open in the slightly flared basal part. The internal cavity extends to the tip. The wall is composed of two layers with amorphous microstructure. The surface of the convex side is smooth, whereas faint transverse folds are present on the concave side. The two sides meet to produce two longitudinal carinae flanked by two thin flanges formed by the outer layer. The flanges reduce gradually towards the tip, where they merge with the carinae.

Stratigraphic and geographic range.—The occurrence of Amphigeisina danica reported herein from the lower part of Cambrian Stage 4 (upper Botoman Stage) is the earliest known from the Siberian Platform, where it also occurs in the Toyonian Stage representing the upper portion of Stage 4 (as Protohertzina yudomica, after Demidenko 2006) and in the middle Cambrian Stage 5, in the Kuonamka Formation of the Malaya Kuonamka and Bol'shaya Kuonamka rivers (Kouchinsky et al. 2011). The species is also known from the upper part of Series 2 of Kazakhstan (Gridina 1991), Avalonia, Britain (Hinz 1987), and Laurentia (Landing 1974; John S. Peel, personal communication 2010) and from Series 3 of Baltica (Poulsen 1966; Bengtson 1976) and South and North China (Qian et al. 2004; Steiner et al. 2007).

Type species: Hertzina? bisulcata Müller, 1959; upper Cambrian, Sweden.

Material.—Four illustrated sclerites, SMNH X 4764–4767, and 12 additional specimens from sample 1/9; carbonate concretions of the basal Kuonamka Formation, Malaya Kuonamka River; Bergeroniellus expansus Zone, upper Botoman Stage.

Description.—Slender, gently curved, simple bilaterally slightly asymmetrical, and laterally flattened calcium phosphatic sclerites. The longitudinally convex side of the sclerite has a strongly convex surface, whereas the opposite, longitudinally concave side is transversely gently convex or flattened. The two sides have almost the same width. Two longitudinal lateral furrows extend from the basal opening towards the tip. The internal cavity extends to the tip of sclerite. The wall has a lamellar fibrous structure (Fig. 53D₂).

Stratigraphic and geographic range.—The occurrence of Gapparodus bisulcatus reported herein from the lower part of Cambrian Stage 4 is the earliest known from the Siberian Platform, where it also occurs in Stage 5 (Kuonamka Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; Müller et al. 1995; Kouchinsky et al. 2011). The species ranges from Series 2 through the Furongian Series in South and North China (Dong and Bergström 2001; Babcock et al. 2007; Steiner et al. 2007). It is also known from the middle—upper Cambrian of Baltica (Bengtson 1976; Müller and Hinz 1991), upper Series 2—Series 3 of Laurentia (Clark and Miller 1969; Clark and Robison 1969; Landing 1974; John S. Peel, personal communication 2010), Cambrian of Western Gondwana (Turkey, Özgül and Gedik 1973), upper Cambrian of Eastern Gondwana (South Korea, Lee 1975), and middle—upper Cambrian of Kazakhstan (Abaimova and Ergaliev 1975).

Type species: Protohertzina cultrata Missarzhevsky, 1977; upper Atdabanian Stage; lower reaches of the Sinyaya River, middle Lena River basin, Siberian Platform.

Material.—Single fragment SMNH X 4768 from sample 7/39.5; two specimens, including SMNH X 4769, from sample 7/47.5; Emyaksin Formation, Bol'shaya Kuonamka river; correlated with the Delgadella anabara Zone, Atdabanian Stage. Three sclerites, including SMNH X 4770, from sample 1/9; carbonate concretions of the basal Kuonamka Formation, Malaya Kuonamka River; Bergeroniellus expansus Zone, upper Botoman Stage.

Description.—Gently curved, simple, laterally narrowed and bilaterally symmetrical calcium phosphatic sclerites. The convex side forms a longitudinal median flange. The internal cavity extends to the apex.

Stratigraphic and geographic range.—Hagionella cultrata is known from the Atdabanian Stage (Stage 3; Missarzhevsky 1977 and herein) and Cambrian Stage 5 of Siberia (Kouchinsky et al. 2011), Series 2 of Kazakhstan (Bengtson 1983), lower-middle Cambrian of South and North China (see Qian et al. 2004; Li et al. 2004a; Steiner et al. 2007), middle Cambrian of Baltica (Bengtson 1976), upper Series 2 of Laurentia (John S. Peel, personal communication 2010), and probably Avalonia, Britain (Hinz 1987).

Material.—Single apical fragment, SMNH X 4771 from sample 1/9; carbonate concretions of the basal Kuonamka Formation, Malaya Kuonamka River; Bergeroniellus expansus Zone, upper Botoman Stage.

Description.—Gently curved bilaterally symmetrical apical fragment of a calcium phosphatic sclerite, 0.8 mm long. Cross-section rhomboidal (or star-shaped), with concave sides. Two distinct keels are situated in the middle of the longitudinally convex (more prominent keel) and concave sides of sclerite. Two other keels run along the middle of the lateral sides of sclerite, but reach the concave side near the apex.

Material.—Single apical fragment, SMNH X 4772 from sample 1/9; carbonate concretions of the basal Kuonamka Formation, Malaya Kuonamka River; Bergeroniellus expansus Zone, upper Botoman Stage.

Description.—Gently curved smooth bilaterally symmetrical apical fragment of a calcium phoshatic sclerite, ca. 1 mm long. Cross-section oval, laterally compressed, with lateral sides shallowly invaginated in the middle. The depression disappears towards the apex.

Material.—Two incompletely preserved calcium phosphatic sclerites, SMNH X 4773 and 4774, from samples 6/3.9 and 6/6.8 (respectively); Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Dokidocyathus regularis Zone, Tommotian Stage.

Description.—Gently curved sclerites with nearly circular cross-section. The surface of the longitudinally concave side of the sclerite shows distinct growth lines sharply bent towards the base of the sclerite laterally, where they run further nearly parallel along the lateral sides.

Material.—Single incompletely preserved calcium phosphatic sclerite, SMNH X 4775 from sample 7/33; Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Sclerite gently curved, with a broad base and laterally compressed apical part. The longitudinally convex side is smooth at the base, with a semi-circular cross-section. A ridge emerges in the apical part, where the sclerite becomes laterally compressed. The longitudinally concave side of the sclerite is significantly shorter, with its broad basal part open, but becoming close and smooth towards the apex. The outer surface is covered with growth lines.

Type species: Fengzuella zhejiangensis He and Yu, 1992; Qiongzhusian Stage?, lower Cambrian; Hetang Formation, Wujialing, Jiangshan County, Zhejiang Province, South China.

Material.—Single laterally flattened bilaterally symmetrical sclerite, SMNH X 4776, from sample 7/45 (Fig. 54A) and two curved bilaterally symmetrical sclerites, SMNH X 4777 and 4778, from samples 7/55.8, and 7/64 (respectively). Emyaksin Formation, Bol'shaya Kuonamka; correlated with the Delgadella anabara and Judomia zones, Atdabanian Stage. An additional specimen, SMNH X 4779, derives from sample 2001-11b, collected along the Sinyaya River, tributary of the Lena River in the middle reaches (Rozanov and Missarzhevsky 1966), Atdabanian Stage, but the exact stratigraphic position of the sample is not known.

Description.—Calcium phosphatic sclerites of planispiral and curved types. The planispiral sclerite is ca. 0.3 mm in diameter and consists of 1¼ whorls. The curved sclerites are 0.5–0.8 mm long. Along each side of the sclerites, the wall has two longitudinal depressions separated from each other by a ridge. The central cavity extends from the aperture to the apex, and has an oval or rhomboidal, star-shaped cross-section with convex sides. The wall is multi-layered and consists of basally secreted and adapically overlapping thin growth lamellae (Fig. 54A₄, B₂, D₄).

Remarks.—Steiner et al. (2003) noted that curved sclerites predominate (85%) and the planispiral type is rare. It has been also suggested that the scleritome consisted of densely packed growing sclerites with their proximal parts attached to a soft tissue. Presumably, the sclerites had originally a sclerotised organic (non-mineralised?) composition. The outer parts of the growth lamellae were deposited externally, probably within an invaginated tissue.

Stratigraphic and geographic range.—Problematic sclerites of Fengzuella zhejiangensis He and Yu, 1992 were originally described from South and North China, from beds that possibly correlate with the Anabarites trisulcatus-Protohertzina anabarica Assemblage Zone of the lower Meishucunian Stage (Steiner et al. 2003). That correlation is not certain, however, and the beds might alternatively be correlated with the Qiongzhusian Stage of China and Atdabanian Stage of Siberia (Steiner et al. 2003, 2007). A single planispiral sclerite similar to Fengzuella is reported, as a coiled and flat discoidal problematicum, from lower middle Cambrian strata of the Kuonamka Formation, Khorbusuonka River, Olenyok Uplift, Siberian Platform (Müller et al. 1995: 114, fig. 5E, F). Planispiral and transitional sclerites of, probably, a new species of Fengzuella are also available from the upper Series 2 of Laurentia (North Greenland; AK and John S. Peel, unpublished observation).

Type species: Microdictyon effusum Bengtson, Matthew, and Missarzhevsky, 1986; Rhombocorniculum cancellatum Zone, lower Cambrian; Geres Member of the Shabakty Group, Ushbas River, Lesser Karatau, Kazakhstan.

Remarks.—For discussion of the validity of the genus and species see Demidenko (2006: 237).

Material.—One complete sclerite, SMNH X 4780 and two fragments from sample 7/54. Emyaksin Formation, Bol'shaya Kuonamla River; correlated with the middle Judomia Zone, Atdabanian Stage.

Description.—The sclerite is weakly convex, with a subcentrally situated apex; thin and rhombic in plan view, with a maximum dimension of ca. 1 mm. The sclerite (as illustrated in Fig. 55A₂) has semicircular upper and lower margins and protrudes into distinct angulations laterally. The upper (as illustrated in Fig. 55A₂) margin of the sclerite forms a flattened limb covered with ca. 5 µm tubercles oriented to the right in Fig. 55A₅. The opposite margin has a weakly expressed reentrant (Fig. 55A₂). The sclerite has distinct, almost circular holes surrounded by 5–7 nodes. The holes range from ca. 10 µm in diameter at the margin of the sclerite to 80 µm in its central part. The nodes have an initial diameter of ca. 10 µm at the margin of the sclerite, increasing up to at least 80 µm centrally. These nodes start at the margin as low flattened tubercles and progress into larger cones, >25 µm tall, with flared bases and tips shifted towards the centre of the sclerite (Fig. 55A₃). The holes are not closed from below.

Remarks.—In articulated lobopodians from Chengjiang and their reconstructions (e.g., Hou and Bergström 1995), the sclerites are approximately bilaterally symmetrical across a vertical plane representing their anatomical orientation along the worm-like body of the animal, and with their eventual spines pointed in dorsal direction. Therefore, the upper ends of the nodes would rather also point towards the dorsal margin of the sclerite. For that reason and in accordance with Kouchinsky et al. (2011: 159), orientation of sclerites in Fig. 55 is presumed to be anatomically dorso-ventral. It is not clear, however, which of the margins herein (lower or upper, as illustrated in Fig. 55) is dorsal or ventral, for the nodes tend to point towards the centre of sclerite.

The holotype of Microdictyon rhomboidale Bengtson, Matthews and Missarzhevsky, 1986 (SMNH X 2111), derives from the Bateny Hills, Tamdytau, Altai-Sayan Folded Area; upper Atdabanian Stage (Bengtson et al. 1986). The sclerite of Microdictyon cf. M. rhomboidale herein has a strong similarity to the holotype and paratypes of M. rhomboidale (Bengtson et al. 1986: fig. 4A–D; Fig. 55B herein), but has somewhat larger nodes and holes, and an almost indistinct reentrant (compare lower parts of Fig. 55A₂, B₃). Both are distinguished from sclerites of other lobopodian species by their rhombic outline and a flattened tuberculated limb. The latter has not been previously described from any of the species.

Remarks.—Microdictyon rhomboidale was described from the upper Atdabanian (Stage 3) of the Altai-Sayan Folded Area in Siberia (Tamdytau Mountains, Kyzyl-Kum). Microdictyon cf. M. rhomboidale is reported from the Sekwi Formation, Mackenzie Mountains, Canada and the upper Campito Formation, White Mountains, California, both from the lower Nevadella Zone, correlated with the middle Atdabanian Stage of Siberia (Bengtson et al. 1986: 98). Microdictyon aff. rhomboidale also is described from Shaanxi Province in China, from the upper Eoredlichia-Wutingaspis Zone, correlated with the upper Atdabanian Stage of Siberia (Zhang and Aldridge 2007). The new occurrence in Siberia is roughly equivalent in age to those above. Chemostratigraphy (Fig. 2 and Kouchinsky et al. 2001) indicates that it belongs to the middle Judomia Zone, or middle Nochoroicyathus kokoulini Zone of the upper Atdabanian Stage.

Material.—Single fragments from samples 3/60 and 6/66.2; two, SMNH X 4782 and 4783, from sample 7/34.7 and six fragments from sample 7/39.5; Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; correlated with the upper Delgadella anabara Zone (samples 7/34.7 and 7/39.5) and Judomia Zone, Atdabanian Stage.

Description.—Fragments of Microdictyon sclerites with partly preserved margin (margin in the lower part of the Fig. 56A₂, A₃, B₄). The fragments have distinct rounded, almost circular holes surrounded by 5–8 cap-like nodes. Additionally, smaller nodes are present along the margin of sclerite (Fig. 56A₂, A₃). The diameter of the holes increases from ca. 15 µm at the very margin of the sclerite to at least 70 µm centrally. The nodes have an initial diameter of ca. 10 µm at the margin of the sclerite, increasing up to at least 50 µm proximally. These nodes start at the margin as tubercles and progress into larger cones, >30 µm tall, with flared bases and tips shifted towards the centre of the sclerite. The holes are not closed from below.

Remarks.—Microdictyon sp. described herein is similar to Microdictyon rhomboidale, described above, but determination of its affinity is hampered by incomplete preservation. Chemostratigraphy (Fig. 2 and Kouchinsky et al. 2001, 2012) indicates that Microdictyon sp., from the upper Delgadella anabara Zone, Atdabanian Stage, is the earliest lobopodian fossil known from Siberia. This occurrence is correlated chemostratigraphically (Dilliard et al. 2007) with the regional first appearance of Microdictyon sp. and Microdictyon cf. M. rhomboidale in the lower Nevadella Zone of Laurentia (see above, Bengtson et al. 1986; Voronova et al. 1987) and, probably, with the first occurrence of M. sphaeroides Hinz, 1987 in the Camenella baltica Zone of Avalonia (Hinz, 1987) and Microdictyon depressum Bengtson in Bengtson et al., 1990 in the Abadiella huoi Zone of Australia (Topper et al. 2011; Kouchinsky et al. 2012).

Type species: Hadimopanella oezgueli Gedik, 1977; middle Cambrian, Taurus Mountains, Turkey.

Material.—Hundreds to thousands of individual sclerites, including SMNH X 4784–4797, from samples K1-2B, 1/5, 3/60, 3/61.5, 3/62.5, 3/63.5, 3/65, 6/66.2, 7/55, 7/55.8, 7/60, 7/62, and 7/64. Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; correlated with the lower Judomia Zone, Atdabanian Stage-Calodiscus-Erbiella Zone, Botoman Stage.

Description.—Disk-like circular to oval phosphatic sclerites with one side convex and smooth, and the opposite side consisting of a conical surface capped with one or several tubercles. Smaller sclerites with a single or a few tubercles are circular or slightly oval, 80–100 µm in diameter. The tubercles form a circle, sometimes around a central group of 3–5 others. In larger sclerites, a group of up to tens of closely situated tubercles form a flat surface with a rounded or oval outline parallel to the margin of sclerite, with the latter up to 200 µm in larger diameter. The conical surface is either smooth or covered with a fine granulation and straight and fine radial striae. The two surfaces pass into each other with a distinct angulation, marked in some specimens by numerous short subradial rugae.

Remarks.—Disarticulated button-shaped palaeoscolecidan sclerites without affinity to known palaeoscolecidan scleritomes are usually attributed to the form genus Hadimopanella Gedik, 1977 (Ponomarenko 2005; Topper et al. 2010; García-Bellido et al. 2013; Barragán et al. 2014a, b). The sclerites described herein fall within the size variation of H. knappologica and are most similar to specimens described by Bengtson (1977). Sclerites from scleritomes of paleoscolecids known from the Botoman Stage of the southeastern Siberian Platform show a different sculpture (Ivantsov and Wrona 2004). H. apicata Wrona, 1982 from the upper Stage 3-Stage 4 of Gondwana and Laurentia is, however, similar in size and morphology to smaller single-tubercle individuals available in the collection, but attributed herein to H. knappologica. The number of tubercles in H. apicata varies from one to four in larger specimens, with the diameter of larger sclerites reaching ca. 150 µm. In addition, sclerites of H. antarctica Wrona, 1987 co-occur with those of H. apicata and would have formed part of the same scleritome (Topper et al. 2010), although Australian sclerites are significantly smaller (11–26 µm) than those from the type material of H. apicata. For that reason, a preliminary definition of the earliest palaeoscolecid sclerites from the Emyaksin Formation as H. apicata (see Kouchinsky et al. 2012) is here considered untenable. The sclerites are therefore redefined herein as H. knappologica. Similar sclerites are also described from the middle Cambrian Kuonamka Formation, Anabar Uplift by Kouchinsky et al. (2011) and as Hadimopanella aff. Н. oezgueli Gedik, 1977 by Müller et al. (1995). Morphologically similar sclerites of the type species, Hadimopanella oezgueli described from the middle—upper Cambrian of Gonwana and Kazakhstan are probably also available from Siberia (see Geyer et al. 2014). Ranges of morphological variability in both form species are, however, broad and overlap, so that it is not possible to associate them with a single or several natural species (Barragán et al. 2014a, b).

Stratigraphic and geographic range.—Lower and probably middle Cambrian, Siberian Platform.

Type species: Mongolitubulus squamifer Missarzhevsky, 1977; Botoman Stage, lower Cambrian; Salany-Gol River, Mongolia.

Remarks.—It has been suggested that Mongolitubulus-like hollow ornamented spines may represent detached fragments of different panarthropod origin, such as bradoriids or phosphatocopids (Melnikova 2000; Skovsted and Peel 2001; Topper et al. 2007), lobopodians (Dzik 2003; Caron et al. 2013), or trilobites (Li et al. 2012). Employment of the name Mongolitubulus Missarzhevsky, 1977 for such fossils should therefore be verified by investigation of the type or topotype material of Mongolitubulus squamifer Missarzhevsky, 1977 from western Mongolia.

Material.—Ten specimens from samples 6/59.3, 6/66.2, 7/54, and 7/55, including SMNH Ar 63450 from sample 7/55; correlated with the Judomia Zone, upper Atdabanian Stage; ten specimens, including SMNH Ar 63451, from sample 7/70; Calodiscus-Erbiella Zone, lower Botoman Stage; Emyaksin Formation, Bol'shaya Kuonamka River. Seven specimens, including SMNH Ar 63452 and 63453, from sample 1/9; carbonate concretions in the basal Kuonamka Formation, Malaya Kuonamka River; Bergeroniellus expansus Zone, upper Botoman Stage.

Description.—Gently curved hollow calcium phosphatic spines nearly circular in cross-section. The wall consists of two layers. The outer layer has a cancellate ornamentation consisting of irregular transversal rows of polygons with serrated edges. The polygons are extended longitudinally, with their apical part somewhat raised and pointed towards the apex of the spine. Inner layer without sculpture, smooth and folded at the base of spine (Fig. 58D₂). The basal part is flaring (Fig. 58A). The specimens reach 3 mm long and are up to 0.2 mm in diameter.

Remarks.—In addition to the type species, M. squamifer Missarzhevsky, 1977, such other specias as M. aspermachaera Topper, Skovsted, Harper, and Ahlberg, 2013, M. henrikseni Skovsted and Peel, 2001, M. reticulatus Kouchinsky, Bengtson, Clausen, Gubanov, Malinky, and Peel, 2011, M. spinosus (Hinz, 1987), M. unialata (Zhang, 2007), and M. unispinosa Topper, Skovsted, Brock, and Paterson, 2007 are included within Mongolitubulus Missarzhevsky, 1977, shown to represent the spines of a bradoriid (Skovsted and Peel 2001; Topper et al. 2007, 2013).

Ornamentation of M. spinosus herein is very similar to that of M. spinosus from Britain, described as Rushtonites spinosus by Hinz (1987: 11) and Brasier (1989a: pl. 7.2), as well as to the spines attributed to Tubuterium ivantsovi Melnikova, 2000 from Siberia (lower Kutorgina Formation, middle Lena River, Botoman Stage; Melnikova 2000: pl. 8; Ponomarenko 2005: pls. 29, 30). The apically raised edges of polygons in Rushtonites spinosus (Hinz 1987: pl. 11: 2, 4, 7, 9) appear to be more rounded, however, than in M. spinosus herein and in Tubuterium ivantsovi, but this can be a preservational effect. Tommotitubulus grausmanae (Vasil'eva 1994: pl. 1: 1 therein and Vasil'eva 1998: pl. 46: 2, 5, 6) from the lower Botoman Stage of Siberia is considered herein to be a junior synonym of M. spinosus.

M. spinosus differs from the other, younger, species from the Kuonamka Formation, M. reticulatus, in ornamentation and in having a straight, not bent apical part. The outer layer in M. reticulatus has a reticulate ornamentation consisting of longitudinally extended polygons forming a regular pattern of straight or gently spiralling rows directed towards the apex (Kouchinsky et al. 2011: figs. 33–35). The wrinkled outer surface of M. aspermachaera from the Furongian of Baltica bears tubercles (Topper et al. 2013). The ornamentation of M. henrikseni Skovsted and Peel, 2001 from the beds equivalent to the Botoman Stage in Greenland and Australia (Skovsted and Peel 2001; Skovsted et al. 2006) and M. squamifer Missarzhevsky, 1977 from beds in Mongolia, Kazakhstan, and Greenland correlated with the upper Atdabanian-Botoman Stages (Missarzhevsky 1977; Missarzhevsky and Mambetov 1981; Skovsted and Peel 2001) represents well-defined scale-like protuberances. The outer surface of M. unispinosa spines bears short second-order spines and rhombic scales inclined adapically, but subdued in the apical area (Topper et al. 2007). A flaring basal part is known from all species attributed herein to Mongolitubulus, except for M. squamifer (Skovsted and Peel 2001).

Stratigraphic and geographic range.—Mongolitubulus spinosus (Hinz, 1987) occurs in the Lower Comley Limestones (Beds Ac2–Ac3, after Brasier 1989b), Callavia Zone, Britain, correlated with the upper Atdabanian-lower Botoman Stages. On the Siberian Platform, it is known from the Judomia Zone of the upper Atdabanian Stage and from the Botoman Stage.

Material.—Tuberculated cuticular fragments, including spines and low cones: single spine from sample 3/65; two spines from sample 6/59.3; two spines, including SMNH Ar 63454, seven cones, including SMNH Ar 63455, and one fragment with a short smooth spine SMNH Ar 63456 from sample 6/66.2; two spines and one cone from sample 7/47.5; one cone from sample 7/48.5; over thirty cuticular fragments from sample 7/54, including sixteen cones and five spines, including SMNH Ar 63457 and 63458; two cuticular fragments with paired cones, SMNH Ar 63459 and 63461, thirty two cones and eighteen spines from sample 7/55; basal part of a large tuberculated spine SMNH Ar 63460, one additional spine and one cone from sample 7/55.8; uppermost Delgadella anabara and Judomia zones. Carapace? fragments from samples K3-24B and 7/27.5, including SMNH Ar 63462; correlated with the Repinaella Zone. All from the Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; Atdabanian Stage.

Remarks.—Phosphatised cuticular fragments, including prominent spines and low cones, that are ornamented with tubercles and polygonal network. Evidence of elastic deformation (Figs. 59A₃, 60) suggests an originally non-mineralised or weakly mineralised wall. The wall is subdivided into a thin outer layer and a much thicker inner layer. The outer layer produces folds and reticulation covering the outer surface (Fig. 59A, C). The outer layer is partly exfoliated, and the sculpture is fragmentarily and indistinctly preserved. The inner layer is composed of thin laminae. Lamination is present in fracture sections of the spines (Fig. 59A). Individual laminae are ca. 0.5 µm thick and run parallel to the surface of the wall (Fig. 59A₁). Concentric lamination is also observed in broken-off tubercles (Fig. 59A₂).

Similar cap-like tuberculated cones of calcium phosphatic composition were described as Cambrocassis verrucatus Missarzhevsky, 1977, from the Atdabanian Stage of Siberia and Mongolia (Missarzhevsky 1977; see also Cambrocassis sp. in Missarzhevsky 1989: pl. 30: 3). The specimens described herein are also very similar to Nikolarites spasskyi Vasil'eva, 1994 from the upper Atdabanian Stage of the Lena-Aldan region in Siberia (Vasil'eva 1994, 1998). These fossils probably represent parts of arthropod, probably bradoriid carapaces (Dzik 2003; Skovsted et al. 2006). Low tuberculate cones are twice as abundant as tuberculate spines in the collection (62 cones and 32 spines can be recognised). This may suggest that there were at least two low cones per tuberculate spine on each side of carapace. This is also consistent with the presence of cuticular fragments with paired cones (Figs. 59D, 60A₃).

Two fragments representing almost isodiametric valves presumably formed an originally bivalved carapace (Fig. 61). The outer surface of the valves is convex and covered with a subdued reticulation. A duplicature is present along the inner side of one of the valves (Fig. 61A₄).

Material.—Several hundred brachiolar (and thecal) plates from sample 7/42.3, including figured SMNH Ec 31488–31495, and several tens of plates from sample 7/45. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage. The plates occur as accumulations within a ∼2.5 m thick interval of the Emyaksin Formation. Their abundance sharply declines in a silty, sandy limestone sample 7/45, in comparison to an older clayey limestone sample 7/42.3. Echinoderm plates from samples 7/42.3 and 7/45 are mainly replacements of the original calcite by dolomite.

Description.—The brachiolar plates are preserved as articulated pairs (Fig. 62A–C, G, H) or isolated elements (Fig. 62E, F) that are crescent- or hoof-shaped in section. The plates may reach 1 mm in length, but commonly are ca. 0.5 mm long. Most of the plates are elongated, with a length-to-width ratio of 1–2 (Fig. 62), taper distally and are slightly to strongly twisted longitudinally (Fig. 62A₂). Two opposed plates abut along two narrow longitudinal ridges (Fig. 62A₁, A₄, B₂, H), which delimit a large, oval aboral canal, up to ½ of the articulated pair in width (Fig. 62G, H). The articulation between the two opposite plates is median and symmetric. The aboral surface is convex. The oral surface of the articulated pairs is highly concave, with variable oral opening and high lateral walls (about ½ of ossicle in height). The oral surface is composed of a median furrow bordered by fine, slightly developed ridges and flanked by two variably developed lateral depressions (Fig. 62A₁, B₁, C). The distal and proximal articulation surfaces range from almost flat (Fig. 62A, G, H) to irregularly undulose (Fig. 62B, C) and have a depression surrounding the aboral canal (Fig. 62G, H).

Remarks.—At least one species with blastozoan feeding appendages is present in the studied assemblage. The absence of brachioliferous thecal, “basal” and columnal plates from the assemblage precludes, however, any reconstruction of the theca, whereas the associated thecal plates (see below; Fig. 62G–L) may derive from non-eocrinoid species. The elongated conical shape of most of the brachiolar plates suggests that each brachiole, unbranched and helically twisted, consisted of a relatively few stacked biserial plates. Helically twisted brachioles are known from different gogiids, including the middle Cambrian Gogia spiralis (Robison, 1965). The oral groove of the brachiolar plates represents a complex tripartite food groove, probably originally lined with the radial water vessel in the middle and two lateral neural strings (Clausen et al. 2009). The large longitudinal canal penetrating the brachioles and underlying the food groove presumably housed a neural branch (Clausen et al. 2009; Sprinkle 1973). Sprinkle (1973) described such a canal from middle Cambrian gogiids, such as Gogia spiralis and a younger derived eocrinoid (Sprinkle 1973: fig. 5c), but noted that the canal is usually absent from blastozoan appendages.

The described plates from the middle Atdabanian Stage of Siberia are among the earliest record of echinoderm skeletal elements (Clausen and Peel 2012; Kouchinsky et al. 2012; Zamora et al. 2013). Together with an elongated fragment of a brachiolar plate from the upper Atdabanian Stage of Siberia (Rozhnov et al. 1992: pl. 6: 12), they are probably the oldest brachiolar remains reported so far. Most authors agree that blastozoans (“brachiole bearing echinoderms” ) are probably derived (e.g., David et al. 2000; Paul and Smith 1984) from edrioasteroids (without feeding appendages). The oldest known edrioasteroids are also reported from the upper part of Cambrian Stage 3-lower Stage 4 (see below; Zamora et al. 2013). Therefore, the earliest brachioliferous echinoderms from Siberia demonstrate a very fast but still poorly illustrated initial diversification of echinoderms during Stage 3.

Material.—Several tens of disarticulated plates from sample 7/70, including figured SMNH Ec 31500–31503, samples 7/64 and K1-3B. Emyaksin Formation, Bol'shaya Kuonamka and Malaya Kuonamka rivers; Judomia Zone (sample 7/64), Atdabanian Stage and Calodiscus-Erbiella Zone, lower Botoman Stage. The plates discussed here and below preserve the original stereom microstructure coated and/or replaced by phosphate.

Description.—The brachiolar plates are preserved as disarticulated elements, crescent- or hoof-shaped in cross-section (Fig. 63_A, B₁, C₂, D₂) and slightly tapering distally (<5°). The plates reach 0.5 mm in length, but are commonly 0.3–0.4 mm long. The elongation of the brachiolars varies, from a lengthto-width ratio >1 (Fig. 63D₁) to ⅓ (Fig. 63B₂). Two opposed plates abutted along two flattened articulation surfaces (Fig. 63A–D, arrowed) which delimited an aboral canal. The posterior articulation surface can be significantly larger than the anterior one and forms up to half the ossicle in height. The concave oral surface of the articulated pair of brachiolars was delimited by lateral walls and consisted of a median furrow flanked by two furrows located in the lateral walls. In lateral view, the lateral walls are generally straight along the oral aperture, but the lateral articulation (between the two opposite rows of plates) varies from almost flat (Fig. 63D₁, left side) to triangular (Fig. 63C₁, right side). Proximal and distal facets carry a depression surrounding the aboral canal and are delimited by a peripheral rim.

Remarks.—The ossicles constituted a biserial, probably unbranched appendage, rounded in cross-section, slowly tapered distally, but not twisted, like most eocrinoid and blastozoan appendages. Within the biserial brachioles the plates were opposite (with a straight suture and flat lateral articulation) to offset (undulose suture and triangular lateral articulation). Similar to the brachiolar plates A (see above), there was a complex tripartite food groove, probably lined with a radial water vessel in the middle and two lateral neural strings; a large longitudinal canal underlying the food groove presumably housed a neural branch. This described anatomy of the brachiolar plates might have been common in the earliest blastozoans (but see Sprinkle 1973). The brachiolar plates B may belong to an unknown eocrinoid species. However, based on the difference in articulation and associated brachioliferous plates without aboral canal as well as brachial plates (see below), several brachioliferous taxa may have been present in the assemblage.

Material.—Two disarticulated plates SMNH Ec 31504 and 31505 from sample 7/70. Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage. The plates are secondarily phosphatised.

Description.—Brachioliferous plates, rounded to polygonal, with a concave interior and convex exterior surface. The exterior convexity leads to a single (Fig. 63J, arrowed) or double (Fig. 63K₁, K₂, arrowed) lateral insertion facet for brachioles. The insertion facets are circular to triangular, with an oblique food groove opening to the plate margin. There is no definitive evidence for an aboral canal. Epispires either absent or more or less developed rounded, without peripheral rim (Fig. 63K₃).

Remarks.—On account of overall similarities, the nomenclature adopted below is based on the terminology generally dedicated to crinoid arm description. It should not be interpreted as a tentative assignation to this taxon, and the plates are described under open nomenclature. Uniserial unbranched ossicles and bifurcating elements can be recognised in the studied material, but their external features differ significantly, so that they may represent different taxa. For this reason, they are described separately in the following section. These plates also exhibit characters known from both blastozoan and crinozoan appendages (Clausen et al. 2009, 2012; Guensburg et al. 2010). The diagnostic features or apomorphies of both groups are debatable, since the plasticity of appendage structure of Cambrian pelmatozoans has been convincingly illustrated (Zamora and Smith 2011).

Material.—Five plates, including SMNH Ec 31506-31509, from sample 7/70. Uppermost Emyaksin Formation, Bol'shaya ya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Arm ossicles (brachials), cylindrical with a circular to slightly oval cross-section, slightly convex latus and a concave oral furrow. They are 0.4–0.6 mm in length and 0.3–0.5 mm in width/height, with a length-to-width ratio of 0.7–1.3. A longitudinal food groove runs along the entire length of the oral surface between relatively high (ca. ⅓ of the ossicle height) lateral walls. It is composed of a central furrow bounded by two relatively low ridges and flanked by a pair of lateral shallow furrows. Proximal and distal articulation surfaces are hoof-shaped and bounded by a distinct and raised peripheral rim, which extends to the edges of the food groove. The rim delimits a central concavity that leads to an oval to crescent-shaped canal 0.05–0.15 mm wide, up to ⅓ of the the articulation surface in width and height, slightly shifted towards the median furrow of the food groove. The canal is more or less filled (with a secondary claustrum-like stereom?) at a distance from the articulation surface. The distal and proximal articulation surfaces range from flat to highly irregularly undulose. Up to four “culminae” and “crenella” are observed on the undulose articulum (Fig. 63F₃). The stereom of the latus is generally obliterated and secondarily filled. The stereom underlying the oral food groove represents a coarse and open, labyrinthine meshwork. The articulation concavity of well-preserved specimens apparently carries aligned structures (Fig. 63H₂), probably representing the galleried stereom (for insertion of ligament bundles).

Material.—Single specimen, SMNH Ec 31510, from sample 7/70. Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—The ossicle is cylindrical and tapers slightly distally, ca. 0.8 mm high, ca. 1 mm long and wide, almost quadrangular in oral view, with a slightly ovate cross section. A thin, shallow, longitudinal food groove runs between low-relief oblique lateral walls along the entire length of the oral surface. The groove branches heterotomously at ca. ⅔ length from the proximal facet, producing distal bifurcation of the ossicle. Proximal and distal branches of the furrow are similar in organization. They are composed of two parallel, thin and shallow furrows (∼50 µm in width and depth) separated by a median, relatively large ridge (∼100 µm wide) with flat to convex upper surface, without transverse channels. The furrows are bounded laterally by oblique, slightly convex walls, which bear rounded notches prolonged to the food groove (Fig. 63I, arrowed). The notches alternate on the two sides of the ossicles. There are four or five notches on the non-bifurcating side, whereas there are two notches below and one above the bifurcation on the opposite side. The two distal facets are confluent adaxially. The larger distal facet is aligned with the proximal facet, whereas the smaller one (∼0.4 mm wide) is inclined at about 45°. Articulation facets are flat and oval, with a large crescent-shaped pit on the aboral part. A faint median depression (median canal?) is visible on a best-preserved, larger distal-facet (Fig. 63I₂, arrowed), but seems to be filled by secondary, labyrinthine dense and coarse stereom.

Remarks.—The uniserial ossicles described herein are the earliest known so far. They differ from each other by the presence or absence of the aboral canal and by the structure of their oral food groove. They may come from different taxa or from different portions (proximal/distal) of the same appendage. A possible interpretation is that these uniserial plates would represent distal/proximal portions of a biserial appendage represented by the brachiolars described above. However, the uniserial and brachiolar plates clearly differ in terms of the organization of the oral food groove and lateral walls. Uniserial brachials are also described from the middle Cambrian of Australia (Clausen et al. 2009) and Laurentia (Clausen and Peel 2012), but their affinity is still debated (see Guensburg et al. 2010). Uniserial plates are also known from Ordovician blastozoans (e.g., Rhipidocystites; Guensburg et al. 2010) and paracrinoids (Parsley and Mintz 1975). However, as in Rhipidocystites, blastozoan uniserial brachioles have the morphology of two fused brachiolar plates, most probably representing biserial arrangement with a “suppressed” biserial suture (still visible in some cases; Sprinkle 1973: fig. 5C). In Rhipidocystites, the uniserial plating is not constant. In Haimacystis (Sumrall et al. 2001), this apparent uniseriality is present only distally on the appendage. Rare solute blastozoans also exhibit uniseriality, probably owing to cryptic aboral suture (Minervaecystis Caster, 1968). In paracrinoids, the primary uniserial arms have a large aboral cavity, which connects with the thecal cavity (e.g., Comarocystites; Parsley and Mintz 1975). The cavity disappears distally and in the “pinnules” (lateral branches of the arms). However, the structure of the feeding groove in paracrinoids is still poorly illustrated, and comparison with the peculiar feeding groove of the axillary plate described herein is not possible.

Recent data based on disarticulated remains (including the newly described Siberian assemblage) demonstrate that the feeding appendages of the earliest echinoderms are actually more diverse in structure than previously assumed and that the uniserial feeding appendage is present since the Cambrian Stage 3-Stage 4 transition. Brachiolar plates described herein also demonstrate that the aboral canal was present in the earliest blastozoans and indicates that the absence of such a structure in some early Ordovician crinoids and numerous blastozoans may be a derived state.

Material.—Five disarticulated plates, SMNH Ec 31511–31515, from sample 7/70, two plates from sample K1-3B, and one, SMNH Ec 31516, from sample 7/64. Emyaksin Formation, Bol'shaya Kuonamka and Malaya Kuonamka rivers; uppermost Judomia Zone, upper Atdabanian Stage (sample 7/64) and Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Plates triangular or trapezoidal, nearly symmetrical in plan view and curved claw-like in the lateral view. They consist of a concave and an opposite convex triangular or trapezoidal surface, perpendicular to a slightly concave and smooth face surrounded by a rounded rim (Fig. 64B₃, C₁, D, arrowed). The two sides converge towards an embayed rounded edge. The embayments are produced by four to six more or less distinct ridges radiating from the face on the concave surface of the plate. These ridges are divided into two sets and are situated almost bilaterally symmetrically on the plate. The median pair of ridges runs almost parallel, whereas the lateral ones diverge. The opposite surface lacks ridges (Fig. 64B₂).

Remarks.—Similar plates were illustrated by Hinz (1987: pl. 15; 11), from the Lower Comley Limestone Callavia Zone of Avalonian Britain (correlated with the upper Atdabanian-lower Botoman Stages (Brasier 1989b; Brasier et al. 1992), by Landing and Bartowski (1996: 755, fig. 8) and Skovsted (2006: figs. 7.7, 7.8), from the Bonnia-Olenellus Zone of Laurentia correlated with Cambrian Stage 4. Two apparently similar plates were also described by Rozhnov et al. (1992: pl. 6: 8, 9) from the uppermost Atdabanian Stage of southeastern Siberian Platform. The plates were tentatively regarded by Landing and Bartowski (1996) as edrioasteroid ambulacral plates, based on comparision with the middle Cambrian Totiglobus Bell and Sprinkle, 1978 and with the late early Cambrian echinoderm plates illustrated by Sprinkle (1973: pl. 25) that were later identified as edrioasteroid plates by Sprinkle (see Landing and Bartowski 1996: 755). Plates described by Hinz (1987), Landing and Bartowski (1996), and herein are different, however, from the plates of the mouth frame of Totiglobus, which carry sets of more regular deeper vertical parallel ridges and notches oriented along the plate peristomal margin. They are more similar to the plates from the basal attachment disc of Totiglobus, which bear parallel to radiating ridges along their inner and outer surfaces (Bell and Sprinkle 1978; Clausen and Peel 2012). Together with the ambulacral plate (see below), the edrioasteroid plates described herein are among the oldest occurrences of edrioasteroids, demonstrating that the latter were already widespread in Cambrian Stage 4 (Clausen and Peel 2012; Zamora et al. 2013).

Material.—Single plate, SMNH Ec 31517, from sample 7/70. Uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Ambulacral flooring plate, hourglass-shaped in plan view, ∼0.5 mm long. The external (upper) side can be divided into a wider and relatively flat face (adradial?; ∼0.4 mm wide) and an elongated, narrower portion (abradial?; ∼0.25 mm wide) carrying two poorly delimited elongated lateral sutural pores/gaps (shared with adjacent ambulacral plates when articulated; Fig. 64G₁, arrowed). The abradial(?) portion bears on its upper surface a wide groove that opens abradially(?), bordered by high lateral rims. The rims form short prongs at the extremity of the abradial(?) portion (Fig. 64G₂, arrowed). The adradial(?) facet appears to consist of two flattened portions: the lower one built of an open coarse labyrinthine to reticulated stereom and the upper one of a denser labyrinthine stereom (Fig. 64G₁, G₂, lower part).

Remarks.—An hourglass-shaped plate was also described and illustrated by Hinz (1987: pl. 15: 18) from the Lower Comley Limestone of Avalonian Britain, Callavia Zone, but preservation precludes undoubted assignment of the plate. Although the fossil described herein is poorly preserved, it is possible to recognise it as one of the oldest occurrences of ambulacral flooring plates in the fossil record (see also discussion in Kouchinsky et al. 2011, Clausen and Peel 2012, and Zamora et al. 2013). Edrioasteroidea Billings, 1858 and Helicoplacoidea Durham and Caster, 1963, both possess ambulacral plates similar to the above described. The oldest representatives of both classes are known from the Cambrian Stage 4 of Laurentia, where helicoplacoids are endemic. Erosion of both extremities of the plates precludes any definite recogniton of the adradial face, but abradial prongs, described from some edrioasteroids, are unknown from helicoplacoids.

Material.—Several hundred thecal plates from sample 7/42.3, including figured SMNH Ec 31496–31499, and several tens of plates from sample 7/45. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage. Occurrence and preservation same as described above for the Atdabanian brachiolar plates.

Tens of secondarily phosphatised disarticulated plates from sample 7/70, including figured SMNH Ec 31518–31526 and 31529–31535; several plates from samples K1-3B (including SMNH Ec 31527) and 7/64; one fragmentary plate from sample 7/55. Emyaksin Formation, Bol'shaya Kuonamka and Malaya Kuonamka rivers; Judomia Zone, Atdabanian Stage (samples 7/55 and 7/64) and Calodiscus -Erbiella Zone, lower Botoman Stage.

Remarks.—It is generally difficult to determine the systematic affinity of disarticulated thecal plates. For example, epispire-bearing thecal plates are described from crinoids, eocrinoids, stylophorans, edrioasteroids, and cinctans. The described assemblage obviously embraces different, but unidentifiable taxa, as demonstrated by the diversity of plate ornamentation and epispire morphology. They are illustrated without detailed description in order to complete the fossil assemblages and facilitate future studies.

The thecal plates are rounded or polygonal in outline, have a plane to concave (presumably internal) and plane to convex (external) surfaces. The plates thin out at the margins, but in some specimens a flattened abutment(?) face occurs. The internal surface is relatively smooth (Fig. 65B, C), whereas the external surface can be ornamented with ridges (Fig. 65A), meshwork (Figs. 62J, K; 65E–G, J; 66A₁), tubercles (Figs. 62L₁, 65D, I₁, 66C₂), or pyramidal and stellate sculpture (Fig. 65H). Epispires are indistinct in the Atdabanian thecal plates (Fig. 62G–L), but can be well developed in the Botoman plates (Fig. 65A–D). Some plates are trapezoidal in outline and have a truncated margin, which bears comb-like structures of the sutural gap, whereas the rest of the margin is smooth (Fig. 66A, C–F). Rounded thecal plates with a similar truncated margin carrying comb-like structures were described by Rozhnov et al. (1992: pls. 5, 6) from the upper Stage 3 of Siberia and attributed to eocrinoids. One plate of an oval outline has a straight truncated margin with four notches (Fig. 66A). Another single plate has a triangular outline and one side convexly curved (Fig. 66B).

Type species: Hyolithes micans Billings, 1871; Bonnia-Olenellus Zone, Dyeran Stage; Troy, New York State, USA.

Material.—Several calcium phosphatic tubes, including SMNH X 4798–4801, from samples 7/19, 7/36.7, and 7/39.5. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Dokidocyathus lenaicus Zone, Tommotian Stage (sample 7/19) and Delgadella anabara Zone, Atdabanian Stage.

Description.—Straight, slightly diverging, a few mm long, fragments of tubes with circular cross-section, 0.2–0.6 mm in diameter (Fig. 67A, C, D, F). The external ornamentation consists of weak concentric ribs and faint longitudinal ridges and furrows undulating between the ribs.

Remarks.—The forms herein are similar to Hyolithellus insolitus Grigor'eva in Voronin et al., 1982 described from the lower Cambrian of Mongolia (see also Esakova and Zhegallo 1996) and southeastern Siberian Platform (Sokolov and Zhuravleva 1983). Other forms illustrated herein under Hyolithellus sp. (Fig. 67B, E, G) lack longitudinal striation. H. insolitus or similar tubes with transversal annulation and longitudinal stiation are also reported from the lower Cambrian of India (Brasier and Singh 1987), South China (Esakova and Zhegallo 1996), Australia (Skovsted and Peel 2011), lower-middle Cambrian of Laurentia (Skovsted and Holmer 2006), and middle—upper Cambrian of Kazakhstan (Esakova and Zhegallo 1996).

The tubes from samples 7/36.7, 7/39.5, and an eroded specimen of Hyolithellus sp. from sample 7/25.6 (Fig. 67E), have numerous circular or oval to rounded sub-rectangular holes, 20–60 µm in diameter, in their wall. These are interpreted to result from bioerosion which occured before deposition into the sediment (see Discussion in Torellella below). According to Skovsted and Holmer (2006), the species may be a junior synonym of H. grandis Missarzhevsky in Rozanov et al., 1969, but the holotype of the latter is much larger and preserved on the rock slab.

Type species: Hyolithes laevigatus Linnarson, 1971; lower Cambrian, Sweden.

Material.—Several calcium phosphatic tubes, including SMNH X 4805–4810, from samples 3/25, 6/6.6, 6/14, 7/25.5, 7/27.5, 7/33, and 7/36.7. Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; correlated with the Dokidocyathus regularis Zone, Tommotian Stage-Delgadella anabara Zone, Atdabanian Stage.

Description.—Undulating and/or curved tubes, diverging at 10–15°, with transversal outline rounded at the initial part (Fig. 67H₂, L) but lenticular at the aperture. The external ornamentation consists of frequent concentric growth lines. A longitudinal continuous narrow furrow (Fig. 67L, M) marks each narrow keel-like side of the tubes.

Remarks.—The fossils are also similar to T. lentiformis (Sysoev, 1960), and the latter may be a junior synonym of T. laevigata (Linnarson, 1871). Longitudinal ornamentation of the narrow sides is not, however, described from the latter two forms. Commonly attributed to Torellella Holm, 1893, the fossils are also very similar to those described from the Qiongzhusian-lower Canglangpuan Stages of China as Sphenothallus Hall, 1847 (Li et al. 2004b). The middle Qiongzhusian Torellella bisulcata Li, 2004 and lower Cambrian T. lentiformis have a narrow sulcus along each of the narrow sides, similar to longitudinal furrows in Torellella cf. T. laevigata herein. A single apical fragment with a flared open attachment structure is also present in the material described (Fig. 67I).

Fragments of Torellella sp. (Fig. 68) demonstrate that the wall, originally phosphatic in composition, consists of multiple layers arranged in a plywood stucture, where adjacent layers have orthogonal orientation of the constituent fibers (Fig. 68D₁; see also Bengtson et al. 1990, Skovsted and Peel 2011). Tube fragments of Torellella from samples 6/66.2, 7/25.6, 7/30, 7/39.5, and 7/60 are eroded and perforated by numerous rounded-squared or rounded-rectangular holes, 20–60 µm in diameter (Fig. 68). In some fragments, the holes incompletely penetrate the wall from the inside of tube (Fig. 68A) and, therefore, are unlikely the result of attacks by a predator from outside. In other specimens, the perforations are combined with eroded outer and inner surfaces (Fig. 68D–G). Some of the holes have rounded broken-off edges (Fig. 68B₂), demonstrating that erosion had taken place after the holes were formed and before deposition into the sediment. These holes expand moderately inwards into the tube wall, and their margins are oriented parallel to the orthogonal direction of fibers in the wall (Fig. 68C). Similar holes in the wall are found in Hyolithellus, but other fossils described herein. They are also observed in hyolithelminths from Series 2 of Laurentia (Christian Skovsted and AK, unpublished observation 2012). Thus, it appears that the perforations have a biological origin (e.g., sponge galleries or holdfasts attachment sites), made before deposition of abandoned tubes into the sediment.

Type species: Anabarites trisulcatus Missarzhevsky in Voronova and Missarzhevsky, 1969; Anabarites trisulcatus Zone, Nemakit-Daldynian Stage, lower Cambrian; mouth of the Kotujkan River, Anabar Uplift, Siberian Platform.

Material.—Two calcium phosphatic internal moulds, SMNH X 4818 and 4819, from sample 7/25.6. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Profallotaspis jakutensis Zone, lower Atdabanian Stage.

Description.—Irregularly curved, slowly expanding internal moulds with tri-fold transverse profiles. Three rounded lobes are separated by narrow and shallow but distinct, grooves. The smaller mould (presumably a younger growth stage) carries transverse growth lines (Fig. 69E).

Remarks.—Anabarites spp. described herein are the first occurrences of anabaritids from Stage 3 in Siberia. These forms often occur within the Terreneuvian Medvezhya and lower Emyaksin Formations of the Anabar Uplift (see Kouchinsky et al. 2009). Anabarites trisulcatus is indicated from the lower part of the section by Val'kov (1975: 12; see also Val'kov 1987: fig. 3). Outside the Siberian Platform anabaritids are reported to range into the Cambrian Series 2, Atdabanian-Botoman equivalent strata, in Mongolia (Esakova and Zhegallo 1996), South and North China (Feng 2005; Li et al. 2007; Steiner et al. 2007), Australia (Bengtson et al. 1990), Baltica (Kouchinsky et al. 2009), and possibly in the Mediterranean region (Elicki 1994).

Stratigraphic and geographic range.—Lower Cambrian worldwide (see Kouchinsky et al. 2009).

Material.—One calcium phosphatic internal mould, SMNH X 4820 (Fig. 69B), from sample 3/53.5; one from sample 6/6.8; one, SMNH X 4821 (Fig. 69C), from sample 6/59.3. Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; correlated with the Dokidocyathus regularis Zone, Tommotian Stage, Delgadella anabara and Judomia zones, Atdabanian Stage.

Description.—Small straight internal moulds with rounded triangular cross-section, shallow faint longitudinal grooves, and thin straight growth lines.

Material.— One specimen, SMNH X 4822, from sample 3/59. Emyaksin Formation, Malaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Internal mould flattened laterally, with two shallow and wide longitudinal depressions and one of the lobes more extended than the other two sides, and curved in one plane. The mould is oval in cross-section, but rounded and more isodiametrical in the initial growth stage.

Material.—Several tens of spicules total, from samples 3/44, 3/49, 3/49.5, 3/50, 6/39.1, 6/41.7, 7/33, 7/34, 7/34.7, 7/34.8, and hundreds, including SMNH Sp 10301–10303 from sample 7/36.7. Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers; correlated with the Delgadella anabara Zone, Atdabanian Stage. One, from sample 6/0.3; Emyaksin Formation, Bol'shaya Kuonamka; correlated with the lower Dokidocyathus regularis Zone, Tommotian Stage.

Description.—Siliceous spicules composed of five rays containing axial canals. One of the rays is much longer, sometimes curves distally (Fig. 70M), and is covered with low, irregularly situated, and longitudinally extended tubercles. The ray can be somewhat swollen at the base (Fig. 70N), but commonly only slightly reduced in diameter distally from its base. Four other rays taper gently at 60–120° to each other in plan view and are inclined at 20–30° away from that plane, perpendicular to the base of the longer ray. The rays are covered with less distinct and smaller tubercles.

Remarks.—The specimens described herein are most similar to those illustrated by Fedorov in Shabanov et al. (1987: pl. 35: 6) and by Vasil'eva (1998: pl. 24: 2) from the Atdabanian Stage, upper Emyaksin Formation, southeastern flanks of the Anabar Uplift and to those reported by Kouchinsky et al. (2011: fig. 40N–Q) from Stage 5 beds of the Malaya Kuonamka River. The tubercles probably represent weathered distally directed barbs similar to those described from some pentacts and hexacts of Cambrian hexactinellids (Rigby 1975; Bengtson 1986b: fig. 8; Bengtson et al. 1990: fig. 15A–F). Similar spicules, but preserved in carbonate surrounded by organic sheaths are reported from the lower Cambrian Forteau Formation of Newfoundland (Harvey 2010).

Material.—Over ten spicules, including SMNH Sp 10304 and 10305, from samples 7/33, 7/34.7, 7/34.8, and 7/36.7. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Siliceous spicules composed of six rays containing axial canals. Four gently tapering rays meet at ca. 90° to each other in one plane, perpendicular to the base of the long ray and to an opposite shorter ray. The longer perpendicular ray tapers slightly from its base and may curve distally. It is covered with longitudinally extended tubercles. The other rays are relatively smooth and straight, but the four of them situated in one plane are covered with less distinct and smaller tubercles.

Material.—Several spicules from samples 3/46.5, 7/33, 7/34.7, and several tens from samples 7/34.8 and 7/36.7, including SMNH Sp 10306. Malaya Kuonamka and Bol'shaya Kuonamka rivers, Emyaksin Formation; Delgadella anabara Zone, Atdabanian Stage.

Description.—Siliceous spicules, 0.8–1 mm in size, composed of five tapering rays with a smooth surface and axial canals. Four of the rays are situated in nearly the same plane at 80–100° between their proximal parts. The fifth ray is almost perpendicular to the other four. Some of the rays curve distally.

Material.—Seven spicules, including SMNH Sp 10307, from samples 7/33 and 7/70. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage (sample 7/33) and Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Siliceous spicules, ca. 0.5 mm in size, composed of six morphologically similar tapering rays with smooth surface and axial canals. The opposite rays are situated respectively along three axes crossed at nearly right angles with each other.

Remarks.—Similar spicules were reported from the middle Cambrian of Siberia (Kouchinsky et al. 2011: fig. 40E–G), lower Cambrian of Laurentia (Skovsted and Holmer 2006: fig. 5G, H), middle—upper Cambrian of Australia (Bengtson 1986b: fig. 9E; Mehl 1998: pl. 1: 1) and South China (Dong and Knoll 1996: fig. 7.1), and upper Cambrian of Argentina (Heredia et al. 1987).

Material.—Thirty spicules, including SMNH Sp 10308–10313, from samples 3/49 and 3/49.5. Emyaksin Formation, Malaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Siliceous spicules, 0.3–0.5 mm in size, consisting of five rays with smooth surface and axial canals. Four of the rays diverge at nearly right angles to each other in one plane, and the fifth ray is relatively perpendicular or somewhat inclined. The surface opposite to the perpendicular ray is convex. The rays commonly have an inflated appearance.

Remarks.—Similar spicules were described by Fedorov (in Shabanov et al. 1987: pl. 34: 14 and pl. 35: 1–4, 7) from the Atdabanian Stage, upper Emyaksin Formation, southeastern flanks of the Anabar Uplift and as Cjulanciella asimmetrica Fedorov in Fedorov and Pereladov, 1987, from the Cambrian Stage 4, Kuonamka Formation, northeastern Siberian Platform (see also Rozanov and Zhuravlev 1992: 228; Vasil'eva 1998: 96; Buslov et al. 2002: 787; Ponomarenko 2005: 54). Another similar spicule attributed to Thoracospongia follispiculata Mehl, 1996 is illustrated by Mehl (1998: pl. 1: 3) from the middle Cambrian of Australia. In some of the spicules, however, the angles of divergence between adjacent rays situated in one plane may vary between 45° and 160° (Fedorov in Shabanov et al. 1987: 131).

Material.—Several spicules, including SMNH Sp 10314–10317, from sample 3/46.5. Emyaksin Formation, Malaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Siliceous spicules composed of six rays. Four of the rays are covered with irregularly scattered tubercles, can be a little inflated at a short distance from the base, taper distally, cross at ca. 90° to each other in the plan view, but inclined at ca. 20° away from that plane. The other two straight smooth tapering rays are situated along one axis perpendicular to the latter plane. One of those rays is usually reduced and the four acanthose rays are inclined away from it. A single aberrant spicule has the fifth acanthose ray intercalated among the other four (Fig. 70W).

Material.—Three spicules, including SMNH Sp 10318 and 10319, from sample 6/0.3. Emyaksin Formation, Bol'shaya Kuonamka River; lower Dokidocyathus regularis Zone, Tommotian Stage.

Description.—Rare siliceous spicules composed of six rays situated at right angles to each other. One or two of the rays are reduced, bulbous and covered with acute tubercles. The other rays are smooth, longer and gradually taper distally.

Material.—Several hundred pentactines of the first type, including SMNH Sp 10320 and 10321 (see below) and several tens of pentactines of the second type, including SMNH Sp 10322 and 10323, are available from sample 6/0.3; correlated with the lower Dokidocyathus regularis Zone, Tommotian Stage. Pentactines similar to the second type derive from samples 3/46.5, 3/49.5, 7/33, 7/34.7, and 7/36.7, whereas several hexactines with slim rays, including SMNH Sp 10324–10326, are found in samples 3/46.5, 3/49, 7/33, 7/34.7, and 7/36.7; correlated with the Delgadella anabara Zone, Atdabanian Stage. Emyaksin Formation, Malaya Kuonamka and Bol'shaya Kuonamka rivers.

Description.—Pentactines of the first type (Fig. 70A) have slim rays, four of which are directed away from the fifth ray, at ca. 60° to the plane perpendicular to that ray. Pentactines of the second type (Fig. 70D) have slim rays, with four of the rays directed away from the fifth ray, at ca. 10–15° to the plane perpendicular to that ray.

Remarks.—One spicule, otherwise a pentactine of the first type, shows the sixth ray situated on the same axis as the fifth ray and opposite to the latter (Fig. 70B). A single polyactine from sample 6/0.3 (Fig. 70C), probably represents an aberrant pentactine with three additional rays. Hexactines with slim long rays usually co-occur with pentactines of the second type.

Material.—A single spicule, SMNH Sp 10327, from sample 7/37.8. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Siliceous spicule composed of six rays. Four of them radiate from the base at ca. 90° to each other and strongly curve in the same direction. Two straight rays opposite to each other are situated perpendicular to the base. One is wider than the other, conical and bears eight longitudinal ridges. The ridges are also present on the lateral rays and make them angular in cross-section.

Material.—Several spicules, including SMNH Sp 10328–10333, from samples 7/34.7, 7/34.8, and 7/36.7. Emyaksin Formation, Bol'shaya Kuonamka River; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Remarks.—Simple spindle-like, monaxonous siliceous spicules with a bulbous termination at one end and the opposite end pointed (Fig. 71B, C?, and G). In some specimens, the pointed end carries barbs (Fig. 71A) or is blunt, with five slightly curved pointed rays (Fig. 71F). The spicules are most similar to Tanchaiella sp. illustarted by Vasiljeva (1998: pl. 25: 1, 4, 5) from the Emyaksin Formation of the Malaya Kuonamka River, Atdabanian Stage and tylostyles from Cambrian Stage 3 Eoredlichia-Wutingaspis Zone of South China (Zhang and Pratt 1994: fig. 3C, D). Ponomarenko (2005: 48) noted similarity between Tanchaiella and “barbed pentactines” (pl. 14: 5, 6 therein) from the Botoman Stage of middle Lena River and reported that Tanchaiella is nomen nudum being provided by Vasiljeva (1998) without diagnosis and type species. Simple monaxonous spicules are also similar to those of uncertain affinity from the Silurian of Wisconsin (Watkins and Coorough 1997: fig. 4). One of the specimens (Fig. 71D) represents a broken off termination with six recurved rays similar to that in Zhang and Pratt (1994: fig. 3B, C) or in Nabaviella-like spicules known from the Cambrian strata worldwide (Mostler and Mosleh-Yazdi 1976; Bengtson 1986b; Bengtson et al. 1990; Zhang and Pratt 1994; Dong and Knoll 1996; Ponomarenko 2005 and references therein).

Material.—Hexaradiate spicules with a bunch of partial rays on one side (seven, from sample 7/70, including SMNH Sp 10334, 10336, and 10337, and eleven, from sample K1-3B, including SMNH Sp 10340) and one hexaradiate with a massive perpendicular ray, SMNH Sp 10344, from sample K1-3B; tetraradiate spicules with a bunch of partial rays on one side (one, from sample 7/70; eight, from sample K1-3B, including five ones with a single massive perpendicular ray on the same side, SMNH Sp 10338 and 10343), and polyactinous spicules with short rays with or without one elongated ray (SMNH Sp 10335 and 10339, from samples 7/70 and K1-3B). Several tetraradiate hexactines, including SMNH Sp 10342, from samples 7/70 and K1-3B are also considered here as probably related. Uppermost Emyaksin Formation, Bol'shaya Kuonamka and Malaya Kuonamka rivers; Calodiscus-Erbiella Zone, lower Botoman Stage.

Remarks.—Secondarily phosphatised spicules, presumably of original calcareous composition, with four, six, or more rays. The four-rayed spicules consist of four slender smooth rays with rounded ends diverging at ca. 90° from the base and at 20–30° to the basal plane (Fig. 72E, J). The central part of the convex side of the spicules carries several short rays or nodes and, in some, a single longer ray (Fig. 72J). The sixrayed spicules occur more often and consist of six slender smooth rays diverging at ca. 60° from the base and at 10–20° from the basal plane (Fig. 72A, C, D, G, H). The central part of the convex side of the spicule carries several nodes. The latter type is similar to the spicules of Eiffelia Walcott, 1920 (see e.g., Bengtson et al. 1990) known to appear globally in the Cambrian Stage 3 (Kozur et al. 2008). In the latter spicules nodes scatteded on the convex side are not as prominent as in the form reported herein, although tetractines are also described from Eiffelia (Botting and Butterfield 2005). Polyactinous (Fig. 72B, F) and hexactinous (Fig. 72I) spicules co-occur in the assemblage discussed herein. Polyactines consist of several short rays diverging from the same center, but one of the rays can be a long rhabd (Fig. 72B₁). Eiffelia has been attributed to stem-group sponges in which the body wall is formed of a single layer of spicules with internal canals and includes smaller tetraradiate spicules (some of them hexactines) in spaces among larger hexaradiate spicules without perpendicular rays (Botting and Butterfield 2005).

The spicules also resemble those of Conwaymorrisispongia spp. from the Abadiella huoi Zone of Stage 3 of South Australia (Kozur et al. 2008), but the latter are characterised by eight massive rays instead of the six in our material. In accordance with the morphological terminology used by Kozur et al. (2008), four or six main rays radiating from the base are paratangential, and nodes in the central part represent branching of the distal ray into partial rays, whereas a proximal ray is not developed. In another spicule, the proximal ray is very massive and six massive paratangential rays radiate from its base (Fig. 72K). In polyactine spicules, all types of rays can be reduced, but some of the spicules carry a longer proximal ray and four short paratangential rays in a perpendicular plane. Rare phosphatised regular hexacts (Fig. 72I) similar in the morphology of individual rays co-occur with the spicules discussed above and may derive from the same sponges, although their hexactinellid affinity cannot be excluded.

Type species: Aetholicopalla adnata Conway Morris in Bengtson et al., 1990; Abadiella huoi Zone, lower Cambrian; Curramulka, Yorke Peninsula, Australia.

Material.—One specimen, SMNH X 4823, from sample 7/70; uppermost Emyaksin Formation, Bol'shaya Kuonamka River; Calodiscus-Erbiella Zone, lower Botoman Stage.

Description.—Phosphatised globule, ca. 1 mm in diameter. The surface is semi-spherical, densely covered with bumps, 20–40 µm wide, having open or broken-off tips. The other side is smooth and slightly concave, with rare bumps and a curved channel-like cast in the middle (Fig. 73A₂).

Remarks.—While otherwise similar to Aetholicopalla adnata, the fossil does not display a double-wall or tubular pillars. The fossil also resembles single-walled Archaeooides in having a regular pustulose surface, but has a flattened portion typical of Aetholicopalla adnata and presumably related to an encrusting habit. Globules frequently occur in the samples, but their preservation prevents identification. Globules, 0.5–1 mm in size, with sphaerical or distorted wall often bearing tubercles occur in samples K7-6.3B, 6/16, 3/8, 3/11, 3/16.5, 3/23, 3/34, 3/40, 3/53.5, from the Emyaksin Formation of the Malaya Kuonamka and Bol'shaya Kuonamka rivers, ranging from the Dokidocyathus regularis Zone, Tommotian Stage to the Delgadella anabara Zone, Atdabanian Stage. Smaller globules of unknown affinity, 0.15–0.3 mm in size, of which deformation often produces a tri-lobate shape (Fig. 73B), occur in samples 3/29, 3/49, 3/50, 3/62.5, 6/59.3, K7-6.3B, 7/27.5, and 7/34.7 from the Tommotian and Atdabanian strata of the same localities.

Stratigraphic and geographic range.—Aetholicopalla adnata is also described from Cambrian Series 2 of the Montagne Noire, France (Kerber 1988) and the Görlitz Syncline, eastern Germany (Elicki and Schneider 1992; Elicki 1998), Australia (Bengtson et al. 1990; Gravestock et al. 2001), Antarctica (Wrona 2004), and from the Series 2–3 boundary interval of Sardinia (Elicki and Pillola 2004).