Cambroclavus Mambetov, 1979

Kouchinsky, Artem, Alexander, Ruaridh, Bengtson, Stefan, Bowyer, Fred, Clausen, Sébastien, Holmer, Lars E., Kolesnikov, Kirill A., Korovnikov, Igor V., Pavlov, Vladimir, Skovsted, Christian B., Ushatinskaya, Galina, Wood, Rachel & Zhuravlev, Andrey Y., 2022, Early-middle Cambrian stratigraphy and faunas from northern Siberia, Acta Palaeontologica Polonica 67 (2), pp. 341-464 : 390-394

Genus Cambroclavus Mambetov in Mambetov and Repina, 1979

Type species: Cambroclavus antis Mambetov in Mambetov and Repina, 1979; Rhombocorniculum cancellatum Zone , upper part of Cambri- an Stage 3; Beshtash Formation, Beshtash Creek, Talass Alatau Range, Kazakhstan.

Cambroclavus sp.

Fig. 34A–J .

Material.—Single phosphatised sclerite from sample 19/5.5 and 15 phosphatic internal moulds and partial phosphatic casts of the external surface of sclerites, including figured SMNH X11060–11069, from samples 19/10.25, 19/11.75, 20/1B, Erkeket Formation, Khorbusuonka River, Siberia, Russia. Lower Botoman stage (correlated with the lower part of Cambrian Stage 4).

Description.—Isolated sclerites, 0.3–0.7 mm in length, representing elongated basal shields with growth increments on the surface and imperfect bilateral symmetry. They are slightly sigmoidally curved in the plan view and flattened. Lateral margins subparallel, occasionally with a variably developed constriction. Dorsal side distinguished by a rounded elevated facet situated on its presumably anterior part. Facet delimited by a circular rim and anteriorly inclined at ca. 45º towards the longitudinal axis of sclerite. The facet representing attachment site of a spine carries a single almost circular hole, 30–60 μm in diameter. Posterior part of the shield elongated with blunt termination. A broad median ridge is present on the dorsal surface in some specimens. Ventral (lower) surface of the shield smooth. Wall of the basal shield composed of aciculate elements radiating from attachment site of the spine, perpendicularly to growth increments of the shield. The aciculate elements are longitudinally oriented in the spine.

Comparison.—Given morphological variation within the same fossil assemblage of sclerites, it is premature to describe a new species with the material currently available, until more specimens representative of the entire morphological variation will be accumulated. It seems the Siberian forms are different from Cambroclavus absonus Conway Morris in Bengtson et al., 1990, in the absence of flaring of the posterior part of the basal shield which has in C. absonus a distinct hourglass-like shape. Prong-like extensions available in C. bicornis ( Qian and Xiao, 1984) have not been found either. The type species, Cambroclavus antis exhibit prominent longitudinal ridges on the upper and lower surfaces of the basal shield ( Mambetov and Repina 1979). It was also noted that inflated sclerites can be minor components of Cambroclavus scleritomes (Duan 1984; Qian and Xiao 1984; Wotte 2009; Na and Li 2011).

Remarks.—Cambroclavids are documented herein from the Siberian Platform. Cambroclavid sclerites were presumably calcareous with a fibrous composition of the wall, closely and regularly spaced on the available fragments of their articulated arrays (Bengtson et al. 1990; Conway Morris and Chen 1991).

The nature of cambroclavid sclerite-bearing animals is yet poorly known. A possible cambroclavid scleritome described by Conway Morris and Peel (2010) from Sirius Passet in Greenland represents a compression of an externally segmented worm-like body. The latter is mainly covered with rows of cambroclavid-type sclerites and presumably zhijinid-type sclerites on its only preserved (anterior?) end. The transition between these two zones is abrupt, which agrees with presence of distinct and often co-occurring cambroclavid and zhinzhinid morphs of sclerites in the small shelly fossil assemblages.

Right- and left-handed sclerites cannot be distinguished but Qian et al. 2000), which rather agrees with a supposed worm-like body rounded in cross-section, rather than with a flattened slug-like animal with externally well-defined left and right sides. Laterally alternating longitudinal rows of diagenetically phosphatised imbricated sclerites are rarely preserved articulated, as reported from Deltaclavus Conway Morris and Chen, 1991 (text-figs 8, 9, 11 therein). Similarly preserved fossils reported by Brock and Cooper (1993: fig. 11) and Zhang et al. (2021) may represent folded parts of cambroclavid scleritomes consisting of Deltaclavus -like sclerites lacking spines.

Genus Zhijinites Qian, 1978

Type species: Zhijinites longistriatus Qian, 1978 ; Meishucunian stage; Yunnan, Sichuan, and Guizhou provinces, South China .

Zhijinites clavus (Mambetov in Mambetov and Repina, 1979)

Fig. 34K–T .

1979 Cambroclavus clavus Mambetov sp. nov.: Mambetov and Repina 1979: 122–123, pl. 13: 3, 5, 7–9.

1981 Cambroclavus clavus Mambetov in Mambetov and Repina, 1979; Missarzhevsky and Mambetov 1981: 71, pl. 9: 1, 2, 8.

Material.—Over 20 phosphatic internal moulds, phosphatised sclerites and their partial phosphatic casts, including figured SMNH X11070–11079, from samples 19/10.25, 19/11.75, 20/1B, 20/4B. Erkeket Formation, Khorbusuonka River, Siberia, Russia. Botoman stage (correlated with the Cambrian Stage 4).

Description.—Disarticulated nail-shaped sclerites consisting of a circular or oval tablet-like basal shield and relatively thick and long shaft, straight or slightly curved in one plane. The shaft is circular in cross-section. Basal shield with a slightly concave upper (dorsal) surface bearing an articulation facet for the shaft. The latter has a circular attachment site adpressed into the upper surface of basal shield. Walls relatively thick delimiting hollow space inside the spine. Two concentric rims delimit inner and outer surfaces of a relatively thick wall that often preserves acicular units. The units are elongated longitudinally, subparallel to the surface of the shaft. The latter can reach at least 1.6 mm in length in incompletely preserved specimens. The shaft is inclined at ca. 80º towards the upper surface of the basal shield. Its tip is not preserved. The external surface was probably longitudinally striated or smooth. Articulation facet in the anterior part occupies more than half of the length of the basal shield. Its posterior part is rounded and smooth. The lower ventral) surface of the shield is smooth.

Remarks.—Similar to Parazhijinites guizhouensis Qian and Yin, 1984 , and material illustrated by Conway Morris et al. 1997: 179, pl. 3) as P. cf. guizhouensis , but the latter has a radial or nodular ornamentation, angle of inclination of the shaft to the basal disk from 55° (where the base is broader) to 90° (in association with a reduced base). There are neither transversally compressed spines in our material, nor longitudinal ribbing, nor a median ridge.

Stratigraphic and geographic range.— Rhombocorniculum cancellatum and Microcornus parvulus zones, Kazakhstan and lower Botoman stage (correlated with the Cambrian Stage 4), Siberia, Russia.

Phylum and class uncertain

Order Chancelloriida Walcott, 1920

Family Chancelloriidae Walcott, 1920

Genus Stellispinella Vasiljeva in Vasiljeva and Sayutina, 1993

Type species: Chancelloria lenaica Zhuravleva and Korde, 1955 ; Tommotian Stage ; Siberian Platform, Russia .

Stellispinella lenaica ( Zhuravleva and Korde, 1955) Fig. 35 .

1955 Chancelloria lenaica Zhuravleva and Korde sp. nov.; Zhuravleva and Korde 1955: 476 – 477, text-fig. 1и–у (Russian).

1955 Chancelloria aldanica Zhuravleva and Korde sp. nov.; Zhuravleva and Korde 1955: 476, text-fig. 1ф–ю (Russian).

1985 Stellaria lenaica ( Zhuravleva and Korde, 1955) ; Vasiljeva 1985: 121, pl. 44: 7, 8.

1985 Stellaria aldanica ( Zhuravleva and Korde, 1955) : Vasiljeva 1985: 121–122, text-fig. 2, pl. 44: 4, 5.

1989 Ginospina araniformis Missarzhevsky sp. nov.: Missarzhevsky 1989: 205–206, pl. 21: 2 and pl. 22: 10, 13.

1998 Stellispinella lenaica ( Zhuravleva and Korde, 1955) : Vasiljeva 1998: 100, pl. 19: 10, 11, 13.

1998 Stellispinella aldanica ( Zhuravleva and Korde, 1955) : Vasiljeva 1998: 100–101, pl. 19: 3, 8, 12, 14.

Material.—Over a hundred phosphatised articulated sclerites, their fragments and phosphatic coatings, including figured SMNH X11081 – 11093, from samples 11/2B, 11/16.05,11/16.1, basal Erkeket Formation, Khorbusuonka River, and samples 21/21 and 21/22, Tyuser Formation, Lena River. Siberia, Russia; Dokidocyathus regularis Zone, Tommotian stage (correlated with the upper part of Cambrian Stage 2).

Description.—Articulated sclerites consist of 9–12 lateral rays curved up to 30º away from the plane of the basal facet and a central disk, which is a subdued central ray. Surface covered with scattered granules or short rods, ca. 10 µm in diameter. Lateral rays taper from massive bases. Base of the central disk is up to twice larger in diameter than bases of lateral rays. Basal facet of each ray flattened, with a central circular foramen.

Remarks.—Central disk present in this species is not known from any other species and genera of chancelloriids (see discussion on synonymies below). The type species of Stellispinella was not well-defined or described by Zhuravleva and Korde (1955). Chancelloria lenaica was redescribed by Vasiljeva (1985), who defined its neotype and selected it as a type species of a new genus Stellaria Vasiljeva, 1985 . The latter proved to be preoccupied and renamed Stellispinella Vasiljeva in Vasiljeva and Sayutina, 1993. The main distinction between two available and simultaneously described species, Chancelloria lenaica Zhuravleva and Korde, 1955 , and Chancelloria aldanica Zhuravleva and Korde, 1955 , is that their articulated sclerites are, respectively, rounded in the plan view with 10–12 lateral rays versus ovaliform with 7–10 lateral rays ( Vasiljeva 1985). Otherwise similar in morphology, size and granular ornamentation, both forms co-occur in the material from the Tommotian stage of the Siberian Platform and can be attributed herein to the same species, Stellispinella lenaica Zhuravleva and Korde, 1955 ) as well as Ginospina araniformis Missarzhevsky, 1989 (see Vasiljeva 1998).

Several other types of co-occurring sclerites without central discs may represent parts of the same scleritome. Those consist of 5 to 8 lateral rays connected with each other without centrally differentiated unit ( Fig. 35E, I, J ). Frequently occurring disarticulated and partially articulated sclerites of those types attributed herein to Stellispinella lenaica are available from the basal beds of the Erkeket and the middle Tyuser formations.

Such other sclerites, which may also be parts of the same scleritome are known to co-occur with Stellispinella lenaica in the Ulakhan-Ald’yarkhay section are represented by Diplospinella zintchenkoi Vasiljeva in Vasiljeva and Sayutina, 1988. The latter is the type and only species of Diplospinella Vasiljeva in Vasiljeva and Sayutina, 1988, with its younger synonym, Sissospina irregularis Missarzhevsky, 1989 . There are two such sclerites from the type locality, from sample 21/21 ( Fig. 35L, M ). They have straight basal facet, 1–2 relatively massive long gently curved rays and 3–5 intermediate rays subperpendicularly situated with respect to the basal facet of sclerite. Much smaller and shorter rays are attached to the base of sclerite. These forms are known from the Tommotian stage of the Siberian Platform.

Stratigraphic and geographic range.—Tommotian stage correlated with the upper part of Cambrian Stage 2), Siberia, Russia.

Genus Chancelloria Walcott, 1920

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

Figures

Fig.34.Sclerites of cambroclavids from the lower Cambrian Erkeket Formation,Khorbusuonka River, Siberia,Russia; samples 19/5.5 (I), 19/10.25 (A–E, J), 19/11.75 (K–T), and 20/1B (F–H). A–J. Cambroclavus sp., SMNH X11060–11069, respectively. A1, A2, B2, C2, E1, G1, I1, J2, view on the upper surface of sclerites; B1, C1, E2, G2, I2, J1, lateral view. K–T. Zhijinites clavus (Mambetov in Mambetov and Repina, 1979), SMNH X11070–11079, respectively. K1, view on the upper surface of sclerite; K2, lateral view. Scale bar: 500 µm (Q), 250 µm (A2, B–H, J–L, N, O, Q–T), 125 µm (A1, I, M, P).

Fig.35.Sclerites of chancelloriids fromthe lower CambrianErkeket Formation, Khorbusuonka River (F–I) andTyuser Formation,lower reaches of the Lena River (A–E, J–L), Siberia, Russia; samples 11/16.1 (F), 11/2B (G), 11/16.05 (H, I), and 21/21 (A–E, J–M).A–D, F–H, K. Stellispinella lenaica (Zhuravleva and Korde, 1955), sclerites with a central disk, SMNH X11081–11088, respectively.A1, plain view;A2, oblique lateral view. E, I, J. Stellispinella lenaica (Zhuravleva and Korde, 1955), sclerites without central disk, SMNH X11089–11091, respectively. I1, lateral view; I2, plain view. L, M. Diplospinella zintchenkoi Vasiljeva in Vasiljeva and Sayutina, 1988, SMNH X11092 and X11093, respectively. L1, L2, lateral view. Scale bar 500 µm.