Rhombocorniculum insolutum Missarzhevsky

Kouchinsky, Artem, Bengtson, Stefan, Clausen, Sébastien & Vendrasco, Michael J., 2015, An early Cambrian fauna of skeletal fossils from the Emyaksin Formation, northern Siberia, Acta Palaeontologica Polonica 60 (2), pp. 421-512 : 471-476

Rhombocorniculum insolutum Missarzhevsky in Missarzhevsky and Mambetov, 1981

Figs. 49–52.

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, G 1, H 1 ), 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. 51I 2 , 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. 49B 2 ). 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. 51G 3 ), 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 showing fibrous composition of the wall; B 3, close-up of B 4; B 4, surface of the lumen of sclerite with fragments of transversal ridges, probably representing growth stages; B 5, view showing ornamentation of the outer and inner surfaces of the wall. C. SMNH X 4726; lateral view. D. Holotype, GIN 4296/31, sample 897/1; D 1, overall view of the broken specimen; D 2, view on the basal part of sclerite with a rounded attachment structure. Scale bar:A 1, B 1, C, D, 300 μm; A

2

, B

4

, B

5

, 120 μm, A

3

, 60 μm, B

2

, B

3

, 30 μm.

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 Rombocornicuthe tubules of the proximal end of sclerite. H. SMNH X 4737; H 2, close-up of the proximal end of sclerite in H 1. I. Broad sclerite, SMNH X 4740, sample 7/25.6; I 1, concave (ventral?) surface of sclerite; I 2, proximal end of sclerite with lumen. J. Broad sclerite, view on the convex (dorsal?) side, SMNH X 4738; sample 7/27.5. Scale bar: 300 μm, except G 2, H 2, 60 μm, G 3 –G 5, 15 μm.

lum 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 δ 13 C 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).

Figures

Fig. 50. Problematicum Rhombocorniculum insolutum Missarzhevsky in Missarzhevsky and Mambetov, 1981 from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/19. A. Sclerite with twinned apical parts, SMNH X 4727; A1, oblique lateral view; A2, close-up of A1; A3, close-up of basal end showing microstructure; A4–A6, lateral views; A7, oblique apical view. B. SMNH X 4728; lateral view. C. SMNH X 4729; C1, general view; C2, view on the transversally compressed sclerite. D. SMNH X 4730; D1 and D2, lateral views. Scale bar: A1, A2, A4, A5, A7, 120 μm; A3, 30 μm; A6, B–D, 300 μm.

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

Fig. 52. Problematicum Rhombocorniculum insolutum Missarzhevsky in Missarzhevsky and Mambetov, 1981 from lower Cambrian Emyaksin Formation, Malaya Kuonamka River, Anabar Uplift, Siberia; SMNH X 4741; sample 3/42. A. View on the convex (dorsal?) side. B. Lateral view. C. View on the proximal end with broad lumen. D. Apical end with tubular microstructure and central lumen. E. External ornamentation. F. Close-up of E, longitudinal fibres. Scale bar: A–C, 300 μm; D, 30 μm; E, 60 μm; F, 15 μm.

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

Fig. 3. Helcionellid mollusc Yochelcionella cf. Y. greenlandica Atkins and Peel, 2004 internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/36.7. A. SMNH Mo 160417; lateral (A 1, A5), sub-apical (A 2), upper (A 3), and supra-apical (A 4) views; A 6, polygonal texture on the surface of snorkel mould. B. SMNH Mo 160418; lateral (B 1), sub-apical (B 2), and upper (B 3) views; B 4, enlargement of B 2, with a shallow groove between the snorkel and aperture; B 5, apical part of the mould with tubercles; B 6, close-up of B 5. C. SMNH Mo 167648; lateral (C 1), sub-apical (C ), and upper (C ) views. D. SMNH Mo 167649; lateral view. Scale bar 600 μm, except A , 60 μm; B , 150 μm; B , 300 μm; B , 120 μm.