Microdictyon, Bengtson, Matthews, and Missarzhevsky, 1986

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 : 481

Microdictyon cf. M. rhomboidale Bengtson, Matthews, and Missarzhevsky, 1986

Fig. 55 .

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. 55A 2 ) has semicircular upper and lower margins and protrudes into distinct angulations laterally. The upper (as illustrated in Fig. 55A 2 ) margin of the sclerite forms a flattened limb covered with ca. 5 μm tubercles oriented to the right in Fig. 55A 5 . The opposite margin has a weakly expressed reentrant ( Fig. 55A 2 ). 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. 55A 3 ). 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. 55A 2 , B 3 ). 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.

Figures

Fig. 55. Lobopodian Microdictyon cf. M. rhomboidale Bengtson, Matthews, and Missarzhevsky, 1986 from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia compared to the paratype of M. rhomboidale from lower Cambrian of the Altai-Sayan Folded Area. A. Microdictyon cf. M. rhomboidale Bengtson, Matthews, and Missarzhevsky, 1986; SMNH X 4780, sample 7/54; A 1, A 2, general views of sclerite; →

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. 5. Helcionellid mollusc Parailsanella sp. 1 internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/36.7. A. SMNH Mo 160415; lateral (A 1), sub-apical (A 2), and upper (A 3) views. B. SMNH Mo 167655; lateral (B 1), sub-apical (B 2), and upper (B 3) views. C. SMNH Mo 167656; lateral (C 1), upper (C 2), and sub-apical (C 3) views. D. SMNH Mo 160416; lateral (D 1), oblique sub-apical D ), and upper (D ) views. Scale bar 600 μm.

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.