Microdictyon sp.

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 : 481-483

Microdictyon sp.

Fig. 56 .

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. 56A 2 , A 3 , B 4 ). 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. 56A 2 , A 3 ). 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

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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).

Figures

Fig. 56. Lobopodian Microdictyon sp. from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/34.7. A. Outer surface of the fragment, SMNH X 4782; A 1, close-up of A 2; A 2, view on the outer surface of sclerite; A 3, oblique view on the lower margin of sclerite. B. SMNH X 4783; B , B , general views of the fragment; B , inner surface; B , close-up of the edge of sclerite. Scale bar: 200 μm, except A , B , 60 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.

Fig. 4. Helcionellid mollusc Bemella? sp. internal moulds from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River,Anabar Uplift, Siberia; sample 7/36.7. A. SMNH Mo 167650; lateral (A1), apertural (A2), and sub-apical (A3) views. B. SMNH Mo 167651; lateral (B1) and upper (B2) views. C. SMNH Mo 167652; apertural (C1), lateral (C2), and sub-apical (C3) views. D. SMNH Mo 167653; D1, close-up of apertural margin in D3; upper (D2) and lateral (D3) views. Scale bar 1200 μm, except D1, 300 μm.