Fordilla, Jell, 1980

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 : 440-441

Fordilla cf. F. sibirica Krasilova, 1977

Fig. 21 .

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. 21A 1 ). Dentition is represented by casts of a single weakly impressed tooth and corresponding socket on each valve ( Fig. 21A 2 , A 3 ). Ligament area elongat- ed, straight, opisthodetic, occupying more than half of the dorsal margin. In lateral view, the angle between dorsal and posterior margins is ~135° ( Fig. 21A 1 ). 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. 21A 4 ). 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. 21A 4 ). 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).

Figures

Fig. 21. Internal moulds of bivalved molluscs from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/70. A. Fordilla cf. F. sibirica Krasilova, 1977, SMNH Mo 167690; right lateral (A1), dorsal (A2), and oblique dorsal (A3) views; A4, close-up of A1. B. Undetermined fordillid bivalve, SMNH Mo 167691, left lateral view. C. Fragment of a larger specimen, interpreted herein as Fordilla cf. F. sibirica, SMNH Mo 167692, lateral view. Scale bar 1000 μm, except A4, 100 μm.

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

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.