Fordilla Barrande, 1881

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 : 379-383

Genus Fordilla Barrande, 1881

Type species: Fordilla troyensis Barrande, 1881 ; Upper Elliptocephala asaphoides assemblage, upper Stage 3 or 4; Middle Granville Formation, continental slope debris flow in Taconian allochthon, eastern New York State, USA.

E, F. Obtusoconus cf. mirabilis Vasiljeva, 1990 , internal moulds, SMNH Mo194705 and 194706, respectively. G, H. Parailsanella sp. , internal moulds, SMNH Mo194707 and Mo194708, respectively. I. Protowenella flemingi Runnegar and Jell, 1976 , internal mould, SMNH Mo194712. M, N. Bemella jacutica (Missarzhevsky in Rozanov and Missarzhevsky, 1966), internal moulds, SMNH Mo194703 and 194704, respectively. J, K. Helcionellidae indet., internal moulds, SMNH Mo194715 and 194716, respectively. K 1, subapical view; K 2, upper view. L, O, P. Horsegullia sp. , internal moulds, SMNH Mo194709–194711. L 1, subapical; L 2, O 1, lateral; L 3, upper; and O 2, oblique apertural views. Scale bar: 1 mm (K 2), 500 µm (A–C, E–H, J, K 1, L –P), and 250 µm (D, I).

Fordilla sibirica Krasilova, 1977 Figs. 26 , 27A–C .

Material.—Several calcium phosphatic internal moulds, including figured SMNH Mo167596 ( Fig. 26B ), Mo194719, 194720, 194723, and 194724 from samples 19/4.25, 19/11.75, 19/12.75, 19/40, 20/1B, from the Erkeket Formation, Khorbusuonka River, Botoman–Toyonian stages (correlated with the Cambrian Stage 4). Several dozens of calcium phosphatic internal moulds including figured SMNH Mo194721 and 194722, from samples 21/46.1 and 21/54, Tyuser Formation, Lena River, Delgadella anabara – Nevadella Zone, Atdabanian stage (correlated with the Cambrian Stage 3). Siberia, Russia.

Remarks.—Shell ca. 2.0 mm in length, equivalved. Valves suboval with anteriorly shifted umbones. Dentition represented by casts of a single weakly impressed tooth and corresponding socket on each valve. Ligament area straight, opisthodetic, along more than half of the dorsal margin. Anterior pallial muscles are evidenced by five nodular scars on the left and right sides of an internal mould as well as a larger anterior adductor is preserved ( Fig. 27A : arrowed). Muscular scars are similar to those reported from Fordilla sibirica by Krasilova (1977, 1987). According to Elicki and Gürsu (2009), Fordilla can be distinguished from Pojetaia Jell, 1980 , in consistently larger size and having the umbo shifted towards the anterior margin. For that reason, the internal mould ( SMNH Mo167596 from sample 19/12.75, Fig. 26B ) described by Vendrasco et al. (2011a: pl. 1: 1, 3–6) as Pojetaia runnegari is reinterpreted herein as Fordilla sibirica , in a way similar to other specimens from the same sample. Microstructures of Pojetaia and Fordilla differ therefore more significantly than suggested by Vendrasco et al. (2011a: 838). Fordilla ( Fig. 26A, B ) displays a staircase-like texture throughout the entire surface, in contrast with Pojetaia with a more developed polygonal aspect of the texture. The texture on the available moulds of the left and right valves has the same clockwise imbricating pattern. Such a consistent arrangement of microstructural elements with respect to antero-posterior direction of the mollusc is presumably similarly expressed in the lineage from Anabarella via Watsonella to Fordilla (see discussion by Vendrasco et al. 2011a), and Watsonella can consequently be interpreted as belonging to the stem group Bivalvia (not Rostroconchia, see also Peel 2021c).

Stratigraphic and geographic range. —Cambrian Series 2, Siberian Platform.

Genus Pojetaia Jell, 1980

Type species: Pojetaia runnegari Jell, 1980 ; Cambrian Stage 3, Parara Limestone ; South Australia .

Pojetaia dentifera Kouchinsky, Bengtson, Clausen, Gubanov, Malinky, and Peel, 2011

Fig. 27D–F .

Material.—Five phosphatised shell fragments, including figured SMNH Mo194725–194727, from sample 22/50, Sekten Formation, Ulakhan-Ald’yarkhay, Lena River, Siberia, Russia. Upper Toyonian stage (correlated with the Cambrian Stage 4).

Remarks.—Shell fragments with four peg-like hinge teeth. Dentition is identical to that in Pojetaia dentifera described from the Drumian Stage of Siberia by Kouchinsky et al. (2011: fig. 3), consisting of 3–5 peg-like teeth decreasing in size anteriorly. A single internal mould of a fordillid bivalve ( Fig. 27F ) elliptical in outline with centrally placed umbo co-occurs and probably represents the same species, although its dentition consisting of at least two teeth is incompletely preserved.

Stratigraphic and geographic range.— Upper Toyonian and Amgan stages (correlated with the upper Stage 4 to lower Drumian Stage), Siberian Platform.

Class uncertain

Order Pelagiellida Mackinnon, 1985

Family Aldanellidae Linsley and Kier, 1984

Genus Aldanella Vostokova, 1962

Type species: Pleurotomaria (Raphistoma) attleborensis Shaler and Foerste, 1888 ; Lower Cambrian ; Hoppin Hill, Massachusetts, USA .

Aldanella sibirica Parkhaev and Karlova, 2011

Figs. 28 , 29 .

Material.—About 200 calcium phosphatic internal moulds with partially phosphatised shells and phosphatic crusts, including figured SMNH Mo194728–194737, from samples 11/16.05, 11/16.1, 11/2B, lower Erkeket Formation, Khorbusuonka River and samples 21/21, 21/22, Tyuser Formation, Lena River. Siberia, Russia; Dokidocyathus regularis Zone, Tommotian stage (correlated with the upper part of Cambrian Stage 2).

Description.—Dextrally coiled moderately expanding shell. Available specimens include up to two whorls and are less than 1 mm in diameter. Spire does not protrude above the last whorl. Apertural cross-section represents an extended oval with width/height ratio 1.6–1.8. The last whorl has gently convex adapical (upper) and umbilical (lower) surfaces. Gently rounded lateral periphery of the whorls spirally ornamented with fine undulating plication perpendicular to the growth lines ( Fig. 29A 1 ). Protoconch, ca. 100 μm in diameter, not clearly separated from the adult shell ( Fig. 29 ), has an extended apical termination ( Fig. 29A ).

Remarks.—In the holotype, the spiral ornament cover the entire whorl surface excluding protoconch ( Parkhaev and Karlova 2011: pl. 11: 9). In the specimens described herein, spiral plication is available near the sutural lines of the whorls and is probably eroded on the rest of the shell surface. The spiral ornament probably represents phosphatised periostracum, which is rarely preserved. A polygonal texture, with units having variable relief and 20–40 μm in diameter, present on the surface of the first whorl in some internal moulds of Aldanella ( Fig. 28B ) cannot therefore derive from periostracal texture, nor from a prismatic microstructure in the ca. 5 μm thick wall (see also Kouchinsky et al. 2017: figs. 18A, B, 20A, F, 23H).

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

Figures

Fig. 26. Bivalve mollusc Fordilla sibirica Krasilova, 1977, from the lower Cambrian Erkeket Formation, Khorbusuonka River, Siberia, Russia; sample 19/12.75. A–C. Internal moulds, SMNH Mo194719, 167596, and 194720, respectively. A1, B1, C, lateral view; A2, dorsal view, A3–A5, B2–B4, close-ups showing surface textures. Scale bar: 830 µm (C), 500 µm (A1,A2, B1), 100 µm (A3–A5, B2–B4).

Fig. 27. Bivalve mollusc Fordilla sibirica Krasilova, 1977 from the lower Cambrian Tyuser (A, B) and Erkeket (C, D) formations and Pojetaia dentifera Kouchinsky, Bengtson, Clausen, Gubanov, Malinky, and Peel, 2011, from Sekten Formation (E–G), lower reaches of the Lena River (A, B, E–G) and Khorbusuonka River (C, D), Siberia, Russia; samples 21/54 (A, B), 20/1B (C), 19/40 (D), and 22/50 (E–G). A–G. Internal moulds, SMNH Mo194721– 194727, respectively.A1, Scanning electron microscope image showing muscle scars on the surface of internal mould, left valve.A2,A3, light-microscope micrographs showing muscle scars (arrowed), left and right sides of internal mould corresponding, respectively, to inner surfaces of left and right valves; G1, dorsal; C2, oblique dorsal; and C1, G2, lateral views. Scale bar: 500 µm (A1, B–G) and 830 µm (A2, A3).

Fig. 28. Pelagiellid mollusc Aldanella sibirica Parkhaev and Karlova, 2011, from the lower Cambrian Erkeket Formation, Khorbusuonka River, Siberia, Russia; samples 11/16.05 (A, B, E–H) and 11/2B (C, D). A–H. Phosphatised shells, phosphatic coats and partially preserved phosphatic interal moulds, SMNH Mo194728–194735, respectively. A1, adapical view; A2, apertural view. B. Internal mould with probably diagenetic polygonal structures; B2, close-up showing polygons. Scale bar: 500 µm (F–H), 250 µm (A, B1, C–E), and 125 µm (B2).

Fig. 29. Pelagiellid mollusc Aldanella sibirica Parkhaev and Karlova, 2011, from the lower Cambrian Tyuser Formation, lower reaches of the Lena River (A) and Erkeket Formation, Khorbusuonka River (B), Siberia, Russia; samples 21/21 (A) and 11/16.1 (B). A. Phosphatised shell, SMNH Mo194736. A1, A2, enlargements showing protoconch;A3, adapical view;A4, oblique adapical view.B. External mould, SMNH Mo194737, with sculpture of the shell surface. Scale bar: 250 µm (A3, A4), 125 µm (A1, A2, B).

Fig. 1. Locality maps. A. Map of the Siberian Platform. B. Khorbusuonka River. Localities referred to in the main text: 96-1, 96-2, 96-3, Malaya Kuonamka (MK) and 96-6, 96-7, 96-8, Bol’shaya Kuonamka (BK) rivers, eastern flank of the Anabar Uplift; 3, 11, 15, 19, 20, Khorbusuonka River, Olenyok Uplift; 21, 22, Lena River lower reaches, Ulakhan-Ald’yarkhay Brook; Le, Lena River middle reaches, Tiktirikteekh and Bydyangaya Brooks; Al, Aldan River, between sections “Dvortsy” and Ulakhan- Sulugur. Margin of the Siberian Platform is shown by a dashed line in A.