Allatheca

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 : 443-445

Allatheca ? cf. A. corrugata Missarzhevsky in Rozanov et al., 1969

Fig. 22F–I .

Material.—Eleven phosphatised conchs and internal moulds, including SMNH X 4609–4612, from sample 7/36.7. Emyaksin Formation, Bol’shaya Kuonamka; correlated with the Delgadella anabara Zone, Atdabanian Stage.

Description.—Slightly dorso-ventrally curved conch, 1–2 mm long, with convex dorsal and flattened ventral sides. Dorso-lateral part is semi-circular in transverse section, and passes via rounded margins into a slightly convex ventral part. In dorsal view, the apical angle is ca. 20°. The aperture has an apparently straight dorsal margin. The conch is ornamented dorso-laterally with distinct narrow transverse ribs. The ventral surface is smooth.

Remarks.—The cross-section and ornamentation of the conchs are typical of Allatheca corrugata Missarzhevsky in Rozanov et al., 1969 (tentatively synonymised herein with A. anabarica Val’kov, 1975). The illustrated individuals are therefore interpreted as probable juveniles of typically larger, several cm-sized Allatheca corrugata , which are reported from the middle part of Emyaksin Formation ( Val’kov 1975, 1987).

Figures

Fig. 22. Hyolith conchs from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/36.7. A–E. Majatheca tumefacta Missarzhevsky in Rozanov et al., 1969, conchs with phosphatised microstructure of the walls. A. SMNH X 4600 (Mo 160442 in Kouchinsky 2000b: fig. 3); dorsal view. B. SMNH X 4601; apertural view. C. SMNH X 4602 (Mo 160441 in Kouchinsky 2000b: fig. 3); oblique lateral (C1) and dorsal C2) views. D. SMNH X 4603; dorsal view. E. SMNH X 4604; apertural (E1) and dorsal (E2) views; E3, septum in the apical part; E4 and E5, phosphatised fibrous bundles near the aperture. F–I. Allatheca? cf. A. corrugata Missarzhevsky in Rozanov et al., 1969. F. SMNH X 4609; lateral view. G. SMNH X 4610; lateral (G1), dorsal (G2), and apertural (G3) views. H. SMNH X 4611; dorsal (H1), apical (H2), and apertural (H3) views. I. SMNH X 4612; ventral view. Scale bar 600 μm, except C1, 300 μm; E3–E5, 200 μm.