Majatheca tumefacta Missarzhevsky

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 : 441-443

Majatheca tumefacta Missarzhevsky in Rozanov et al., 1969

Figs. 22A–E , 23A, B, 24.

Material.—Twenty phosphatised internal moulds and conchs from sample 7/25.5 (correlated with the Profallotaspis jakutensis Zone ), sample 7/27.5 (correlated with the Repinaella Zone ), samples 7/28.5, 7/29, 7/31.5, and 7/34 (correlated with the Delgadella anabara Zone ); twenty-three phosphatised conchs, including SMNH X 4600–4604, from sample 7/36.7, correlated with the Delgadella anabara Zone, Atdabanian Stage ; Emyaksin Formation, Bol’shaya Kuonamka River. Seven phosphatised conchs, including SMNH X 4605 and 4606, from sample 7/70 and several additional conchs from sample K1-3B; several phosphatised opercula, including SMNH X 4607 and 4608, from samples 1/7.8 and K1- 3B (respectively); Uppermost Emyaksin Formation, Malaya Kuonamka and Bol’shaya Kuonamka rivers; Calodiscus -Erbiella Zone, lower Botoman Stage.

Description.—Conch gently curved dorsally at the initial part, with broadly oval aperture. Ventral and dorsal margins of the aperture protrude slightly. Apical angle is ca. 25°. The initial part is rounded and separated from the main conch by a septum. The wall consists of two layers of fibrous bundles. In the outer one, the bundles are inclined longitudinally towards the conch apex and contain pore canals. The bundles of the inner layer run transversely around the conch.

Operculum outline is oval. On the inner surface of operculum, a well-developed circular ridge delimits a rounded triangular or rounded trapezoidal depression in the middle. The ridge and medial depression carry radial folds and expose radially oriented fibres in places ( Kouchinsky 2000b: fig. 7A, D). The marginal part of the operculum is about onethird of the opercular radius in width and is formed of stacked growth laminae penetrated by pores ( Fig. 24 ; see description of these porous opercula in Kouchinsky 2000b).

Remarks.—The fossils are described by Kouchinsky (2000b: 69, fig. 3) as Allathecid? hyolith, microstructural type A, forms 1 and 2. Missarzhevsky (in Rozanov et al. 1969: 119) noted that the wall of Majatheca tumefacta is built of acicular units (“crystals”) perpendicular to the surface. Opercula, illustrated as Allatheca sp. in Rozanov et al. (1969: pl. 11: 4, 8) and attributed to M. tumefacta in Sokolov and Zhuravleva (1983: pl. 22: 7), as well as Allatheca sp. 2 ( Meshkova 1974: pl. 11: 12) are very similar to those in Fig. 24A herein. Conchs and opercula herein attributed to M. tumefacta co-occur in sample K1-3B. Sample K1-3B from the Malaya Kuonamka section contains a variety of often well-preserved hyolith opercula, in contrast to the coeval sample 7/70 from Bol’shaya Kuonamka that contains only poorly preserved opercula but well-preserved hyolith conchs. The general disparities in the numbers of hyolith opercula versus conchs from the same level is probably explained by local differences in hydrodynamic sorting (see Malinky and Skovsted 2004).

Stratigraphic and geographic range.—Upper Tommotian– lower Botoman Stages, Siberian Platform.

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.

Fig. 24. Opercula of hyoliths Majatheca tumefacta Missarzhevsky in Rozanov et al., 1969 with phosphatised microstructure of the walls from lower Cambrian Emyaksin Formation, Malaya Kuonamka River, Anabar Uplift, Siberia. A. SMNH X 4607 (Mo 160465 in Kouchinsky 2000b: fig. 7), sample 1/7.8; inner (A 1), dorsal (A 2), and ventral (A 3) views. B. SMNH X 4608 (Mo 160467 in Kouchinsky 2000b: fig. 7), sample K1-3B; broken operculum with phosphatised canals preserved. Scale bar: A, 600 μm; B, 300 μm.