Majatheca sp.

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 : 404

Majatheca sp.

Fig. 43 .

Material.—Several tens of phosphatised opercula, including figured SMNH X11159–11163, from samples 11/16.05 and 11/2B, basal Erkeket Formation, Khorbusuonka River and sample 21/21, Tyuser Formation, lower Lena River. Siberia, Russia; Dokidocyathus regularis Zone, Tommotian stage (correlated with the upper part of Cambrian Stage 2).

Description.—Outer surface of the operculum flattened to slightly concave, gently bending in sagittal direction

Fig. 43C 2 ), showing a rounded trapezoidal or rounded pentagonal aspect in the outline with slightly undulating margin. Initial part centrally placed, rounded and slightly protruding on the outer surface of operculum. The latter carries continuous concentric growth lines. Inner surface of the operculum carries a circumferential bilaterally symmetrical slightly dorso-ventrally extended rounded pentagonal ridge, offset from the margin of the operculum, and a gently sloping rim between the ridge and opercular margin.

Outer face of the ridge vertical with respect to the rim. Inner face slopes gently towards the center of operculum and carries bilaterally symmetrical radial folds. Crest of the ridge undulates. In ventral part of the operculum it arches broadly towards the opercular margin, forms two elevated folds laterally and produces a narrow fold dorsally towards the opercular margin.

Remarks.—Similar opercula, Circotheca sp. 1 , are illustrated by Meshkova (1974: pl. 11: 2, 3, 9) from the Tyuser Formation, Chekurovka section (Lena River). A larger specimen probably of the same species is defined by Meshkova (1974: pl. 11: 12) as Allatheca sp. The opercula lack processes and are generally similar to Majatheca tumefacta ( Fig. 51F herein and Kouchinsky et al. 2015a: fig. 24) from lower Cambrian Stage 4 beds of the Siberian Platform. Majatheca tumefacta opercula have, however, more laterally extended broad oval shape and a clearly developed rounded trapezoidal aspect of the circumferential ridge on the inner side of the operculum.

Figures

Fig.43. Hyolith opercula Majatheca sp. from the lower Cambrian Erkeket Formation, Khorbusuonka River (A, C–E) and Tyuser Formation, lower reaches of the Lena River (B), Siberia, Russia; samples 11/16.05 (A, C, E), 11/2B (D), and 21/21 (B). A–E. Phosphatised opercula, SMNH X11159–11163, respectively. C1, E1, outer surface; C2, E2, oblique ventral view; D1, inner surface; D2, oblique dorsal view. Scale bar 500 µm.

Fig. 2. Stratigraphic columns of sections 96-1 and 96-2, Malaya Kuonamka River and section 96-8, Bol’shaya Kuonamka River, Siberia, Russia (after Kouchinsky et al. 2011: fig. 2). Arrows with numbers indicate derivation of fossiliferous samples. Botoman Stage is correlated herein with the Cambrian Stage 4, in accordance with Geyer (2019).Asterisk A, indicates occurrences of Davidonia anabarica, Enigmaconus? pyramidalis, Pelagiella sp. 1 and 2, Majatheca tumefacta, Tetratheca clinisepta, “Lenatheca groenlandica”, Conotheca cf.mammilata, Parkula cf. esmeraldina (see Kouchinsky et al.2015a). Asterisk B, indicates occurrences of Anabaroconus sibiricus, Eotebenna viviannae, Kuonamkaella oliva, Leptostega cf. hyperborea, Pelagiella sp. cf. Cambretina mareki, Pelagiella sp. cf. Costipelagiella zazvorkai, Pojetaia dentifera, Protowenella sp., Pseudomyona queenslandica, and Yochelcionella angustiplicata (see Gubanov et al. 2004b and Kouchinsky et al. 2011). Abbreviations: Ac., Acadoparadoxides; An., Anabaraspis; B., Bergeroniellus expansus; P., Paramicmacca; Fm., Formation; H., Homotreta; O-S, Ovatoryctocara–Schistocephalus; T., Triplagnostus.

Fig. 51. Phosphatised hyolith opercula from the lower Cambrian Sekten Formation, lower reaches of the Lena River (A–D) and Erkeket Formation, Khorbusuonka River (E–H), Siberia, Russia; samples 19/10.25 (E, H), 19/11.75 (F, G), and 22/50 (A–D). A, B. Parkula bounites Bengtson in Bengtson et al., 1990, SMNH X11207 and 11208, respectively. C, D. Hyolithid operculum 1, SMNH X11209 and X11210, respectively. E, H. Triplicatella sp., SMNH X11211 and X11212, respectively. F. Majatheca tumefacta Missarzhevsky in Rozanov et al., 1969, SMNH X11213. G. Triplicatella papilio Kouchinsky in Kouchinsky et al., 2015, SMNH X11214. A1, D2, inner surface; B1, C1, E3, H2, outer surface; A2, oblique lateral; B2, C2, D1, E1, H1, lateral; C3, E2, dorsal; and A3, C4, ventral views. Scale bar 500 µm.