Protoconus, Yu, 1979

Kouchinsky, Artem, Bengtson, Stefan, Landing, Ed, Steiner, Michael, Vendrasco, Michael & Ziegler, Karen, 2017, Terreneuvian stratigraphy and faunas from the Anabar Uplift, Siberia, Acta Palaeontologica Polonica 62 (2), pp. 311-440 : 373

Protoconus ? sp.

Figs. 33G , 34A, B.

Material.—Three internal moulds, SMNH Mo182338, 182340, 182341, from samples 2/A (section 2, Fig. 1) and 3/0.1 (section 3, Fig. 3), upper Nemakit-Daldyn and basal Medvezhya formations, western Anabar Uplift, Siberian Platform, Russia. Correlated with the upper Fortunian Stage.

Description.—Bilaterally symmetrical, low cap-shaped shell with irregularly ovoid, broad aperture. Apex not clearly distinct from rest of conch, situated near the middle of the shell in plan view, displaced towards the narrower side of the aperture. Inner surface of the shell (replicated by the internal mould) with two parallel longitudinal thickenings that define a wide median buttress flanked by furrows that extend from the apex to the wider side of the aperture. The subapical side is laterally slightly concave and forms a low broad, somewhat arched margin. The opposite side is gently convex.

Remarks.—The apex has a somewhat nodose texture (Fig. 34A 1) similar to that in co-occurring Purella antiqua moulds. The same texture occurs laterally to the apex. The distal parts of the mould are covered with a polygonal texture. The top of the mould is smooth with rare tubercules. These types of textures are distributed on the internal mould in a bilaterally symmetrical pattern (Fig. 34A 1). Protoconus cf. crestatus (see above) is different in having a more marginally located apex and broader median buttress.

Figures

Fig. 33. Molluscs Purella antiqua (Abaimova, 1976) (A–F, H), Protoconus? sp. (G), and Yunnanopleura sp. (I), from early Cambrian Emyaksin (A–F) and Medvezhya (H–I) formations, Anabar Uplift, Siberia, Russia; samples 4/27.5 (A–D), section 96-4; 5/0 (E, F), section 96-5; 3/0 (H) and 3/0.1 (G, I), section 3. A–F, H. Internal moulds with partly phosphatized shell (B and D), SMNH Mo182331–182336 (A–F, respectively), SMNH Mo182337 (H). G, I. Internal moulds, SMNH Mo182338, 182339, respectively. A1, B, C1, E, F, G2, H3, I2, upper; A2, C2, G1, H1, I3, lateral; C3, D, G3, H2, I1, subapical views. Scale bar 250 μm (C1), 500 μm (A, B, C2, C3, D–I). Fig. 34. Molluscs Protoconus? sp. (A, B) and Purella antiqua? (Abaimova, 1976) (C), internal moulds, from early Cambrian Nemakit-Daldyn Formation, → western Anabar Uplift, Siberia, Russia; sample 2/A, section 2. A–C. SMNH Mo182340–182342, respectively. A1, B1, C2, upper; A2, C3, supra-apical; A3, B3, subapical; A4, B2, C1, lateral views. Scale bar 250 μm (A, C), 500 μm (B).