Lapworthella dentata Missarzhevsky, 1969

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 : 465-466

Lapworthella dentata Missarzhevsky in Rozanov et al., 1969

Fig. 44C–H .

Material.—Single sclerites from samples 3/53.5, 3/55.5, 6/41.7 (correlated with the Delgadella anabara Zone, Atdabanian Stage ), and 1/5 ( Calodiscus -Erbiella Zone, lower Botoman Stage), but more common in samples 7/60, 7/62, and 7/64 (correlated with the upper Judomia Zone , upper Atdabanian Stage); SMNH X 4716–4721. Emyaksin Formation, Malaya Kuonamka and Bol’shaya Kuonamka rivers.

Description.—Pyramidal, asymmetrically coiled sclerites (broken off fragments up to 1 mm long, but can be somewhat longer), with oval or subrectangular slightly undulating aperture (up to 1 mm wide), commonly elongated transversely to direction of coiling. Apex represents a simple smooth cone with circular cross-section.Apical angle varies between ca. 15° and 45°. External ornamentation consists of distinct transversal ridges bearing short, apically oriented tubercles (denticles). The density of denticles is 35–40 per mm.

Remarks.— L. dentata Missarzhevsky in Rozanov et al., 1969 from Stage 3 of Siberia is most similar to L. schodackensis ( Lochman, 1956) from Laurentia ( Landing, 1984) and L. cornu ( Wiman, 1903) from Baltica and Avalonia, from Cambrian

Series 2, Stage 4. Each of these species is represented by diverse morphologies with variable apical angle, apertural cross-section (circular to compressed oval or rectangular), axial curvature and spacing of growth ridges ( Bengtson 1980; Landing 1984). Missarzhevsky (in Rozanov et al. 1969: 164) indicated denticles as the only difference between L. dentata and L. schodackensis . The denticles are present, however, in L. schodackensis (35–144 per mm, after Landing 1984). Landing (1984) also showed that this feature may be obscured by diagenesis and/or preparation in some specimens. Specimens of L. cornu ( Wiman, 1903) from Sweden typically carry 10–20 denticles per mm, whereas L. dentata has an intermediate number of denticles, 25–45 per mm ( Bengtson 1980). Landing (1984) suggested that L. schodackensis should be synonymised with L. dentata and L. cornu . This synonymisation would be acceptable if only the presence or absence of denticles is considered, with L. cornu ( Wiman 1903) the oldest available name. Otherwise, they are different in the relative density of denticulation. Poorly preserved single specimens of Lapworthella cf. L. tortuosa Missarzhevsky in Rozanov and Missarzhevsky, 1966 and Lapworthella sp. are also available in our material ( Fig. 44A, B ).

Stratigraphic and geographic range.—Lower Cambrian, Atdabanian and lower Botoman Stages, Siberian Platform.

Figures

Fig. 44. Tommotiids Lapworthella spp. from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River and Malaya Kuonamka River, Anabar Uplift, Siberia. A. Lapworthella cf. L. tortuosa Missarzhevsky in Rozanov and Missarzhevsky, 1966, SMNH X 4714, sample K6-8.1B; lateral (A1) and basal (A2) views. B. Lapworthella sp., SMNH X 4715, sample 6/4.9; lateral (B1) and apical (B2) views. C–H. Lapworthella dentata Missarzhevsky in Rozanov et al., 1969. C. SMNH X 4716, sample 7/60; apical view. D. SMNH X 4717, sample 7/60; oblique apical view. E. SMNH X 4719, sample 7/62; lateral (E1) and basal (E2) views. F. SMNH X 4718, sample 7/60; lateral view. G. SMNH X 4720, sample 7/64; apical (G1) and lateral (G2) views. H. SMNH X 4721, sample 1/5; lateral view on the fragment. Scale bar 300 μm, except H, 600 μm.