Pelagiella sp. 2
publication ID |
https://doi.org/ 10.4202/app.2012.0004 |
persistent identifier |
https://treatment.plazi.org/id/03C3891D-153D-C260-FCBF-F88CCADAF9B5 |
treatment provided by |
Felipe |
scientific name |
Pelagiella sp. 2 |
status |
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Figs. 19B, C View Fig , 20 View Fig .
Material.—One calcium phosphatic internal mould, SMNH Mo 160429 and one fragmentary external mould, SMNH Mo 167688 of the same specimen, from sample K1-3B. Two internal moulds SMNH Mo 167686–167687 from sample 7/70. Uppermost Emyaksin Formation, Malaya Kuonamka and Bol’shaya Kuonamka rivers; Calodiscus -Erbiella Zone, lower Botoman Stage.
Description.—Rapidly expanding shell consisting of ¾ whorl. Aperture is asymmetrical oval ( Figs. 19B View Fig 2 View Fig , C 3 View Fig , 20A View Fig 2 View Fig ). The peripheral surface of the whorl is rounded and passes without angulation into convex spiral and umbilical surfaces of the whorl. Ornamentation of the shell exterior ( Fig. 20B View Fig ) consists of fine radial striations, 1–3 μm wide, and transversal narrow ribs that appear on the surface from ca. ½ revolution. Distance between ribs increases with growth and towards periphery of the whorl, up to ~25 μm.
Remarks.—The fossils are discussed as Pelagiella cf. subangulata by Kouchinsky (2000a: 135, fig. 14). On one of the internal moulds, a low polygonal texture is developed unilaterally as a radially extended area transitional between the peripheral and spiral surfaces of the whorl ( Fig. 20A View Fig 4 View Fig , A 5 View Fig ; Kouchinsky 2000a: fig. 14A, arrow E). The texture consists of polygons, up to 50 μm in width, with tubercles, 5–10 μm in diameter, at their centres ( Fig. 20A View Fig 4 View Fig , A 5 View Fig ).
Among other pelagiellids, the forms described herein are distinguished by the ornamentation of the shell exterior with comarginal ribs thought to be diagnostic of Costipelagiella Horný, 1964 . The ribs are twice as densely spaced as in the middle Cambrian C. zazvorkai Horný, 1964 and Pelagiella sp. cf. Costipelagiella zazvorkai from Siberia ( Kouchinsky et al. 2011), although the shell exterior was apparently similarly smooth over the initial half revolution. C. nevadense Skovsted, 2006 from Laurentia and Costipelagiella sp. cf. C. zazvorkai from the middle Cambrian of New Zealand ( MacKinnon 1985) are more laterally compressed, but have similarly spaced ribs, which do not extend, however, to the peripheral portion of the whorl in the former form.
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