Anabarella plana Vostokova, 1962
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publication ID |
https://doi.org/ 10.4202/app.00289.2016 |
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persistent identifier |
https://treatment.plazi.org/id/03AD87A8-FF86-6D07-FFF3-F95B67B480FF |
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treatment provided by |
Felipe |
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scientific name |
Anabarella plana Vostokova, 1962 |
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Anabarella plana Vostokova, 1962 View in CoL
Figs. 14, 15.
Material.—About 100 phosphatic internal moulds, including figured SMNH Mo 160430, 182218–182234, from samples 5/10, 5/10B, 5a/1.2 (section 96-5 and 96-5a, Fig. 2 View Fig ), 1/45, 1/46, 1/29.2, 1/6 (section 1, Fig. 4 View Fig ), K2/21, K2/22, K2/24, K2/25, K2/26, 3/12, 3/12.2 (section 3, Fig. 3), Kugda-Yuryakh, Medvezhya, and lower Emyaksin formations, Anabar Uplift, Siberian Platform, Russia. Correlated with the upper Fortunian and lower part of Cambrian Stage 2.
Description.—Bilaterally symmetrical, coiled up to 3/4 of a whorl, rapidly expanding, laterally compressed shell, up to 5 mm in length. Aperture elongate oval with length/width ratio of ca. 3 in planar view, curved in sagittal plane. Subapical margin of aperture arched. Apex curved, no obvious protoconch boundary, projects beyond the apertural margin. Subapical side very short. Supra-apical side long and evenly convex. Inner surface of shell (replicated on internal moulds) smooth with frequent irregular, gentle comarginal folds.
Remarks.—The dorsum of the internal moulds from the Kotuj River (Fig. 14) is often longitudinally depressed (Fig. 14C 4, F 1) and carries a shallow polygonal microtexture (Kouchinsky 1999). Polygonal patterns were also observed on the apex, including the subapical area (Kouchinsky 1999). The shells of Anabarella plana are slightly involute and reach 10 mm in length ( Gubanov and Peel 2003). The internal moulds range from an involute Oelandiella -like type (Fig. 15) to a more evolute Watsonella -like type (Fig. 14; Gubanov et al. 1999; Gubanov and Peel 2003; see the latter report for comparison between the outer shell surface and internal mould). The morphological differences between the Oelandiella - and Watsonella -like forms include a longitudinal dorsal depression in Watsonella -like internal moulds from the Medvezhya Formation compared to a smooth rounded dorsal surface in the more involute forms (Fig. 15) from the lower Emyaksin Formation. Additional material is needed, however, to determine if two different morphs or subspecies of A. plana , or even species of Anabarella are represented.
The earliest Anabarella plana is reported from the upper Nemakit-Daldyn Formation of the western Anabar Uplift (Kotuj River), from the Purella antiqua Zone by Khomentovsky and Karlova (1993), but the fossil is not illustrated. Laterally compressed internal moulds indeed occur in our material from the upper Nemakit-Daldyn Formation (see Fig. 9E), but their affinity with A. plana remains questionable. A. plana reported from the upper Ust’-Yudoma Formation (ca. 40 m below the top) from the Dzhanda River (Pel’man et al. 1990: 16: 2) can also be attributed to the Purella Zone. A. plana frequently occurs in the Medvezhya and lower Emyaksin formations and disappears from the fossil record by the upper part of Cambrian Stage 2 in Siberia. A. cf. plana also occurs in the Lontova Formation ( Platysolenites antiquissimus Zone ) of Baltica (Mens and
Fig. 14. Mollusc Anabarella plana Vostokova, 1962 , internal moulds, from early Cambrian Medvezhya Formation, Kotuj River, western Anabar Uplift, → Siberia, Russia; samples 3/12.5 (A), K2/25 (B–F, H), K2/26 (I), section 3; and 1/29.2 (G), section 1. A–I. SMNH Mo182218 (A), SMNH Mo182219 (B), SMNH Mo160430 (C), SMNH Mo182220–182225 (D–I, respectively) (compare C with Kouchinsky 1999: fig. 1A). A 1 –C 1, D 2, E 1, F 2, G 2, H 1, I 2, lateral; A 2 –C 2, D 1, E 3, G 4, H 3, I 1, subapical; B 3, D 3, E 2, F 3, G 3, H 2, upper; G 1, supra-apical view; C 4, F 1, oblique lateral views showing upper furrow. Scale bar 1 mm.
Isakar 1999; Isakar and Peel 2007), which can be broadly correlated with the Terreneuvian Series (Gubanov 1998, 2002). Anabarella spp. morphologically similar to A. plana (Landing et al. 1989; Gubanov and Peel 2003) are reported from the upper Fortunian–Cambrian Stage 2 equivalent deposits of the Bayan-Gol Formation of western Mogolia (Voronin et al. 1982; Esakova and Zhegallo 1996; Brasier et al. 1996; Gubanov 1998, 2002; Demidenko and Parkhaev 2014) and from Assemblage Zones 2 and 3 of South China (Luo et al. 1982; Xing et al. 1983). In Western Gondwana ( Spain), Anabarella plana Vostokova, 1962 and Oelandiella korobkovi Vostokova, 1962 are reported from the upper part of the lower Corduban Stage, probably also representing the upper Fortunian–Cambrian Stage 2 interval (Gubanov 1998, 2002; Vidal et al. 1999; Kouchinsky et al. 2012). Anabarella sp. reported from the uppermost part of the Watsonella crosbyi Zone in Avalonia ranges much higher, through the middle Camenella baltica Zone (Cambrian Stage 2–lower Cambrian Stage 3; Landing et al. 1989).
Stratigraphic and geographic range.—Upper Fortunian and lower part of Cambrian Stage 2; Siberia, western Mongolia, Baltica, Gondwana (South China, Spain), and Avalonia (eastern Newfoundland).
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Department of Paleozoology, Swedish Museum of Natural History |
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