Drepanoistodus basiovalis

Drepanoistodus basiovalis (Sergeeva, 1963)

Fig. 6A–D

Oistodus basiovalis Sergeeva, 1963: 96, pl. 7 figs 6–7, text-fig. 3.

Drepanoistodus basiovalis – Lindström 1971: 43, text-figs 6, 8. — Stouge & Bagnoli 1990: 15, pl. 5 figs 18–24. — Dzik 1990: fig. 12; 1994: 78, pl. 16 figs. 16–20, text-fig. 12a; 2020: fig. 7A–E. — Rasmussen 1991: 277, fig. 6l; 2001: 71–73, pl. 5: 9 (cum. syn.). — Löfgren 1994: fig. 6.30; 2000a: fig. 4w; 2006: figs 3n, 3ab. — Viira et al. 2001: fig. 5z. — Zhen & Percival 2004: 93, fig. 11a–j. — Tolmacheva et al. 2013: pl. 3, fig. 24.

partim Drepanoistodus basiovalis – Löfgren 1978: 55–56, pl. 1 figs 11–16 (only), non 17 (= D. contractus (Lindström, 1955)). — Olgun 1987: 49, pl. 6w (only). — Landing et al. 2003: fig. 4e (only). — Zhen et al. 2011: 222–227, fig. 12a?, b–n, p–q (only).

cf. Drepanoistodus basiovalis – Zhang 1998: 61–62, pl. 5 figs 5–12 (unusually short upper margin of the base).

? Drepanoistodus basiovalis – Lehnert et al. 1998: 55, pl. 3 figs 6, 12 (12 may belong to Paroistodus originalis (Sergeeva, 1963)). — Boncheva et al. 2009: text-fig. 3.8 (broken element). — Albanesi & Ortega 2016: fig. 7(6) (shares characters with D. basiovalis and D. cf. balticus). — Feltes et al. 2016: fig. 3ac. — Wu et al. 2018: fig. 5e (unusually long base compared to the cusp).

non Drepanoistodus basiovalis – Gutiérrez-Marco et al. 2008: 153, figs 3.29–3.31 (may be Drepanoistodus cf. basiovalis or Drepanoistodus cf. suberectus (Branson & Mehl, 1933). — Hints et al. 2012: fig. 6h (= Drepanoistodus cf. suberectus). — Wu et al. 2017: fig. 7u (= Drepanoistodus contractus (Lindström)). — Lindskog et al. 2020: fig. 7v–w (= Drepanoistodus viirae sp. nov.).

Original diagnosis (translated from Sergeeva, 1963 [in Russian])

Inclined conodonts, almost symmetrical, with a wide shortened base, the edge of which is rounded.

33 geniculate elements including 24 from the Lynna section.

Original description, slightly shortened (translated from Sergeeva, 1963 [in Russian])

Medium-sized conodonts (0.52–0.92 mm), inclined; the degree of inclination of the cusp is 45–60°, sometimes up to 80°. Base high, not very long, elongated along the CD; base length 2.5–3 times its height (comment by the authors: “ we find the meaning of the latter measure ambiguous ”). Base wall slightly transparent near the edge, rounded. The angle between the sides AC is more than 90°; angle between AD 40–45°; corners are smoothly obtuse. Transverse in cross section, the base is oval, elongated along CD and compressed along L1L2. From the C side, the base is compressed, sometimes with a thin keel near the tip, with a small keel on side D. The sides of the base L1 and L2 are smooth and flat. Basal cavity is not always visible, it is wide, but not deep, without visible tops. The cusp is long, straight or slightly curved towards L1, sharply tapering towards the tip; compressed. The sides of the cusp are almost flat, with a welldeveloped longitudinal, wide carina on L1 and less developed carina on the side L2. The carinae usually run from the base to the tip of the cusp. Thin keels occur on the lower (D) and upper (C) parts of the cusp.

In her original diagnosis, Sergeeva (1963) only included geniculate elements with a rounded basal margin in “ Oistodus ” basiovalis, which is also evident from the species epithet: basiovalis (meaning oval base). This interpretation of the geniculate element in Drepanoistodus basiovalis is followed here. Additional typical characters that may be added to the original species description include: anterior margin and upper anterior corner rounded or weakly rounded; cusp usually straight; a median or median to lower, longitudinal carina present on the inner (sometimes slightly concave) side of the element. Carina is more distinct in Darriwilian specimens than in Dapingian ones. Whereas angle A (Fig. 3) between the cusp and the upper margin is 29.6° with a standard deviation at 5.6, the mean ratio between the length of the free upper margin and free cusp (b/c ratio) reaches 0.40 with a standard deviation of 0.1 (Fig. 5B).

Drepanoistodus basiovalis occurs from the L. antivariabilis Zone (sample LY12-9) to the interzone (“uncertain interval”) between the L. variabilis Zone and the Y. crassus Zone sensu Lindskog et al. (2020) in the Lynna River section (sample LY12-21b; between LY12-21 and LY12-22). In addition, D. basiovalis has been reported from several other localities in Baltoscandia and Poland, and also outside the Baltica palaeocontinent, e.g., New Brunswick, Argentina, Australia and China (for references, see the synonymy list above).