Drepanoistodus cf. suberectus ( Branson & Mehl, 1933 )

Rasmussen, Jan Audun, Eriksson, Mats E. & Lindskog, Anders, 2021, Middle Ordovician Drepanoistodus (Vertebrata, Conodonta) from Baltica, with description of three new species, European Journal of Taxonomy 774, pp. 106-134 : 127

Drepanoistodus cf. suberectus ( Branson & Mehl, 1933)

Fig. 6T

Drepanoistodus cf. suberectus ( Branson & Mehl, 1933) – Löfgren 2003: fig. 7s–u. — Mellgren & Eriksson 2010: fig. 7k (only). — Hints et al. 2012: fig. 6h.

Material examined

Four geniculate elements including three from the Lynna section .

Remarks

Drepanoistodus cf. suberectus is included in the present work because it superficially resembles Drepanoistodus viirae sp. nov. Originally, D. suberectus was described as Oistodus suberectus from the Upper Ordovician strata of Missouri, USA, by Branson & Mehl (1933), but it was not until 1966 that conodont specialists included the geniculate element in the apparatus (see Bergström & Sweet 1966 and Webers 1966, for details). The D. suberectus type locality near Ozora, Missouri, was located and restudied by Bergström & Leslie (2010) who documented the conodont fauna and illustrated three different elements of D. suberectus , including the geniculate element. The Upper Ordovician geniculate D. suberectus elements (e.g., Stauffer 1935; Nowlan 2002; Bergström & Leslie 2010) are generally more rounded anteriorly and carry more pronounced keels on the cusp than the three geniculate elements at hand, thus leading us to leave the Lynna River specimens in open nomenclature.

Drepanoistodus cf. suberectus occurs only sporadically in the Lynna River section samples. It is characterised by a short upper margin of the base compared to the free cusp (b/c ratio near 0.20 in the three specimens found). Angle A between the upper margin of the cusp and the carina on the cusp (see Fig. 3 ) varies considerably (41–52°) but it is wider than that of the other Drepanoistodus species described here. Moreover, it is typified by a convex basal margin; weakly rounded anterior margin, and a weakly developed carina on the straight cusp, which is located on the lower half part of the cusp.

Superficially, D. cf. suberectus resembles D. viirae sp. nov. because of the relatively short base, but the latter species is distinguished by a narrower angle A (see Fig. 3 ); wider sides anteriorly on the cusp; laterally compressed cusp with distinct keels, and a median, as opposed to a lower, carina.

Occurrence

The Yangtzeplacognathus crassus Zone at Lynna River (samples LY12-34, LY14-2 and LY14-5). Drepanoistodus cf. suberectus has also been documented from the L. variabilis Zone of Hällekis, Sweden (Mellgren & Eriksson 2010).

Figures

Fig. 6. Light microscopy photographs of selected geniculate elements of species of Drepanoistodus, as outlined herein, from the Lynna River section, Russia. A–D. Drepanoistodus basiovalis (Sergeeva, 1963). A. LO 12476t, inner view, sample LY12-16, L. variabilis Zone. B. Same specimen as A in outer view. C. LO 12477t, inner view, sample LY12-9,L. antivariabilis Zone.D. LO 12478t, inner view, sample LY12-21b, L. variabilis Zone. E–H. Drepanoistodus iommii sp. nov. E. LO 12479T, holotype, inner view, sample LY12-16, L. variabilis Zone. F. Same specimen as E in outer view. G. LO 12480t, inner view, sample LY12-21b, L. variabilis Zone. H. LO 12481t, inner view, sample LY12-3, L. antivariabilis Zone. I–L. Drepanoistodus svendi sp. nov. I. LO 12482t, inner view, sample LY12-13, L. variabilis Zone. J–K. LO 12483T, holotype, inner and outer view, respectively, sample LY12-14, L. variabilis

Fig. 3. Geniculate Drepanoistodus basiovalis (Sergeeva, 1963) element demonstrating the essential morphological characteristics utilized in the present study. Two characters were measured quantitatively: Angle A between the median line (usually carina) of the cusp and the lower margin of the keel situated on the upper margin of the cusp (A); and the b/c ratio, which is the ratio between the length of the upper margin of the base (b) and the length of the free cusp (c).