Drepanoistodus stougei Rasmussen, 1991

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 : 126

Drepanoistodus stougei Rasmussen, 1991

Fig. 6R–S

Material examined

22 geniculate elements including 17 from the Lynna section .

Remarks

Drepanoistodus stougei was first described by Rasmussen (1991) and further morphological details for all the four element types were subsequently added by Rasmussen (2001). The most distinct features are the rounded anterior margin and the relatively long upper margin on the base. In the material at hand, the b/c ratio (see Fig. 3 ) mean is 0.50 and angle A has a mean of 27.4°. In some cases, D. svendi sp. nov. is characterized by a similarly rounded anterior margin but is distinguished by its (weakly) curved cusp and carina. Drepanoistodus aff. basiovalis morphotypes with a rounded anterior margin are separated from D. stougei by the characteristically extended keel in the upper anterior corner of the former taxon. Because D. stougei is included here just for comparison with the new species described, synonymy or further descriptive details have not been incorporated, but can be found in Rasmussen (1991, 2001).

Occurrence

From the Lenodus antivariabilis Zone (sample LY12-5) to the basal part of the interzone (“uncertain interval”; sample LY12-29) between the L. variabilis Zone and the Y. crassus Zone sensu Lindskog et al. (2020) in the Lynna River section.

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).