Rhenocycloceras denckmanni gen. et sp. nov.

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Fig. 7

Diagnosis

Species of the genus Rhenocycloceras gen. nov. with an orthoconic conch that is an expansion angle of ca 15°; cross section compressed (cw/ch ca 0.85). Annuli extending with a broad projection on the flank and shallow sinuses in the ventral and dorsal areas, standing in distances of ca 0.23 of the conch diameter at the end of the annulate stage. Growth lines fine with linear course; standing in equal distances on annuli and interspaces. Wide septal spacing.

Etymology

Named after August Denckmann (1860–1925), who discovered the fossil locality at Schälk.

Type material

Holotype GERMANY – Rhenish Mountains • Schälk, northern quarry; Edmooroceras pseudocoronula Zone (early Serpukhovian); 1983; Korn leg.; illustrated in Fig. 7; MB.C.32341.

Paratype GERMANY – Rhenish Mountains • Schälk, northern quarry; Edmooroceras pseudocoronula Zone (early Serpukhovian); 1904; Denckmann leg.; BGRB X13498.

Description

Holotype MB.C.32341 is an internal mould with a preserved length of 104 mm; small remnants of the shell are locally retained (Fig. 7). The dorsoventral conch diameter increases from 11 mm to 31 mm over a length of 75 mm, corresponding to an expansion angle of 16° in lateral view and 12° in dorsoventral view. The conch profile is laterally compressed (cw/ch=0.84), with diverging flanks and a ventral side that is distinctly broader than the dorsal.

The specimen bears 20 annuli with gradually increasing spacing. At a dorsoventral conch diameter of 27 mm, the distance/conch diameter ratio of the annuli is 0.23. The annuli follow a slightly undulating course, forming very shallow sinuses on the venter and dorsum. They are most sharply developed in the juvenile stage, becoming progressively weaker and lower during ontogeny. In cross-section, the annular crests are broadly rounded, as are the interspaces. The annuli sweep in a broad convex arc across the flanks and dip into shallow sinuses in ventral and dorsal areas.

Only a few small areas of shell material are preserved; these show fine, evenly developed growth lines, equally pronounced on the annuli and in the intervening spaces. Approximately half of the specimen is chambered. The individual chambers are very long, with suture lines consistently positioned in the middle of every second annulus.

Remarks

Rhenocycloceras denckmanni gen. et sp. nov. is clearly distinguishable from other species of the genus by its laterally compressed conch profile. In addition, it exhibits an unusually wide septal spacing: unlike other species in which the chamber length corresponds approximately to the spacing of the annuli, the chambers in R. denckmanni are twice as long as the distance between successive annuli.

Stratigraphic occurrence

Rhenocycloceras denckmanni gen. et sp. nov. is so far known only from the Edmooroceras pseudocoronula Zone at Schälk and co-occurs there in a cephalopod accumulation with Emstites schaelkensis (Brüning, 1923) (Korn 1988). This zone lies at the base of the classical Namurian Stage (Korn 1996). In case that the Viséan–Serpukhovian boundary will be defined by the conodont species Lochriea ziegleri Nemirovskaya, Perret & Meischner, 1994, however, this zone would be situated at some stratigraphic distance above the base of the Serpukhovian (Wang et al. 2018).