Discoceras damesii (Schröder, 1891)

Figs 6, 10; Table 6

Trocholites Damesii Schröder, 1891: 158, pl. 28(5) fig. 2.

Schroederoceras Damesi – Hyatt 1894: 469–470.

Discoceras damesi – Sweet 1958: text-fig. 13a.

Trocholites damesii – Neben & Krueger 1971: pl. 39 figs 13–15.

Diagnosis

Species of the genus Discoceras in which the shell ornament on inner whorls has sharp, regularly spaced (up to 1 mm apart) and weakly frilled annuli or raised lirae, with symmetric crests, accompanied by extremely fine spiral ornament; shell in later whorls ornamented with irregularly frilled and strongly imbricated transverse elements. Whorl profile rounded, weakly depressed (WWI ~1.20); siphuncle near dorsal shell wall in subadult stage.

Type material

Holotype GERMANY – Brandenburg • Schwedt/Oder; Dalbyan Regional Stage (Sandbian Stage, Late Ordovician); illustrated by Schröder (1891: pl. 5(28) fig. 2) and Neben & Krueger (1971: pl. 39 figs 13– 15), re-illustrated here in Fig. 10; MB.C.11542.

Description

Holotype MB.C.11542 is a phragmocone in the juvenile or early adult growth stage, with a diameter of 56 mm and 2.5 whorls preserved (Fig. 10). The conch is embedded in rock on one side and broken off on the other; therefore, some ratios are estimates. At its largest diameter, the conch is discoidal (CWI ~0.42) and subevolute (UWI = 0.42), with a WER ah of 2.05. The whorl profile is elliptic and weakly depressed (WWI ~1.19). The imprint zone rate appears to increase in the last half-whorl (IZR from 0.00 to approximately 0.07). The siphuncle is positioned close to the dorsal shell wall (RSP ~0.12), with a relative septal foramen height (RSH) of 0.18. In the inner whorls, the shell is ornamented with rather sharp annuli or raised lirae, which are regularly and relatively widely spaced (up to 1 mm apart at dm = 24 mm), weakly frilled and not imbricated; the transverse elements are accompanied by an extremely fine spiral ornament. On the outer whorl, the transverse ornament becomes irregularly frilled and strongly imbricated, while spiral lines are no longer discernible.

Remarks

Discoceras damesii is a species of the genus in which the first 2.5 whorls are characterised by a weakly depressed, elliptic whorl profile and distinct shell ornament. The first at least 1.5 whorls show sharp annuli or raised lirae crossed by an extremely fine, flat spiral ornament. This transitions into irregularly thick, irregularly frilled, and strongly imbricated transverse elements, while spiral elements are no longer present.

The conch ratios and ornament of the holotype of D. damesii are rather similar to corresponding growth stages of the holotype of D. vesenbergense (Balashov 1953) and some representatives of D. saemanni (Kröger 2025: text-figs 50b, 53 and the specimen MB.C.32316 studied herein). However, it appears most similar to some specimens of D. roemeri (Kröger 2025), not only in whorl profile but particularly in the shell ornament, which transitions from thin, sharp ribs to frilled transverse elements. Further comparison is hindered by the insufficient details provided by Balashov (1953) and the lack of knowledge on the late growth stages of the holotype of D. damesii. Additionally, the latter is stratigraphically much older (Sandbian vs Katian to Hirnantian stages) than the representatives of the other three species named above. D. damesi is thus kept separate until more completely preserved material is known.

Geographic and stratigraphic occurrence

Northern Germany; Sandbian Stage, Late Ordovician.