Discoceras paopense Kroger 2025

Discoceras cf. paopense Kröger, 2025 Figs 6, 11–12; Table 7 Material examined ESTONIA • 1 spec.; Ristna cliff (Harju County); Keila Regional Stage (late Sandbian or early Katian Stage, Late Ordovician); Hints Coll.; illustrated in Fig. 11; GIT 789-6 • 1 spec.; Ristna (Hiiumaa Island); Keila Regional Stage (late Sandbian or early Katian Stage, Late Ordovician); illustrated in Fig. 12; GIT 426-443. Description Specimen GIT 789-6 represents a subadult growth stage of a conch with a diameter of 98 mm with 1.5 volutions preserved (Fig. 11). The specimen is slightly worn on one side, and the inner whorls are damaged and compacted. In the last whorl, the conch is discoidal (CWI decreasing from 0.28 to 0.25) and evolute (UWI = 0.55). The coiling rate (WER ah = WER wh in this case) decreases from 2.01 to 1.75. The whorl profile is weakly imprinted dorsally (IZR close to zero) and transitions from compressed to equidimensional (WWI increases from 0.94 to 1.00), with convex flanks and a flattened venter. The penultimate whorl profile (dm = 53 mm) appears to be weakly trapezoidal, with a wider ventral side, though this may result from an oblique natural cut of the specimen. The relative cameral length (RCL) is approximately 0.20 of the corresponding whorl height in the last preserved whorl. The suture line extends with a lateral lobe, an external lobe and a ventrolateral saddle. At a diameter of 53 mm, the siphuncle is subdorsal (RSP = 0.32) with a relative septal foramen height (RSH) of 0.19. Specimen GIT 426-443 is a conch in the subadult or adult growth stage, measuring 116 mm in diameter, with approximately three outer volutions preserved (Fig. 12). The specimen is broken off adaperturally and strongly compacted from one side. The innermost whorls cannot be measured, and the width and shape of the outer volutions can only be estimated from the unaffected flank. At the beginning of the last whorl, the conch is discoidal (CWI ~0.29 at dm ~ 85 mm) with an equidimensional whorl profile (WWI ~1.04 at dm ~ 85 mm), which appears convex laterally and either convex or very weakly flattened ventrally. In the last whorl, the coiling rate (WER wh) slightly increases from ~1.93 to 2.07 between a conch diameter of ~ 85–101 mm) before decreasing again from ~2.07 to 1.89 between a conch diameter of ~ 101–116 mm. At its greatest diameter, the conch is evolute (UWI ~0.52). The whorl profile in the last two whorls appears only very weakly imprinted dorsally or not imprinted (IZR close to zero). The relative cameral length (RCL) is approximately 0.20 of the whorl height in the last 1.5 whorls. The suture lines feature a lateral lobe, a ventral lobe, and a ventrolateral saddle. The siphuncle cannot be precisely measured, but it appears to be subdorsal, at least in the inner whorls. The shell ornament is best preserved on the compacted lateral side of the specimen. It consists of slightly irregularly spaced, oblique ribs or annuli, which appear rounded on top, with weaker transverse elements present between them. On the inner whorls, the ornament appears much finer, though this may be a result of the specimen’s preservation. Remarks The minor differences in whorl expansion rate between the two specimens are interpreted here as being due to inaccuracies in the measurements of the compacted specimen GIT 426-443 and differences in adult size. The conch geometry and ornament in both specimens are most similar to those seen in representatives of Discoceras paopense. However, both specimens are kept in open nomenclature due to incomplete preservation and compaction. Additionally, there is a stratigraphic gap between the specimens studied here (Keila Regional Stage) and representatives of D. paopense (Vormsi Regional Stage). Geographic and stratigraphic occurrence Western Estonia; Keila Regional Stage, late Sandbian or early Katian Stage, Late Ordovician.