Discoceras gubkovense Balashov 1953

Discoceras cf. gubkovense Balashov, 1953 Figs 13–14; Table 8 Material examined GERMANY – Brandenburg • 1 spec.; Schwedt; Nabala Regional Stage (Katian Stage, Late Ordovician); illustrated in Fig. 13; MB.C.32317. Description Specimen MB.C.32317 is a conch in the subadult or adult growth stage, measuring 100 mm in diameter (Fig. 13). In the final half whorl, the conch becomes increasingly discoidal (CWI decreasing from 0.32 to 0.21), more widely umbilicate (UWI increasing from 0.46 to 0.51), and less expanded (WER decreasing from 2.16 to 1.76). The whorl profile transitions from equidimensional to weakly compressed (WWI decreasing from 1.00 to 0.86) and from convex to flattened ventrally and laterally. The final whorl is not uncoiled, but the conch is distorted adaperturally, and it is unclear whether the aperture is preserved. The ornament consists of strongly imbricated lirae, which are irregularly raised to form narrow, transverse ribs. Remarks The conch geometry and ornament of specimen MB.C.32317 are similar to those of specimens of D. paopense from the Vormsi Regional Stage of Estonia. However, in MB.C.32317, the whorl profile transitions from equidimensional to weakly compressed and from convex to flattened ventrally and laterally, and the body chamber does not appear to uncoil. Specimen MB.C.32317 more closely resembles the type specimens of D. gubkovense from the Sandbian Stage of the St. Petersburg Region of Russia as described by Balashov (1953). The similarities include an adult conch size of approximately 100 mm, an extremely discoidal conch shape, very slowly expanding and compressed whorls and shell ornamentation consisting of lirae and low ribs that form a ventral sinus. However, the type material of D. gubkovense described by Balashov (1953) is rather incompletely preserved and originates from a stratigraphically older interval (Sandbian vs Katian Stage). Additionally, internal features cannot be examined in MB.C.32317. As a result, the latter specimen is tentatively assigned to D. gubkovense with a question mark, pending further study. Geographic and stratigraphic occurrence Northern Germany; Nabala Regional Stage, Katian Stage, Late Ordovician.