Discoceras brandenburgense Aubrechtová, Korn & Kröger, 2026, sp. nov.

Discoceras brandenburgense sp. nov. urn:lsid:zoobank.org:act: 26674E19-CF3F-4E2C-B25C-A42F123EE5BF Figs 14, 21; Table 12 Rectanguloceras cf. roemeri – Neben & Krueger 1973: pl. 65 figs 1–2. Discoceras roemeri – Kröger 2013a: 73. Diagnosis Species of the genus Discoceras with an adult conch diameter of about 80 mm. Conch tightly coiled up to diameter of about 64 mm; body chamber and adapertural part of phragmocone widely uncoiled. Conch increasingly discoidal (CWI decreases from 0.40 to 0.30 in last three quarters of a whorl) with decreasing coiling rate (WER wh decreasing from 2.50 to 2.00). Whorl profile subcircular, equidimensional to weakly depressed (WWI ~ 1.05) in last whorl but equidimensional and slightly ventrally flattened in body chamber. Phragmocone chambers short (RCL = 0.18 in last whorl), suture line with broad lateral lobe, straight across venter. Etymology From Brandenburg (Germany), the region of the type locality. Type material Holotype GERMANY – Brandenburg • Niederfinow; Moldåan Regional Stage (early Katian Stage, Late Ordovician); Neben Coll.; illustrated in Fig. 21; MB.C.32324. Description Holotype MB.C.32324 is a conch with a maximum diameter of 82 mm; approximately 3.25 whorls are preserved, with the last ~100 degrees of the outer whorl being uncoiled (Fig. 21). The coiled portion has a diameter of about 64 mm. In the last three-quarters of a whorl, the conch becomes increasingly discoidal (CWI decreases from 0.38 to 0.31) and increasingly evolute (UWI increases from 0.41 to 0.48), while the coiling rate (WER) decreases from 2.51 to 1.97. The whorl profile is nearly circular, transitioning from equidimensional to slightly depressed (WWI increases to 1.03 to 1.08) throughout the last whorl, before becoming equidimensional again at the end of the body chamber, with a slightly flattened venter. The body chamber is preserved over a length of approximately 90 degrees; it is tubular in shape, with only a slightly flared venter. The phragmocone chamber length (RCL) is 0.18 in the last whorl. The shell ornament is preserved only in a small area on the venter at the end of the last whorl, where it consists of fine lirae. Remarks Holotype MB.C.32324 is unique among the species of Discoceras due to its comparatively small adult conch diameter of 82 mm. It is widely uncoiled, slowly expanding last volution, has an equidimensional or slightly depressed whorl profile that is ventrally slightly flattened, and shows the presence of short phragmocone chambers. Neben & Krueger (1973) classified holotype MB.C.32324 as “ Rectanguloceras (Discoceras) cf. roemeri ” (see also Kröger 2013a). However, the terminal conch of D. roemeri is much larger (up to 150 mm in diameter) with an elliptic, depressed to quadrangular whorl profile. In D. roemeri, the last whorl uncoils from the preceding whorl only at the very end of the body chamber, whereas it is much more widely uncoiled in holotype MB.C.32324. Additionally, the phragmocone chambers are longer in D. roemeri (RCL ~ 0.25 vs ~ 0.18 in the last whorl). Holotype MB.C.32324 is similar in umbilical width, conch width and expansion rate to the holotype of D. polonicum sp. nov. (MB.C.11992); the latter also has an uncoiled outer whorl. However, the whorl profile in MB.C.11992 is compressed (WWI ~ 0.90 in the last volution) and ventrally rounded. Additionally, the last 90 degrees of the conch expand more rapidly in MB.C.11992 (WER = 2.27–2.15 vs 2.04– 1.97 in MB.C.32324). Geographic and stratigraphic occurrence Northern Germany; early Katian Stage, Late Ordovician.