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

Discoceras circulare sp. nov. urn:lsid:zoobank.org:act: E909E2E3-131A-4015-8856-5F94EAB67E91 Figs 23–24; Table 13 Diagnosis Species of the genus Discoceras with adult conch diameter of over 100 mm; conch extremely discoidal (CWI ~ 0.30) and subevolute (UWI ~0.40); whorls weakly imprinted dorsally or not imprinted (IZR close to zero); whorl profile remains nearly equidimensional (WWI ~1.00) with convex flanks and venter; whorl expansion rate decreases in ontogeny up to WER ~2.10; relative cameral length (RCL) of 0.25 in the last whorl; body chamber slightly uncoiled adaperturally and slightly flared; siphuncle subdorsal with a relative septal foramen height (RSH) of ~ 0.20 in the last whorl; shell ornament of fine, regularly spaced lirae which are not frilled or weakly frilled and weakly imbricated. Etymology From the Latin ‘circulare’ (adj., neut.) = ‘circular’, referring to the shape of the whorl profile. Type material Holotype ESTONIA • Aluvere quarry (Lääne-Viru County); late Haljala Regional Stage (Sandbian Stage, Late Ordovician); illustrated in Fig. 24; GIT 426-145. Paratype GERMANY – Brandenburg • Schlabendorf (near Calau); early Haljala regional Stage (Sandbian, Late Ordovician); Krueger Coll.; MB.C.32325. Description Holotype GIT 426-145 is a conch of a nearly adult or adult individual with a maximum diameter of 102 mm (Fig. 24). The last 2.25 whorls are preserved; the conch is adaperturally incomplete, with at least 90 degrees of the outermost volution missing and indicated only by remains on the outer surface of the penultimate whorl. During growth, the conch is discoidal (CWI decreases from 0.46 to 0.34) and moderately evolute (UWI increases from 0.28 to 0.41). The coiling rate (WER) decreases from 2.83 at a diameter of 34 mm to 2.13 at a diameter of 83 mm. The whorl profile is weakly imprinted dorsally, subcircular, and equidimensional (WWI ~1.00). The siphuncle has a subdorsal position and a relative septal foramen height (RSH) of about 0.19. The shell ornament consists of well-developed transverse lirae, which are regularly spaced and not frilled or only weakly frilled. In the last half whorl, the lirae are irregularly thickened, with very fine lines present between thicker lines, and low, rounded annuli are indicated. Paratype MB.C.32325 is a conch in the adult growth stage with a diameter of 105 mm with the last 2.5 volutions preserved. The conch is extremely discoidal (CWI = 0.31) and subevolute (UWI = 0.43) with a high coiling rate (WER = 2.19). At its largest diameter, the whorl profile is subcircular and nearly equidimensional (WWI = 0.97). The body chamber is preserved over 95 degrees and becomes uncoiled after about 70 degrees. It appears to be adaperturally slightly flared on its ventral and lateral sides, with a slight indication of a ventral sinus. However, it is not clearly visible whether the aperture is preserved. The relative cameral length (RCL) is 0.25 at dm = 70.9 mm. The ornament consists of regularly spaced lirae, which are not frilled or only weakly frilled and slightly imbricated. Remarks The holotype and paratype of Discoceras circulare sp. nov. are most similar to specimens assigned to Discoceras bandonis, but they differ in having a more circular whorl profile (WWI ~ 1.00 vs 0.93 at the corresponding conch diameter). The conch geometry in the representatives of Discoceras circulare sp. nov. falls within the variability described for the late Katian to early Hirnantian D. roemeri (Kröger 2025). However, D. roemeri differs in having a greater adult size (140 mm), a subquadratic whorl profile in subadult growth stages, and a stronger ornamentation consisting of imbricated lamellae, irregularly spaced costae, and shallow ribs. Geographic and stratigraphic occurrence Northern Germany, Estonia; Haljala Regional Stage, Sandbian Stage, Late Ordovician.