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

Discoceras aseriense sp. nov. urn:lsid:zoobank.org:act: 7E00DA96-8FBD-4160-996E-B0EA30BECF3E Figs 14, 20; Table 11 Diagnosis Species of the genus Discoceras with an adult conch diameter of about 90 mm, terminal body chamber uncoiled. Conch in the last half whorl thinly discoidal (CWI ~0.30) and evolute (UWI ~0.50) with WER of ~ 1.75 in late growth stage. Whorl profile not imprinted dorsally, weakly depressed (WWI ~ 1.10) with rounded or slightly flattened venter and flanks. Phragmocone chambers with RCL ~ 0.25 in last whorl. Suture line straight on the venter or with a shallow ventral and lateral lobes. Shell on inner whorls ornamented with fine lirae, and in the last whorl with regularly spaced, narrow, rounded ribs or flat annuli. Etymology From ‘Aseri’, referring to the type horizon. Type material Holotype ESTONIA • Koila (Lääne-Viru County); Aseri Regional Stage (Darriwilian Stage, Middle Ordovician); Schmidt Coll.; illustrated in Fig. 20; GIT 225-1084. Description Holotype GIT 225-1084 is an adaperturally uncoiled phragmocone in the adult growth stage, with a total diameter of approximately 90 mm; the last 2.5 whorls are preserved (Fig. 20). The body chamber is preserved over a length of about 120 degrees and becomes uncoiled from the phragmocone after about 100 degrees (at a coiled diameter of ~ 82 mm). In the last half whorl, the conch is discoidal (CWI = 0.27) and evolute (UWI increases from 0.48 to 0.52), with a coiling rate (WER) of 1.76 at dm = 82 mm. The whorls are not imprinted dorsally; the whorl profile is weakly depressed to equidimensional (WWI = 1.05), appearing slightly flattened on the ventral side. The relative cameral length (RCL) is 0.25 in the last whorl, with the final phragmocone chambers shortened. The suture line exhibits shallow ventral and lateral lobes. The shell ornament is preserved on the inner whorls, where it consists of fine lirae, and locally on the body chamber, where it comprises rather regularly spaced, narrow, rounded ribs or annuli. Remarks With its occurrence in the Aseri Regional Stage, the holotype of Discoceras aseriense sp. nov. represents the stratigraphically oldest known representative of Discoceras from Baltica, predating the Lasnamägian D. depressum of Norway and the Uhakuan D. rarospirum of Estonia, if not globally. Among other species, the holotype of D. aseriense sp. nov. appears to be most similar in conch geometry and shell ornamentation to a late Darriwilian or early Sandbian specimen from Norway, which Sweet (1958: pl. 8 fig. 7) assigned to “ Discoceras arcuatum?”. However, it is difficult to compare the two based on the available information and the limited data on the internal whorls and siphuncle. Moreover, the validity of D. arcuatum has been questioned (Rasmussen & Surlyk 2012), as both type specimens were likely lost during World War II. Therefore, this matter remains unresolved until more complete specimens are available to allow proper taxonomic assignments. The other Darriwilian representatives of Discoceras differ from the holotype of D. aseriense sp. nov. more significantly. The holotype of D. depressum has a depressed whorl profile even in the late ontogenetic stage (WWI ~1.35). The representatives of D. rarospirum do not uncoil adaperturally. The specimens of D. boreale have shorter phragmocone chambers (RCL 0.14–0.20 vs 0.25), and the whorl profile becomes slightly compressed in later growth stages (WWI = 0.90). The other Darriwilian representatives of Discoceras differ from the holotype of D. aseriense sp. nov. more significantly. The holotype of D. depressum has a depressed whorl profile even in late ontogenetic stages (WWI ~ 1.35). The representatives of D. rarospirum do not uncoil adaperturally. The specimens of D. boreale have shorter phragmocone chambers (RCL 0.14–0.20 vs 0.25) and the whorl profile is becomes slightly compressed in growth (WWI = 0.90 in late ontogeny). Among the specimens studied herein, the holotype of D. aseriense sp. nov. is most similar to specimen MB.C.32328 (D. sp. indet.). However, at a diameter of 70 mm, the whorl profile in MB.C.32328 is weakly subquadrate with a convex venter and slightly flattened flanks. The shell ornamentation in the outer whorl of the holotype appears more distinctly and regularly developed. The two specimens are difficult to compare, however, due to the incomplete preservation of MB.C.32328 and the lack of information on the whorl profile and siphuncle in the holotype of D. aseriense. Additionally, the two specimens are significantly separated stratigraphically (Aseri vs Haljala Regional Stages), which is a further argument for keeping them separate until better-preserved material is available. Geographic and stratigraphic occurrence Northern Estonia; Aseri Regional Stage, Darriwilian Stage, Middle Ordovician.