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

Discoceras angustum sp. nov. urn:lsid:zoobank.org:act: A67ACC33-C790-4987-A16C-FBE9797AC8EE Fig. 22 Diagnosis Species of the genus Discoceras with adult conch diameter of 70 mm or more; conch extremely discoidal (CWI ~ 0.30) and evolute (UWI ~0.45); whorls weakly imprinted dorsally or not imprinted (IZR close to zero); whorl profile changes from nearly equidimensional (WWI ~1) to weakly depressed (WWI ~ 1.10) with convex flanks and slightly flattened venter; whorl expansion rate increases up to WER ~2.3 at ~ 50 mm conch diameter; relative cameral length (RCL) of 0.14 in the last whorl; shell ornament with low and irregularly thick transverse ribs; fine spiral elements in early growth stage. Etymology From the Latin ‘angustum’ (adj., neut.) = ‘narrow’, referring to the remarkably short phragmocone chambers. Type material Holotype ESTONIA • Rakvere (Lääne-Viru County); Keila Regional Stage (late Sandbian or early Katian Stage, Late Ordovician); illustrated in Fig. 22; TUG 2-731. Description Holotype TUG 2-731 is an internal mould of a conch in the subadult or adult growth stage, embedded in the rock matrix on one side (Fig. 22). Internal structures are almost entirely destroyed, but some whorl profiles can be partially reconstructed. The conch has a diameter of 70 mm, with two outer volutions visible. In the last 1.5 whorls, the whorl profile transitions from nearly equidimensional (WWI = 0.97 at whorl height of 9 mm) to weakly depressed (WWI = 1.12 at a whorl height of 21 mm), subquadrate in shape, with convex flanks and an apparently slightly flattened venter. The coiling rate (WER) increases from 1.87 to 2.28, while the conch width index (CWI) increases from 0.27 to 0.33. At its maximum diameter, the conch is evolute (UWI = 0.45). At a whorl width of 9 mm, the siphuncle is positioned close to the dorsal shell wall and has a relative septal foramen height (RSH) of 0.18. The phragmocone chambers are markedly short, with a relative cameral length (RCL) of about 0.14 of the respective whorl height in the last whorl. The suture lines form shallow lateral and ventral lobes. The internal mould shows traces of strongly oblique, low, and irregularly thick transverse ornament elements. Additionally, fine spiral elements are discernible on the flank of the innermost preserved whorl. Remarks The flanks of the outer volution of the conch in specimen TUG 2-731 have a shallow impression with a series of thin grooves, measuring up to 6 mm in total width. Similar structures have been described in coiled cephalopods before and interpreted as evidence of conch damage during earlier ontogeny (Manda & Turek 2018; Aubrechtová & Korn 2025), as a colour band (Kröger & Aubrechtová 2019), or as a type of ornament produced by the elevation of costae (Kröger 2025). However, the shell wall is not preserved in specimen TUG 2-731, and it is unclear how this lateral band manifests on the shell surface. Therefore, further interpretation is avoided here. It appears that the ontogenetic trajectories of several characters in TUG 2-731 are rather unique. The whorl profile becomes more depressed, and the expansion rate increases during growth. The latter may indicate that the specimen does not yet represent the adult growth stage. Specimen TUG 2-731 differs from all other representatives of Discoceras with subquadrate or weakly depressed whorl profiles, such as D. vesenbergense and D. roemeri, in possessing very short phragmocone chambers and fine spiral ornamentation in the early growth stage. Geographic and stratigraphic occurrence Northern Estonia; Keila Regional Stage, late Sandbian or early Katian Stage, Late Ordovician.