Discoceras ievense

Discoceras ievense (Balashov, 1953) Figs 6, 9; Table 5 Schroederoceras ievense Balashov, 1953: 259, pl. 9 fig. 1. Discoceras ievesense – Sweet 1958: 112, pl. 12 figs 2–3, text-fig. 13h — Dzik 1984: 44. Rectanguloceras ievense – Stumbur 1962: 137, 142. Discoceras ievense – Kröger & Aubrechtová 2018: 809, 811; 2019: 966. Diagnosis (after Balashov 1953; Sweet 1958; emended) Species of the genus Discoceras with adult sizes of about 110 mm; up to four tightly coiled whorls; last whorl discoidal with CWI ~0.30 and WER ah ~ 2.00. Whorl profile weakly imprinted dorsally, equidimensional (WWI ~ 1.00), convex in first two whorls but flattened laterally and ventrally (subquadrate) later in ontogeny. Suture line almost straight with broad and shallow ventral and lateral lobe. Shell ornamented with lirae on inner whorls, fine annuli are added on the last whorl, which are most pronounced on flanks. Siphuncle subdorsal with diameter ~ 0.20 of whorl height in late growth stages. Material examined ESTONIA • 1 spec.; Idavere; early Haljala Regional Stage (Sandbian Stage, Late Ordovician); Mark Coll; illustrated in Fig. 9; GIT 426-370. Description Specimen GIT 426-370 is a tightly coiled conch in the subadult growth stage, with a maximum diameter of 66 mm (Fig. 9); a little over one whorl is preserved (minimum diameter ~ 30 mm). At a conch diameter of 54 mm, it is subevolute (UWI = 0.39) and discoidal (CWI = 0.31), with a high coiling rate (WER ah = 2.14). The conch is worn on one side, but it can be recognised that at smaller diameters, the whorl profile is very slightly compressed or equidimensional (WWI close to one), nearly square-shaped with flattened venter and flanks and very weakly embracing (IZR close to zero). In ontogenetically earlier stages of the conch, the flanks are more rounded, but the venter remains flattened; in the earliest visible whorl profile (at dm ~ 30 mm), the venter also appears rounded, with the whorl profile transitioning from nearly circular to nearly square-shaped during ontogeny. The shell ornament is preserved only in small remnants of the shell wall or as faint imprints on the internal mould of the outer whorl; it consists of irregularly thick lirae and/or low annuli. The siphuncle is positioned near the dorsal shell wall but is not in contact with it (subdorsal) and has a relative septal foramen height (RSH) of about 0.20. The septa are closely spaced with the RCL ranging between 0.15 and 0.20 in the outer whorl; the suture line is nearly straight, with only a very shallow lateral and external lobe. Remarks The holotype of Discoceras ievense was not accessible for study herein. It is the specimen illustrated by Balashov (1953: pl. 9 fig. 1), reportedly deposited under coll. no. 40 at the St. Petersburg State University. The holotype originates from the late Haljala Regional Stage (Sandbian Stage, Late Ordovician) of the Tallinn Region of Estonia. Discoceras ievense is readily recognised from all other representatives of the genus by the square-shaped whorl profile with clearly flattened lateral and ventral sides in the outer two whorls and the relatively wide (RSH ~ 0.20), subdorsal siphuncle. The most similar species are D. rarospirum and D. vesenbergense from Estonia. In the late Darriwilian to Katian D. rarospirum, the whorl profile transitions from circular to trapezoidal (with wider dorsal side) and finally square-shaped, but the flanks of the conch remain rounded (Balashov 1953; Kröger & Aubrechtová 2018, 2019). In the Katian (Rakvere Regional Stage) D. vesenbergense, the whorl profile has a flattened venter and flanks, but it is depressed, with the WWI decreasing from 1.35 to 1.08 (Balashov 1953; Kröger & Aubrechtová 2018, 2019). Geographic and stratigraphic occurrence Northern Estonia and Norway (Oslo-Asker and Nes-Hamar districts); Sandbian Stage, Late Ordovician.