Richardsonoceras priscum Kröger 2025

Richardsonoceras priscum (Eichwald, 1860) comb. nov. Fig. 35C–D Cyrtoceras priscum Eichwald, 1860: 1285, pl. 47 fig. 10a–c. Beloitoceras (?) estonicum Teichert, 1930: 292, pl. 6 figs 11–12. Richardsonoceras nikiforovae Balashov, 1962b: 114–115, 128, pl. 46 fig. 5a–b. Richardsonoceras nikiforovae – Zhuravlyeva 1962: pl. 23 fig. 4. “ Richardsonoceras ” nikiforovae – Dzik 1984: 63, 67, text-fig. 18.34. Oonoceras priscum – Dzik & Kiselev 1995: 66, fig. 2 (non fig. 1f–g, fig. 3c–d). Emended diagnosis Richardsonoceras with a relatively weak conch curvature, with an angle of expansion of ca 8°; oval compressed conch cross section with CHI of 1.2–1.6 which increases with conch height; mature conch height ca 18 mm. Material examined ESTONIA • 13 specs; Hiiumaa Island, Kõrgessaare quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-129, GIT 878-130, GIT 426-387, GIT 426-388, GIT 426-390, GIT 426-397 to GIT 426- 399, GIT 426-428, GIT 426-554, GIT 426-607 to GIT 426-609. Type locality and horizon Kõrgessaare quarry, Hiiumaa island, Estonia; Kõrgessaare Formation, Vormsi Regional Stage (Lyckholm Stage of Teichert 1930). Description The most complete specimen (GIT 426-608, Fig. 35C) is a deformed (compressed), curved fragment of a mature body chamber, and possessing at least ten chambers of the phragmocone. The phragmocone increases in height from 11.5 mm to 18 mm (angle of expansion of 8°). The conch height at the base of the body chamber is 14 mm and its preserved length ca 35 mm. The peristome is not preserved. In lateral view, the dorsal and ventral conch margins are concave and convex, respectively, throughout the entire length of the body chamber. The adoralmost part of the body chamber appears to be less curved than the rest of the specimen. The sutures form wide lateral lobes and distinct and sharp saddles on the prosiphuncular side. They are 2 mm apart where the conch height is 12 mm (RCL = 0.17). The two adoralmost septa are crowded. Traces of a narrow siphuncle (0.7 mm in width) are preserved near the conch margin. The siphuncular segments expand slightly into the chambers. The oval compressed conch cross section is well-preserved in specimen GIT 426-399, with a CHI of 1.3 at a conch height of 13 mm. There, the conch margin on the prosiphuncular side is narrower rounded than on the antisiphuncular side. The CHI increases during ontogeny from ca 1.2 at a conch height of 10 mm (specimen GIT 878-129) to 1.5 at conch a height of 18 mm (specimen GIT 426-608). The mean angle of expansion is 10° (range: 6°–15°, n = 5). The high variability results from deformation during the diagenesis of the specimens. The mean RCL is 0.13 (range: 0.1–0.15, n = 6). Remarks The material described herein provides for the first time a comprehensive overview of this species, which has a complicated history. The original description of Cyrtoceras priscum Eichwald, 1860 is based on a specimen from near Kõrgessaare, Hiiumaa Island (Eichwald 1860: 1285). An original from the Eichwald collection was figured in Dzik & Kiselev (1995: fig. 2) and designated as the lectotype of “ Oonoceras priscum (Eichwald, 1861) ” (sic). Earlier, Teichert (1930) had designated a small fragment, also from Kõrgessaare, Hiiumaa Island, as type of his new species Beloitoceras (?) estonicum Teichert, 1930 noting its similarity to C. priscum (Teichert 1930: 271). However, based on the single, fragmentary type of B. (?) estonicum and the single fragment available from the Eichwald collection in St Petersburg, he could only speculate regarding a possible synonymy of the two species. The additional material described above comes exclusively from the Kõrgessaare Formation, Vormsi Stage of Kõrgessaare, the type locality of C. priscum and B. (?) estonicum. The new material shows that B. (?) estonicum must be considered a junior synonym of C. priscum, and that this species is best placed within Richardsonoceras. The placement with Richardsonoceras is preferred over Beloitoceras because of the slender, non-gibbous or only very weakly gibbous mature body chamber of this species. It is now clear that this species is identical in size and shape to R. nikiforovae Balashov, 1962b; described from the Dolborian Regional Stage, Katian, of the Siberian platform and which should be considered as a subjective junior synonym of C. priscum. The complication arises from a misinterpretation of the stratigraphical horizon from which Eichwald’s type originated by Dzik & Kiselev (1995: 66). They literally translated Eichwald’s “calcaire à Orthocératites” as the “ Orthoceras limestone” of later authors, which is late Darriwilian in age. However, Eichwald (1860) generally termed Ordovician limestone strata as “calcaire à Orthocératites”, as is evident from his descriptions of other specimens collected from Late Ordovician strata. There is no evidence that the Kõrgessaare specimens are from glacial boulders of Darrwilian age, and the specimens described above are clearly from a Kõrgessaare Formation lithology. This stratigraphic misinterpretation probably led Dzik & Kiselev (1995) to synonymize a specimen figured in Dzik (1984: text-fig. 15a, pl. 9 fig. 1), which significantly differs from C. priscum in having a lower angle of expansion, a wider chamber spacing and a nearly circular conch cross section and thus represents a different species. Hence, C. priscum is a genuine late Katian Richardsonoceras, presently known only from Kõrgessaare, Hiiumaa Island. Comparison Several specimens are deformed and / or are only fragmentarily preserved, making a definite determination of the conch curvature difficult. However, three comparatively well-preserved specimens (GIT 426-607, -608, GIT 878-129) reveal the relatively low degree of curvature of the conch compared with other species of Richardsonoceras. The species of Richardsonoceras described from the Black River Group (Foerste 1932, 1933) have a stronger conch curvature. All, except R. simplex (Billings, 1857), also have a larger mature conch height (> 20 mm). The mature size of R. bellatulum Sweet & Miller, 1957 from the Cape Phillips Formation, Cornwallis islands, Canada, is similar than that of R. estonicum, but the Cape Phillips species is also more strongly curved (see also Balashov 1962b: 115).

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