Beloitoceras sinuososeptatum ( Roemer, 1861 )

Kröger, Björn, 2025, The Lyckholm acme of cephalopods - Review of the late Katian (Vormsi-Pirgu regional stages) Ordovician cephalopods of Estonia, European Journal of Taxonomy 978, pp. 1-169 : 80-81

Beloitoceras sinuososeptatum ( Roemer, 1861)

Fig. 32A–B

Orthoceras sinuoso-septatum Roemer, 1861: pl. 6 fig. 3a–c.

Beloitoceras heterocurvatum Strand, 1934: 76, pl. 10 figs 8–9 (partim).

Richardsonoceras sinuososeptatum – Dzik 1984: 58, 188, text-fig. 16a–b, pl. 8 fig. 1a–d.

Beloitoceras heterocurvatum – Frye 1987: 84, fig. 3a–d. ― Kröger et al. 2011: 38, fig. 5a–b. Beloitoceras sinuososeptatum – Kröger 2013: 17–20 , figs 9a–b, 11.

Diagnosis

Beloitoceras with compressed cross section and keeled conch margins on concave and convex side of conch curvature throughout entire length; in longitudinal section, antisiphuncular conch margin concave; angle of expansion of phragmocone ca 16–17°; length of body chamber somewhat less than maximum conch height; conch surface ornamented with irregularly spaced, rounded striae, which form a shallow sinus on the prosiphuncular side and a deep V-shaped sinus on the antisiphuncular side; about six chambers similar to the height of the conch cross section (RCL = 0.17); sutures with deep lateral lobes; siphuncle situated near ventral margin, segments ca twice as long as high (SCR = 0.5), with slightly convex vertical outline. (From Kröger 2013.)

Material examined

ESTONIA • 4 specs ; Vormsi Island, Hosholm shore ; Adila Formation , Pirgu Regional Stage; GIT 426- 110 , GIT 840-248 , GIT 840-92 , GIT 840-251 • 3 specs; same data as for preceding; TUG 1743-76 , TUG 1745-10 , TUG 939-59 • 10 specs; Vormsi Island, Hosholm shore (tower locality); Adila Formation , Pirgu Regional Stage; GIT 426-2 , GIT 840-256 , GIT 840-267 , GIT 840-268 , GIT 878-237 to GIT 878- 239 , GIT 878-241 to GIT 878-243 • 2 specs; Sutlepa quarry ; Adila Formation , Pirgu Regional Stage; TAM G5:9 , TAM G149:127 • 8 specs; Vardi manor ( Schwarzen ); Adila Formation , Pirgu Regional Stage; TAM G5:1 to TAM G5:8 • 7 specs; Vohilaid Island, Vohilaid shore (E); Adila Formation , Pirgu Regional Stage; GIT 878-144 to GIT 878-146 , GIT 878-151 , GIT 878-152 , GIT 878-240 , GIT 878-244 • 1 spec.; same data as for preceding; TUG 1723-26 .

Type locality and horizon

Zawidowice by Oleśnica, Poland; erratic boulder, Vormsi–Pirgu regional stages (“Lyckholm Stufe” of Teichert 1930).

Remarks

The species has been revised and described in detail by Kröger (2013). The Estonian material adds to the knowledge of the variability of this species. Combined with the total dataset of 32 measurements, derived from the types from erratic boulders in Poland ( Dzik 1984), the specimens known from Norway ( Strand 1934), and Sweden ( Kröger 2013), a decreasing angle of expansion is now evident for this species with ca 15°–20° in early growth stages and angles lower than 10° in nearly mature phragmocones ( Fig. 33A ). Conversely, the CHI increases during ontogeny (note, however, the possible taphonomic compression of some of the Estonian and Norwegian specimens; Fig. 33B ). As with the Swedish material, the variability of the curvature of the conch varies quite strongly among specimens but distinct groups of weakly and strongly curved conchs could not be identified (compare, e.g., specimens TUG 939-59 and TUG 1723- 26, Fig. 32A–B ). The largest known specimen has a height at the base of the mature body chamber of 44 mm (TUG 939-59, Fig. 32A ).

Figures

Fig. 32. Oncoceratidae Hyatt, 1884 of the Vormsi–Pirgu regional stages, Estonia. A–B. Beloitoceras sinuososeptatum (Roemer, 1861). A. Specimen TUG 939-56, from Haapsalu holm, Pirgu Regional Stage, lateral view. B. Specimen TUG 1723-26, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage, lateral view. C. Beloitoceras cf. sinuososeptatum (Roemer, 1861), specimen GIT 878-231, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage. D. Cyrtorizoceras sp. A., specimen GIT 878-260, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage, lateral view.E–F. Cyrtorizoceras hariense sp. nov., holotype GIT 878-71, from Saxby shore (N), Vormsi Island, Estonia, Vormsi Regional Stage. E. View of prosiphuncular side. F. Lateral view. Scale bar = 20 mm, same scale in all figures.

Fig. 33. Diagrams of morphological variability of specimens of Beloitoceras sinuososeptatum (Roemer, 1861). A. Angle of expansion. B. Conch width index (CHI). Explanation of symbols: White circles = B. sinuososeptatum Estonian specimens, Vormsi–Pirgu regional stages (herein); black circles = B. sinuososeptatum Boda Limestone, Sweden (Kröger 2013); star = type B. sinuososeptatum in Dzik (1984); cross = types B. sinuososeptatum in Strand (1934); triangle = B. cf. sinuososeptatum (herein). See text for discussion.