Beloitoceras siljanense Frye, 1987

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

Beloitoceras siljanense Frye, 1987

Figs 29C , 30A

Beloitoceras siljanense Frye, 1987: 86–88 , figs 3e–f, 4, 6c.

Beloitoceras siljanense – Kröger 2013: 20, figs 9c–d, 11, tab. 2. ― Pohle et al. 2022: fig. 2.

Diagnosis

Beloitoceras with compressed cross section; in longitudinal section, antisiphuncular conch margin nearly straight approaching mature peristome; angle of expansion of phragmocone typically more than 17°; mature conch with maximum height and width at peristome. (Adopted from Frye 1987.)

Material examined

ESTONIA • 1 spec.; Haapsalu holm; Adila Formation , Pirgu Regional Stage; TUG 939-56 .

Type locality and horizon

Kallholn, Dalarna, Sweden; Boda Limestone, late Katian.

Description

The specimen is a fragment of a phragmocone with a conch height that increases from 20 mm to 29 mm and respective conch width increasing from 13 mm to 21 mm (angle of expansion = 18°). The conch cross section is oval compressed with narrower prosiphuncular side (CHI: 1.4–1.5). The conch surface is not preserved. The sutures form shallow lateral lobes and rounded saddles on the venter and dorsum. Mid-laterally, the sutures are 3.3 mm apart where the conch height is 24 mm (RCL = 0.14). The septal necks are cyrtochoanitic. The siphuncle is relatively narrow, and slightly expanded within the chambers ( Fig. 29C )

Remarks

This specimen can be identified as B. siljanense because of the characteristic straightened dorsal conch margin as well as of the relatively large angle of expansion.

Figures

Fig. 29. Median sections of phragmocones of Graciloceratidae Flower, 1950 and Oncoceratidae Hyatt, 1884 of the Vormsi–Pirgu regional stages, Estonia. A. Hiiumoceras hiiuense gen et sp. nov., holotype TUG 1745-11, from Paope quarry, Hiiumaa Island, Vormsi Regional Stage. B. Rizosceras teres sp. nov., paratype TUG 80-508, from Saxby shore, Vormsi Island, Vormsi Regional Stage. C. Beloitoceras siljanense Frye, 1987, specimen TUG 939-56, from Haapsalu holm, Pirgu Regional Stage. D. Dalecarlioceras constrictum Frye, 1987, specimen GIT 878-248, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage. Scale bars = 5 mm.

Fig. 30. Graciloceratidae Flower, 1950 and Oncoceratidae Hyatt, 1884 of the Vormsi–Pirgu regional stages, Estonia. A. Beloitoceras siljanense Frye, 1987, specimen TUG 939-56 from Haapsalu holm, Pirgu Regional Stage, lateral view. B. Rizosceras teres sp. nov., paratype GIT 426-550, from Kõrgessaare quarry, Vormsi Regional Stage, lateral view. C. Piersaloceras (?) sp., specimen TUG 1827-163, from Sutlema old quarry, Vormsi Regional Stage, lateral view. Scale bar = 20 mm, same scale in all figures.