Isorthoceras sp. B

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 : 51

Isorthoceras sp. B

Fig. 12B

Material examined

ESTONIA • 1 spec.; Moe trench ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 878-116 .

Description

Specimen GIT 878-116 is a ca 12 mm long internal mold of parts of a mature body chamber and three phragmocone chambers with a circular conch cross section. The maximum diameter of the specimen is 10 mm at its adoral end. At the base of the body chamber the diameter is 9.8 mm and the adapicalmost chamber has a diameter of 9.6 mm (angle of expansion 2.5°). The three preserved chambers of the phragmocone decrease in length from 2.4 mm to 1.5 mm, to 0.5 mm adorally. The septal foramen is well preserved at 1.7 mm from the conch margin, where the conch cross section is 9.6 mm in diameter (RSP = 0.18), with a diameter of 1.1 mm (RSH = 0.11). The conch surface is apparently smooth, although fine details of the ornamentation are not preserved.

Remarks

This specimen can be related to I. junceum ( Hall, 1847) with respect to the mature conch size, position of the siphuncle, and general conch shape. However, because the crucial details of the conch surface are not preserved, a species level determination is not possible.

Figures

Fig. 12 (next page). Orthocerida Kuhn, 1940 and Pseudorthocerida Flower & Caster, 1935 of the Vormsi– Pirgu regional stages, Estonia. A. Isorthoceras sp. A, specimen GIT 878-128, Sutlema quarry, Vormsi Regional Stage. B. Pleurorthoceras sp., specimen GIT 878-154, from Saxby shore, Vormsi Island, Vormsi Regional Stage. C. Gorbyoceras clathratoannulatum (Roemer, 1861) comb. nov., specimen GIT 878-305, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage. D–E. Gorbyoceras textumaraneum (Roemer, 1861). D. Specimen GIT 878-143 from Vohilaid shore, Vohilaid Island, Pirgu Regional Stage. E. GIT 878-223, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage. F. Isorthoceras sp. C, specimen GIT 878-49, Saxby shore, Vormsi Island, Vormsi Regional Stage. G. Isorthoceras sp. B, specimen GIT 878-116, from Moe trench, Vormsi Regional Stage. H. Gorbyoceras clathratoannulatum, specimen GIT 840-265, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage. I. Isorthoceras sp. D, specimen GIT 878-114, Moe trench, Vormsi Regional Stage. J. Gorbyoceras stumburi (Kröger, 2013), specimen TUG 939-30, from Harilaid, Pirgu Regional Stage. K. Palaeodawsonocerina? sp., specimen TUG 939-78, from Hosholm shore, Vormsi Island, Pirgu Regional Stage. L–M. Gorbyoceras textumaraneum. L. Specimen TUG 1745-193, from Mõnuste quarry, Vormsi Regional Stage, lateral view. M. Specimen GIT 878-306, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage. N. Gorbyoceras sp. A, specimen TUG 42-417, from Kohila, Vormsi Regional Stage, lateral view. Scale bars = 10 mm in all figures. Same scale in A–B, F–G, I, N, and in C–E, H, J, L.