Striatocycloceras cf. hosholmense, Kröger, 2025

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 : 28-30

Striatocycloceras cf. hosholmense

Figs 11F, 13C

Material examined

ESTONIA • 3 specs; Vormsi Island, Hosholm shore; Adila Formation, Pirgu Regional Stage; TUG 899- 81, TUG 939-20, TUG 1745-233.

Description

Specimen TUG 939-20 is a fragment of a phragmocone, 38 mm long, with diameter 23–27 mm (apical angle 8.5°), and four chambers and annulations ( Fig. 13C ). The characters of the outer shell are not preserved. The annulations are directly transverse with deep troughs and sharply accentuated crests. The sutures are parallel to and positioned in the troughs of the annulations. The siphuncle is nearly centrally positioned and slightly expanded within the chambers. It is ca 2.5 mm wide at position of the septal foramen and 3.5 mm wide at mid-way between septa. The septal necks are orthochoanitic, 1.3 mm long (Fig. 11F). No cameral deposits are developed. Two additional specimens are in the Estonian collections which are similar in having a sharply crested annulations, ca three annulations per distance equal to the corresponding conch diameter and an apical angle of ca 10°.

Remarks

The specimens described as Striatocycloceras cf. hosholmense are in all aspect similar to Striatocycloceras hosholmense sp. nov. expect in having a distinctly larger apical angle at comparable conch diameters. However, none of the specimens has its outer shell preserved, making a comparison somewhat questionable. More material is needed to establish this group of specimens as a distinct species. Notably,

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.

a similar pair of species A. anellus and A. paquettense with low and high apical angles are known from Katian strata of North America ( Foerste 1932: 104). This could indicate the presence of sexual dimorphism in this genus.

Figures

Fig. 13. Annulated Orthocerida of the Vormsi–Pirgu regional stages, Estonia. A. Palaeodawsonocerina senckenbergi (Teichert, 1930), specimen TUG 939-81, from Saxby shore (N), Vormsi Island, Vormsi Regional Stage. B, D. Striatocycloceras hosholmense sp. nov., holotype GIT 878-297, from Hosholm shore, Vormsi Island, Pirgu Regional Stage. C. Striatocycloceras cf. hosholmense, specimen TUG 939- 20, from Hosholm shore, Vormsi Island, Pirgu Regional Stage. E. Striatocycloceras sp., specimen TUG 42-414, from Saaremõisa (Lyckholm), Vormsi Regional Stage. Scale bars = 10 mm for all figures, same scale in A, C–E.