Gorbyoceras stumburi ( Kröger, 2013 )

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 : 42-43

Gorbyoceras stumburi ( Kröger, 2013)

Figs 12J , 18

Dawsonoceras stumburi Kröger, 2013: 32–34 , figs 16e, 17b.

Dawsonoceras sp. – Kröger & Isakar 2006: 157, figs 10e, j, 12h.

Diagnosis

Annulated orthocones with angle of expansion of ca 5° and circular cross section; ca five relatively shallow annulations occur in a distance similar to conch diameter; approximately 40 longitudinal lirae occur around shell circumference, more than 10 pronounced transverse striae occur per cycle of annulations; septal necks achoanitic; siphuncle subcentral, fusiform, expanded within chambers. (Adopted from Kröger 2013.)

Material examined

ESTONIA • 1 spec.; Vormsi Island, Hosholm shore ; Adila Formation , Pirgu Regional Stage; TUG 1745- 47 • 1 spec.; Harilaid; Vormsi – Pirgu regional stages; TUG 939-30 • 1 spec.; Vormsi Island, Saxby shore ; Kõrgessaare Formation , Vormsi Regional Stage; TUG 939-21 .

Type locality and horizon

Kallholn, Siljan District, Sweden; Boda Limestone, Boda Core Member, late Katian, Ordovician.

Description

Specimen TUG 1745-47 is a slightly curved fragment of a mature body chamber with a length of ca 35 mm and a maximum diameter of 20 mm. Approximately five annulations occur in a distance similar to the conch diameter. The surface is ornamented with ca 40 longitudinal lirae around the circumference and fine growth lines. At its base, the fragment has a diameter of 19 mm and a chamber of the phragmocone is preserved with two septa which are 5 mm distant. The septa are located in the troughs of the respective annulations and are directly transverse, parallel to the annulations. The septal foramen is poorly preserved but apparently slightly eccentric in position and located toward the convex side of the conch curvature.

Specimen TUG 939-30 ( Fig. 12J ) is a nearly tubular fragment of a body chamber with relatively well-preserved conch surface showing ca 40 longitudinal lirae across the circumference and fine transverse lirae or raised growth lines (ca 10–12 per cycle of annulation). The spacing of the annulation is relatively narrow with 5–6 cycles of annulations per distance similar to the corresponding conch cross section.

Remarks

No new data on the internal characters of this species can be added with the Estonian material. The species was originally placed within Dawsonoceras . However, based on the similarity of the known characters from this species with other species of Gorbyoceras and following the more restricted genus diagnosis of Dawsonoceras of Horny (1956), followed herein (see above), a placement within Gorbyoceras is suggested, herein.

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.