Gorbyoceras, Shimizu & Obata, 1935

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 53

Gorbyoceras sp. A

Figs 16C , 17C

Material examined

Specimen PMU 26772 from Osmundsberget, Siljan District, Sweden; Boda Limestone, Boda Core Member, late Katian.

Description

A fragment of a conch ca. 22 mm long, with a diameter increasing from 22 to 25 mm (angle of expansion ca. 6°). No internal structures are preserved ( Fig. 16C ). The shell is strongly annulated and exhibits four annuli that show varying amplitudes (maximum amplitude of 1.5 mm). About 16 pronounced primary lirae are present around the circumference; between each pair, three to four less pronounced, irregularly spaced secondary striae occur ( Fig. 17C ). About ten transverse lirae are present in a length equal to one cycle of annulation.

Remarks

This small annulated fragment with its 16 primary and more than 48 secondary longitudinal lirae is uniquely ornamented; no Ordovician orthocerid with a similar ornamentation is known. The ornamentation is most similar to species of Gorbyoceras and the specimen is interpreted as a new species of Gorbyoceras . However, this poorly preserved specimen does not provide sufficient data to justify the erection of a new species.

Figures

Fig. 16. Dawsonoceratidae and Proteoceratidae of the Boda Limestone. A-B. Dawsonoceras fenestratum (Eichwald, 1860), Kallholn. A. PMU 26702, nearly mature specimen with adorally decreasing angle of expansion. B. PMU 26712, juvenile specimen. C. Gorbyoceras sp. A, PMU 26772, Osmundsberget. D. Goryboceras sp. B, PMU 26773, Kallholn. E. Dawsonoceras stumburi sp. nov., PMU 26717, holotype, Kallholn. F. Gorbyoceras alternestriatum (Strand, 1934) comb. nov., PMU 26771, Unskarsheden. Scale bars: A-C, E = 10 mm; D = 10 mm; F = 10 mm.

Fig. 17. Shell ornamentation of selected Orthocerida of the Boda Formation. A. Isorthoceras cf. elongatocinctum (Portlock, 1843), PMU 26825, Kallholn, adoral part of the specimen. B. Dawsonoceras stumburi sp. nov., PMU 26717, holotype, Kallholn. C. Gorbyoceras sp. A, PMU 26772, Osmundsberget. D. Isorthoceras cf. elongatocinctum (Portlock, 1843), same specimen as in A, adapical part of specimen. E. Isorthoceras junceum (Hall, 1847), PMU 26834, Kallholn. F. Gorbyoceras alternestriatum (Strand, 1934) comb. nov., PMU 26771, Unskarsheden. G. Isorthoceras curvilineatum sp. nov., PMU 26787, holotype, Osmundsberget. H. Isorthoceras angelini sp. nov., PMU 26774, holotype, Kallholn. I. Isorthoceras suave (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26835, Kallholn. Scale bar = 5 mm for all figures.