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.