Gorbyoceras Shimizu & Obata 1935

Gorbyoceras sp. B.

Fig. 16D

Material examined

One specimen (PMU 26773), Kallholn, Siljan District, Sweden; Boda Limestone, Boda Core Member, late Katian.

Description

Specimen PMU 26773 is a diagenetically deformed and fragmented part of a body chamber and phragmocone 52 mm long and 43 mm in diameter (Fig. 16D). Apically a relatively deep septum is preserved (depth of curvature 8 mm, 0.19 of conch height). The position and size of the siphuncle cannot be determined.

The shell is annulated, with distance between annuli of ca. 11 mm (0.26 of conch cross section, ca. five annuli per distance equal to conch diameter). The shell is 2.1 mm thick at the culminations of the annuli, and 0.5 mm thick on the floors in between.As a consequence of the zones with thickened shell, the surface of the shell is strongly annulated with sharply rounded ridges, while the internal mould is only weakly annulated with much shallower ridges. The shell surface is ornamented with longitudinal lirae, but the preservation of the shell is too poor to conserve the details of the pattern of the ornamentation.

Remarks

This strongly annulated fragment displays a longitudinal ornamentation which is characteristic of Gorbyoceras. However, because details of the ornamentation, such as the number of primary and secondary lirae, are not preserved a species level determination is impossible. This specimen differs from known late Ordovician species of Baltoscandia in the exceptionally high amplitude of the annulations combined with the large diameter of the conch.