Siljanoceras, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 82

Siljanoceras sp. A.

Figs 36 A-B, 38

Material

One specimen ( PMU 26917), Kallholn, Siljan District, Sweden; Boda Limestone, Boda Core Member, late Katian.

Description

Specimen PMU 26917 is a mildly deformed portion of a slightly curved phragmocone, 23 mm long with a conch height of ca. 22–40 mm and a conch width of ca. 20–40 mm (angle of expansion ca. 38°) ( Fig. 36A, B ). The original shape of the conch cross section is difficult to evaluate, but was probably slightly depressed. The conch surface is poorly preserved and no signs of ornamentation are visible. The sutures are nearly transverse and have a distance of ca. 0.17 of the corresponding conch cross section.

The position of the septal perforation is eccentric (SPR 0.4). The siphuncular segments are barrel-shaped and widely expanded with maximum diameter of ca. 0.18 of the corresponding conch cross section. The septal necks are achoanitic.

Remarks

This fragmentarily preserved specimen differs from Siljanoceras varians gen. et sp. nov. in having a less curved, more expanded conch with a less eccentric siphuncle, that appears slightly more expanded within the shell. The general conch form is similar to that of Siljanoceras gen. nov. and it is therefore assigned to this genus. However, a species determination is impossible based on this imperfectly known specimen.

Figures

Fig. 36. Polished median sections of Siljanoceras gen. nov., Boda Limestone. A. Detail of siphuncular segment of Siljanoceras sp. A, PMU 26917, Kallholn. B. Same specimen, total view. C. Siljanoceras varians sp. nov., PMU 26915, Kallholn; detail of apical-most siphuncular segment, section not perfectly in median plane, with adapical septal perforation not preserved. D. Same specimen, total view. Scale bars: A, C = 1 mm; B, D = 5 mm.