Geisonoceras, Hyatt, 1884

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 46

Geisonoceras ? sp.

Fig. 23H

Material

PMU 26758, from Kallholn, Siljan District, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician.

Description

The specimen represents the juvenile part of a phragmocone 45 mm long, having a diameter of 5–8.3 mm (angle of expansion 4.4°), and a circular conch cross section. The shell is ornamented with obliquely transverse adorally imbricated striae ( Fig. 23H ). Adorally the conch widens at each stria, which results in an annulated pattern with a sharp ridge at the adoral end of each of them. The distance between two successive striae varies widely from 0.3 to 1.5 mm. At the adoral end a trace of a healed predation additionally causes irregularities in the distance of the transverse elements.

Remarks

This small specimen might represent the juvenile stage of Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880) comb. nov. Only relatively large fragments of G. wegelini are known. These fragments are ornamented with obliquely transverse bands with highly irregular spacing. The pattern of ornamentation of G. wegelini comb. nov. resembles that of specimen PMU 26758, but is shallower and not imbricated. Both forms are interpreted as separate species, because intermediate growth stages are not available for further study.

Comparison

The specimen PMU 26758 differs from known juvenile specimen of Tyrioceras subannulare ( Portlock, 1843) (see Evans 1993) and from Imbricatoceras ascendens ( Blake, 1882) (see Holland 1993) in having a strongly irregularly spaced transverse ornamentation. Known species of Thyriocerias differ in having a curved and more regularly spaced transverse ornamentation.

Figures

Fig. 23. Orthocerida of the Boda Limestone. A. Pleurorthoceras osmundsbergense sp. nov., NRM-PZ Mo 150126, holotype, Osmundsberget. B. Isorthoceras suave (Angelin inAngelin & Lindström, 1880) comb. nov.,PMU26835,Kallholn.C.Isorthocerasjunceum(Hall,1847),PMU26833,Kallholn.D-E.Isorthoceras cf. elongatocinctum (Portlock, 1843), Kallholn. D. PMU 26814, juvenile growth stage. E. PMU 26825, larger specimen with irregularly spaced transverse ornamentation. F. Isorthoceras curvilineatum sp. nov., PMU26787,holotype,Osmundsberget.G.Isorthocerasangelinisp.nov.,PMU26774,holotype,Kallholn. H. Geisonoceras? sp., PMU 26758, Kallholn. I. Isorthoceras heroyense (Strand, 1934) comb. nov., PMU 26831, Kallholn. Scale bars: A = 10 mm; B-I = 10 mm.