Isorthoceras curvilineatum, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 57-58

Isorthoceras curvilineatum sp. nov.

urn:lsid:zoobank.org:act:7B1F09EA-8B80-44EC-8F80-7E0EDD3F4E3D

Figs 17G , 23F

Diagnosis

Transversally ornamented, straight Isorthoceras with circular cross section and low angle of expansion of less than 2°. Ornamented with ca. ten rounded transverse striae per millimetre, which form characteristic shallow hyponomic sinus with two directly adjacent shallow salients. Adult size clearly less than ca. 10 mm in cross section. Siphuncle central with diameter ca. 0.1 of conch cross section.

Etymology

Refers to the conspicuously curved transverse ornamentation of this species.

Type material

Holotype

PMU 26787 .

Paratypes

Two paratypes: PMU 26788 and 26789, Osmundsberget, Siljan District, Sweden; Boda Limestone, Boda Core Member, late Katian, Ordovician.

Type locality and horizon

Osmundsberget, Siljan District, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician.

Description

Three specimen from Osmundsberget (PMU 26787–26789) are very similar to each other, with a very low angle of expansion of less than 2°, conch diameters between 6 and 8 mm and a very fine transverse ornamentation, with ca. ten rounded striae per millimetre ( Fig. 17G ). The striae are laterally slightly oblique, and form a shallow hyponomic sinus with two directly adjacent shallow projections on the left and right side.

The holotype is a fragment of parts of a phragmocone and body chamber with a length of 35 mm and a diameter of 6.2–7.2 mm (angle of expansion 1.6°) ( Fig. 23F ). The siphuncle is central, with a diameter of the septal perforation of 0.6 mm at the apical end of the fragment. The length of the body chamber cannot be measured, because no traces of septa are visible in the middle parts of the holotype.

Remarks

A very small adult size of less than 10 mm is assumed for this species, based on the very low angle of expansion, which is only 0.7° in the largest specimen, with a maximum diameter of 7.5 mm.

This species is assigned within Isorthoceras because the general conch shape is very similar to other small species of Isorthoceras , such I. angelini sp. nov. However, some uncertainty remains in the generic assignment, because the internal characters are not known. A similar narrow, transverse ornamentation

is known from Isorthoceras elongatocinctum ( Portlock, 1843) comb. nov., which differs in having a directly transverse striation.

Stratigraphic and geographic range

Boda Limestone, late Katian, Dalarna, Sweden.

Figures

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.

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.