Isorthoceras junceum ( Hall, 1847 )

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 63-64

Isorthoceras junceum ( Hall, 1847)

Figs 17E , 23C

Orthoceras junceum Hall, 1847: 204 , pl. 47; fig. 3a-f.

Geisonoceras junceum – Balashov 1956: 199, pl. 4, fig. 4a-b.

Isorthoceras? junceum – Kröger et al. 2011: 46, figs 4a, 9h-i (with synonymy).

Diagnosis

Transversally ornamented, straight Isorthoceras with circular cross section and comparatively low angle of expansion of 5°, chamber distance 0.2–0.3 of corresponding cross section; adult size ca. 10–11 mm in cross section; siphuncle subcentral-central with diameter ca. 0.1 of corresponding conch cross section. (Compiled from Hall 1847.)

Type locality and horizon

Watertown, New York, USA; Trenton Formation, early Katian, Late Ordovician.

Material

Two specimens ( PMU 26833, 26834) from Kallholn, Siljan District, Sweden; Boda Limestone, late Katian, Ordovician.

Description

Specimen PMU 26833 is a fragment of a body chamber 40 mm long, with a diameter increasing from 11 to 12.8 mm (angle of expansion 2.6°). At the adoral end of the specimen, the cast possesses a ca. 3 mm wide apertural constriction, which indicates a mature apertural shell thickening. The shell surface is ornamented with ca. ten fine rounded transverse striae per millimetre.

The second specimen is a 44 mm long portion of a phragmocone, with a diameter increasing from 6.2 to 8.8 mm (angle of expansion 3.4°) and a central thin siphuncle ( Figs 17E , 23C ).

Remarks

The two specimens are assigned to I. junceum based on the general similarity of the conch dimensions, the low angle of expansion, and the characteristic narrow transverse ornamentation. The adult conch diameter of specimen PMU 26833 is slightly larger than in other specimens of this species, which is interpreted as intraspecific variation.

The species was discussed in detail by Kröger et al. (2011). Herein the question mark that was positioned after the genus name by Kröger et al. (2011) is omitted, because a comparison of known species of Isorthoceras clearly indicates that this genus is characterized by the typical conch features that are listed above. The internal characters of the type of this species are still not known, but the external features are so similar to those of other species of the genus, that it justifies assigning O. junceum to Isorthoceras .

Comparison

Isorthoceras romingeri ( Foerste, 1932) is a finely transversely striated Isorthoceras from the Black River Formation of Minnesota, USA, which differs in having a larger adult size (more than 17 mm in cross section) and a narrower chamber spacing (seven chambers occur per distance similar to corresponding cross section).

Stratigraphic and geographic range

Trenton Limestone, early Katian, from New York, USA ( Hall 1847); Rocklandian-Edenian, mid Katian, from north central USA ( Catalani 1987); Martinsburg Shale, late Katian ( Bassler 1919); Utica Shale, late Katian ( Flower 1946); Anderskyi bed, southern Kazakhstan, late Katian ( Balashov 1956); Boda Limestone, late Katian-Glisstjärn Formation, Hirnantian, Siljan District, 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.