Isorthoceras heroyense ( Strand, 1934 ), 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 62-63

Isorthoceras heroyense ( Strand, 1934) comb. nov.

Figs 23I , 28A

Geisonoceras herØyense Strand, 1934: 15 , pl. 3; fig. 16.

Diagnosis

Transversally ornamented, straight Isorthoceras with circular cross section and comparatively low angle of expansion of ca. 5°, chamber distance 0.5 of corresponding cross section; transverse ornamentation consists of rounded, sharp, raised lirae, with distance of ca. six per millimetre; siphuncle eccentric with diameter ca. 0.13 of conch cross section. (Adopted from Strand 1934.)

Type locality and horizon

Herøya near Porsgrunn, Norway; Herøya Formation (“Gastropod Limestone”), late Katian, Late Ordovician.

Material

Two specimens ( PMU 26831, 26832) from Kallholn, Siljan District, Sweden; Boda Limestone, Boda Core Member, late Katian, Ordovician.

Description

Specimen PMU 26831 is a fragment of a body chamber with a length of 35 mm and a diameter of 9.2–12 mm (angle of expansion 4.5°) ( Figs 23I , 28A ). The basal septum of the body chamber is preserved, with a convexity of ca. 3 mm and a septal perforation with a diameter of 1.5 mm (0.16 of conch cross section). The siphuncle is positioned 2.3 mm from the conch margin (SPR 0.25). The conch is ornamented with fine transverse lirae, ca. six in a distance of one millimetre, which are slightly oblique, projecting adorally on the prosiphuncular side of the conch and forming shallow and wide sinus on the antisiphuncular side. Around the circumference several brown bands are preserved ( Fig. 28A ). The bands are ca. 0.5 mm wide; a number of 28 would occur around the total circumference.

Remarks

This species is assigned to Isorthoceras on the basis of the general similarity of the external conch characters to those of other species of this genus. Nothing is known about the adult size of this species, but the decreased angle of expansion of the two specimens from the Boda Limestone suggest an adult size of no more than 15 mm.

Comparison

This species is most similar to I. junceum ( Hall, 1847) with respect to the transverse ornamentation, but differs in having more widely spaced transverse striae (six per millimetre in I. heroyense comb. nov.; ten per millimetre in I. junceum ) and a higher angle of expansion at similar conch diameters. The strongly eccentric siphuncle is unique amongst known species of Isorthoceras .

Stratigraphic and geographic range

Herøya near Porsgrunn, Norway, Herøya Formation (“Gastropod Limestone”), Frognøya Island, Ringerike, Norway, Bønsnes Formation ( Strand 1934); Boda Limestone, Siljan District, Sweden; all late Katian, Late Ordovician.

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.

Fig. 28. Color marks in Isorthoceras Flower, 1962, Boda Limestone. Longitudinal striae of varying width occur in several specimen of Isorthoceras; see text for discussion. A. Isorthoceras heroyense (Strand, 1934) comb. nov., PMU 26831, Kallholn. B. Isorthoceras suave (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26835, Osmundsberget. C. Isorthoceras wahlenbergi Niko, 2008, PMU 26862, Kallholn. Scale bar = 10 mm for all figures.