Isorthoceras suave (Angelin in Angelin & Lindström, 1880 ) Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 64-66

Isorthoceras suave (Angelin in Angelin & Lindström, 1880) comb. nov.

Figs 17I , 23B , 28B

Orthoceras suave Angelin in Angelin & Lindström, 1880: 3 , pl. 7, figs 13-17.

Diagnosis

Isorthoceras with weakly curved conch and circular or slightly compressed cross section; angle of expansion ca. 9° in premature growth stages; ornamented with fine longitudinal raised lirae and faint

transverse irregularly spaced growth lines; mature size more than 20 mm; siphuncle eccentric to subcentral at convex side of conch curvature.

Type material

Lectotype

NRM-Mo 151187 ( Angelin & Lindström 1880: pl. 7, figs 16-17) (designated herein).

Paralectotype

NRM-Mo 151186 ( Angelin & Lindström 1880: pl. 7, fig. 14).

Type locality and horizon

Ostbjörka, Siljan district, Dalarna, Sweden; Boda Limestone, Boda Core Member, late Katian, Ordovician.

Other material examined

Eight specimens: three specimens (PMU 26835–26837), Osmundberget, Dalarna; four specimens (PMU 26838– 26841), Kallholn, Dalarna; all from the Boda Limestone, Boda Core Member, late Katian.

Decription

The lectotype is a fragment of a very slightly curved phragmocone, 42 mm long, with a diameter that increases from 8 to 14 mm (angle of expansion 8.1°). It is ornamented with fine longitudinal raised striae.

Specimen PMU 26835 is a nearly straight portion of phragmocone 43 mm long, increasing in diameter from 10 to 17 mm (angle of expansion 9.2°) ( Fig. 23B ) and a circular conch cross section. The siphuncle is nearly central, 5.5 mm from the conch margin at a corresponding conch diameter of 12.5 mm (SPR 0.44). The chamber height is 3 mm at a diameter of 14.5 (0.21 of conch diameter). The diameter of the septal perforation is 1 mm at a conch diameter of 12.5 mm (0.08 of conch diameter). The specimen is ornamented with fine raised lirae, ca. five per millimeter, and very faint, irregularly spaced growth lines ca. 1 mm apart ( Fig. 17I ). Approximately 34 ca. 0.7 mm wide dark bands run longitudinally around the entire circumference ( Fig. 28B ).

In another specimen (PMU 26836) the siphuncle is more eccentrically positioned. At a conch diameter of 21 mm, the siphuncle is positioned 8 mm from conch margin (SPR 0.38).

The angle of expansion of the six available specimens varies between 5.7 and 9.9°, with no size trend. The chamber height varies between 0.2 and 0.3 of the corresponding conch height.

Remarks

The adult size of this species must be clearly more than 20 mm. The largest specimen (PMU 26836) in the collection is a fragment of a phragmocone with a diameter of 21 mm and the septal spacing shows no signs of septal crowding. The longitudinal ornamentation is crossed only by faint, irregularly spaced growth lines. A total of 34 longitudinal color bands are preserved around the entire circumference of the specimen PMU 26835. This occurrence of longitudinal color bands is known from other species of Isorthoceras (see Kröger et al. 2009; Fig. 28 ), but their significance for taxonomy is not known, because of the rarity of this type of preservation.

Comparison

This species is unique amongst species considered to belong to Isorthoceras in having exclusively longitudinal lirae.

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.

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.