Isorthoceras cf. elongatocinctum ( Portlock, 1843 ), 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 59-61

Isorthoceras cf. elongatocinctum ( Portlock, 1843) comb. nov. Figs 17A, D , 23 D-E, 27

Orthoceras elongato-cinctum Portlock, 1843: 372 , pl. 27, fig. 2a-b. Orthoceras elongato cinctum – Baily 1875: pl. 12, fig. 10 (non pl. 13, fig. 8; see discussion below). Orthoceras elongatocinctum – Blake 1882: 119, pl. 13, figs 7, 8, 8a. ― Evans 1996: 118, pl. 21, figs 1, 2, text-fig. 21f.

Geisonocerina elongatocinctum – Holland 1993: 39, text-fig. 1g, i; 2000: 181, text-fig. 1m. Orthoceras aff. elongatocinctum – Evans 2002: 81, pl. 1, figs 1-3.

Diagnosis

Transversally ornamented, straight Isorthoceras with circular cross section and comparatively low angle of expansion of ca. 4°, chamber distance 0.3–0.4 of corresponding diameter; transverse ornamentation consists of distinctly elevated, sharp, sometimes imbricate lirae, with distance of ca. six to ten per millimetre; siphuncle nearly central with diameter ca. 0.1 of corresponding conch diameter. (Adopted from Holland 1993; Evans 1993.)

Type locality and horizon

Killey Bridge Formation, County Tyrone, Northern Ireland. Katian Stage.

Material examined

Twenty-two specimens, one ( PMU 26809) from Osmundsberget; 20 from Kallholn, Siljan district, Sweden ( PMU 26810–26829); all Boda Limestone, Boda Core Member, late Katian, Late Ordovician. One additional specimen ( PMU 26830) from mid Katian Kullsberg Limestone, Kullsberg, Siljan district, Sweden.

Description

Specimen PMU 26825 is a portion of the phragmocone and a nearly complete mature body chamber, with a diameter increasing from 8 to 13 mm and a length of 75 mm (angle of expansion 3.8°). It is ornamented

with sharp, distinctly raised lirae; apically with ca. four to five lirae per millimetre. Adorally the lirae are increasingly more crowded and irregularly spaced, with ca. ten per millimetre near the aperture. The lirae are slightly oblique and form a distinct, but shallow hyponomic sinus on the antisiphuncular side of the conch. An apertural shell thickening and the low apical angle indicate maturity.

When comparing the seventeen measured specimens, a clear trend of decreasing angle of expansion becomes apparent ( Fig. 27 ). In early growth stages the angle is as high as 7°, but it continuously decreases with increasing conch diameter. The position of the siphuncle is preserved in specimens PMU 26814–26815 (SPR 0.49, 0.46), 26810 (SPR 0.4), 26823 (SPR 0.43) and 26826 (SPR 0.44). In smaller specimens the siphuncle is more eccentric, but in all other specimens it is nearly centrally positioned. The septal spacing varies between 0.3 (PMU 26815) and 0.44 (PMU 26826) of the corresponding conch height.

Remarks

This species was previously known only from the latest Katian sediments of Northern Ireland and Wales. Details of the internal characters were described by Evans (1993: fig. 8), who revised the species based on more than one hundred specimens. Evans (1993: 167) described the septal necks as orthoconic, but his fig. 8 shows septal necks which must be termed “suborthochoanitic” in the strict sense (see Material and Methods, above). The shape of the septal necks, the constricted siphuncle, the general conch shape and the ornamentation are similar to other species of Isorthoceras . Based on this similarity, this species is here assigned to Isorthoceras .

The largest specimen in the Irish material has a diameter of 25 mm. The specimens from the Boda Limestone and the diagram in Evans (1993: fig. 9) suggest a maximum size of less than 15 mm, which is the reason why the Boda specimens are assigned with the addendum “cf.” to this species.

These specimens are identical in external characters to the specimens described by Evans (1993). Additional information on the variability of the apical angle and the ornamentation are provided by the specimens from the Boda Limestone. Specimen PMU 26825 is the most completely preserved fragment and demonstrates the ontogenetic variation of the transverse ornamentation and angle of expansion ( Figs 17A, D ; 23 D-E). The angle of expansion of mature parts of the shell is very small, and the spacing of the transverse lirae is narrower and more irregular in later growth stages.

Comparison

This species differs from other species of Isorthoceras in having an ornamentation of fine, but distinctly elevated, acute transverse lirae, ca. 0.2 to 0.1 mm distant.

Geisonoceras heintzi Strand, 1934 , from the Sørbakken formation in the Ringerike Distict, Norway, probably represents a species of Isorthoceras and is a small, slender transversally ornamented form that differs from I. elongatocinctum comb. nov. in having transverse lirae which are more distantly spaced (distance ca. 0.4 mm). Isorthoceras heroyense ( Strand, 1934) comb. nov. is similar an ornamentation, but differs in having a more eccentric siphuncle (SPR 0.31) and a larger angle of expansion (5–9°).

Stratigraphic and geographic range

Kildare Limestone, Killey Bridge Formation, Portrane Limestone, late Katian, Ireland, Keisley Limestone, late Katian, Cumbria, UK ( Holland 1993; Evans 1996, 2002); Sholeshook Limestone, late Katian, South Wales, UK ( Evans 1996); lower Wether Law Linn Formation, Telychian, Ireland ( Holland 2002); Boda Limestone, late Katian, 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.

Fig. 27. Angle of expansion of 15 measured specimen of Isorthoceras cf. elongatocinctum (Portlock, 1843), Boda Limestone, Late Katian, Dalarna, Sweden.