Gorbyoceras alternestriatum ( 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 : 52-53

Gorbyoceras alternestriatum ( Strand, 1934) comb. nov.

Figs 16F , 17F

Spyroceras alternestriatum Strand, 1934: 19 , pl. 3, figs 5, 6.

Gorbyoceras sp. – Dzik 1984: 122, text-fig. 48d, pl. 35, fig. 1.

Emended diagnosis

Gorbyoceras with slightly cyrtoconic adult body chamber, shell slightly compressed, relatively large adult size of more than 50 mm in conch height; slightly irregularly spaced, pronounced rounded annulations, approximately five to six in a length equal to conch diameter; approximately 30 regularly spaced longitudinal lirae around the entire circumference; in interspaces between primary lirae one or two less pronounced secondary lirae occur, approximately 12–15 fine transverse angular lirae between two successive ridges of annulations; sutures straight, nearly transverse, one suture at each groove of the annulations. Siphuncle eccentric in juvenile, subcentral in more adult growth stages, septal perforation approximately one tenth of the diameter of the shell.

Type locality and horizon

Stavnestangen, Ringerike District, Norway, Bønsnes Formation, latest Katian, Late Ordovician.

Material examined

Two specimens ( PMU 26769, 26770), Kallholn; one specimen ( PMU 26771), Unskarsheden, Siljan District, Sweden; Boda Limestone, Boda Core Member, late Katian, Ordovician.

Description

Specimen PMU 26771 is a nearly complete body chamber of a mature individual with a length of 90 mm and an adoral conch height of 51 mm and 47 mm at the apical end of the specimen (angle of expansion 2.5°) ( Fig. 16F ). The conch cross section is slightly compressed with ratio of the width to height of 0.91. The conch is slightly curved with the siphuncle, eccentric (SPR 0.34) on the convex side of the conch curvature. The septal perforation is 3 mm wide (0.06 of conch height). The septum at the base of the body chamber is shallow, with a depth of 10 mm (0.21 of conch height). The shell is annulated with slightly irregularly spaced and irregularly elevated rounded elevations; ca. six occur in a length equal to conch diameter. Around the circumference ca. 32 regularly spaced, distinctly raised primary lirae occur. In the interspaces one slightly less pronounced longitudinal raised line occurs. Additionally the conch is ornamented with fine, angular transverse lirae, 12–15 occurring between two successive ridges of the annulations, generating a fine reticulate pattern ( Fig. 17F ).

The two other specimens consist of portions of phragmocone with diameters from 18 to 22 mm, and from 22 to 23 mm respectively, with angles of expansion of ca. 8°. The distance between two successive annuli is ca. 0.18 of the corresponding conch cross section. The cross section is nearly circular. In specimen PMU 26769 the distance between the chambers is 0.18, and the suture lies in the groove of the annulation. The surface of this specimen is ornamented with 32 and 28 primary lirae, respectively. Between the primary lirae, one or two less pronounced secondary lirae occur.

Remarks

The specimens described above are assigned to Gorbyoceras alternestriatum ( Strand, 1934) comb. nov., because they agree in detail with the specimens described by Strand (1934) from the Bønsnes Formation of Norway. The new data, available from the adult body chamber PMU 26771, and additional data regarding the internal characters permit a more detailed species diagnosis and emendation.

Comparison

This species is very similar to G. textumaraneum ( Roemer, 1861) comb. nov. Both species have relatively large (more than 40 mm in conch diameter), curved adult body chambers, combined with a relatively widely spaced reticulate ornamentation. G. alternestriatum comb. nov. differs from the latter mainly in having a primary and secondary longitudinal ornamentation.

G. tetreauense Wilson, 1961 from the late Katian, Ottawa Formation, Canada is similar with respect to the angle of expansion and general aspects of ornamentation, but differs in having a more curved, stronger expanding conch with less pronounced annulations.

Stratigraphic and geographic range

Bønsnes Formation, latest Katian, Stavnestangen, Ringerike District, Norway ( Strand 1934); latest Katian, erratic boulder, Międzyzdroje, Poland ( Dzik 1984); Boda Limestone, Boda Core Member, late Katian, Siljan District, Sweden.

Figures

Fig. 16. Dawsonoceratidae and Proteoceratidae of the Boda Limestone. A-B. Dawsonoceras fenestratum (Eichwald, 1860), Kallholn. A. PMU 26702, nearly mature specimen with adorally decreasing angle of expansion. B. PMU 26712, juvenile specimen. C. Gorbyoceras sp. A, PMU 26772, Osmundsberget. D. Goryboceras sp. B, PMU 26773, Kallholn. E. Dawsonoceras stumburi sp. nov., PMU 26717, holotype, Kallholn. F. Gorbyoceras alternestriatum (Strand, 1934) comb. nov., PMU 26771, Unskarsheden. Scale bars: A-C, E = 10 mm; D = 10 mm; F = 10 mm.

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.