Dawsonoceras stumburi, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 32-34

Dawsonoceras stumburi sp. nov.

urn:lsid:zoobank.org:act:1C1B98BD-F65E-4D72-8A31-203A0730E970

Figs 16E , 17B

Dawsonoceras sp. – Kröger & Isakar 2006: 157, figs 10e, j, 12h.

Diagnosis

Dawsonoceras with angle of expansion of ca. 5° and circular cross section; ca. five relatively shallow annulations occur at a distance similar to shell diameter; approximately 40 longitudinal lirae occur around shell circumference, more than 10 pronounced transverse striae occur per cycle of annulations; septal necks achoanitic; siphuncle subcentral, fusiform, expanded within chambers.

Etymology

In honor of Heino Stumbur (Tallinn), an Estonian geologist and paleontologist who first described this form in his unpublished diploma thesis ( Stumbur 1955) under the nomen nudum of Spyroceras paulum .

Holotype

PMU 26717 .

Type locality and horizon

Kallholn, Siljan District, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician.

Description

The holotype is a piece of phragmocone with a length of 72 mm and a diameter of 14–20 mm (angle of expansion 5°), with a circular conch cross section. The sutures are directly transverse, with a distance of 3 mm at a diameter of 15 mm (0.2 of corresponding conch cross section; Fig. 16E ). The conch is ornamented with 40 distinctly raised longitudinal lirae around circumference. Additionally, the conch is annulated. The distance between two successive annulations is 3.5 mm at a corresponding conch diameter of 16 mm (0.22 of distance similar to corresponding conch diameter). Approximately 20 distinct and directly transverse striae occur in a single cycle of annulation ( Fig. 17B ).

Comparison

This species differs from D. fenestratum in having a higher number of longitudinal lirae (ca. 20–30 in D. fenestratum against ca. 40 in D. stumburi sp. nov.). Additionally, the transverse annulation appears to be less pronounced and slightly more widely spaced in D. stumburi sp. nov., but the transverse striae are narrower than in D. fenestratum .

The external characters of D. stumburi sp. nov. are identical to those of Orthoceras clathratum Hall, 1847 from the Katian Middleville Formation of New York (see, e.g., Foerste 1928). But a synonymization is at present not possible, as the internal characters of this species are not known.

Stratigraphic and geographic range

Vormsi Regional Stages, Estonia ( Kröger & Isakar 2006); Boda Limestone, Siljan District, Sweden; late Katian, Ordovician.

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.