Nathorstoceras adnatum, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 42-43

Nathorstoceras adnatum sp. nov.

urn:lsid:zoobank.org:act:58064C88-D4A3-4CA4-B6EA-DE719EE0F724

Figs 10C , 13F , 19A , 20A , 21B

Diagnosis

Nathorstoceras with nearly straight and very weakly annulated conch with angle of expansion of 8°; annulations more pronounced in late growth stages; shell ornamented with regularly spaced transverse lirae and distance between annulations ca. 0.2 of corresponding conch height; annuli and lirae form shallow sinus on prosiphuncular side; conch cross section compressed with width/height ratio of ca. 0.8; septal distance ca. 0.24 of corresponding conch height; siphuncle subcentral with SPR 0.41; septal perforation with diameter of ca. 0.05 of corresponding conch height; siphuncular segments abruptly expanded at septal foramen with maximum diameter of ca. 0.14 of corresponding conch cross section; endosiphuncular deposits unknown.

Etymology

Refers to the strongly expanded, adnate siphuncular segments of this species.

Holotype

PMU 26733 .

Type locality and horizon

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

Description

The specimen is a portion of phragmocone ca. 50 mm long, with a height of 37–44 mm and a width at the adoral end of 30 mm (width/height ratio 0.81, angle of expansion 8°) ( Fig. 10C , 19A ). The conch is faintly annulated with a distance between two successive annuli of ca. 0.2 of the corresponding conch cross section, and ornamented with pronounced, regularly spaced lirae, with a distance between two subsequent lirae of ca. 1. 5 mm. The conch is is nearly straight, with a subcentrally positioned siphuncle; at the apical end of the specimen its distance from the conch margin is 15 mm (SPR 0.41). The connecting ring is much wider than the septal perforation ( Figs 20A , 21B ). At the adapical end of the specimen the septal perforation is ca. 2 mm and the maximum diameter of the siphuncle is 5 mm (0.05 and 0.14 of the corresponding conch cross section, respectively). The sutures are directly transverse and form shallow lateral lobes. At the adapical end of the specimen the distance between two septa is 9 mm (0.24 of corresponding conch cross section). The septal necks are achoanitic and the siphuncular segments are adnate, abruptly expanded at septal foramen. No cameral and endosiphuncular deposits occur.

Comparison

See comparison with Nathorstoceras kallholnense sp. nov. (above).

Family Geisonoceratidae Zhuravleva, 1959

Figures

Fig. 10. Camera lucida drawings of cross sections of selected Boda Limestone cephalopods. A. Cyrtorizoceras thorslundi sp. nov., PMU 26657, Osmundsberget, mature body chamber. B. Tyrioceras cf. kjaerulfi Strand, 1934, PMU 26897, Kallholn. C. Nathorstoceras adnatum sp. nov., PMU 26733, holotype, Kallholn. D. Nathorstoceras kallholnense sp. nov., PMU 26729, holotype. E. Redpathoceras depressum sp. nov., PMU 26927, holotype, Kallholn, cross section of base of mature truncated conch. F. Schuchertoceras fryi sp. nov., PMU 24744, holotype, Kalllholn, cross section of mature aperture (dark grey) and body chamber (medium grey). G. Redpathoceras bullatum sp. nov., PMU 26923, holotype, Kallholn, cross section of base of mature truncated conch. Prosiphuncular side directed downward in all figures. Siphuncles shown as white circles. Scale bar = 10 mm for all figures.

Fig. 13. Polished median sections of Orthocerida of the Boda Limestone. A. Dawsonoceras fenestratum (Eichwald, 1860), PMU 26706, Kallholn. B-C. Striatocycloceras isbergi sp. nov. B. PMU 26681, Kallholn. C. PMU 26661, Osmundsberget. D. Isorthoceras angelini sp. nov., PMU 26784, Kallholn. E. Nathorstoceras kallholnense sp. nov., PMU 26729, holotype, Kallholn. F. Nathorstoceras adnatum sp. nov., PMU 26733, holotype, Kallholn. G. Pleurorthoceras osmundsbergense sp. nov., NRM-PZ Mo, 190102c, Osmundsberget.H.Ordogeisonocerasfoerstei(Strand,1934)comb.nov.,PMU26763,Kallholn. I. Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26740, Kallholn. Scale bars: A-D, I = 1 mm; E-H = 5 mm.

Fig. 19. Orthocerida of the Boda Limestone. A. Nathorstoceras adnatum sp. nov., PMU d1440, holotype, Kallholn, lateral view. B-C. Nathorstoceras kallholnense sp. nov., PMU 26730, Kallholn, complete body chamber. B. Lateral view. C. view from prosiphuncular side. D. Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26747, Unskarsheden, lateral view. E. Ordogeisonoceras foerstei (Strand, 1934) comb. nov., PMU 26762, Kallholn, lateral view. F. Nathorstoceras kallholnense sp. nov., PMU 26729, holotype, lateral view, note siphuncle on left side. Scale bar = 10 mm for all figures.

Fig. 20. Polished median sections of Nathorstoceras sp. nov., Kallholn, Boda Limestone. A. Nathorstoceras adnatum sp. nov., PMU 26733, holotype, Kallholn. B. Nathorstoceras kallholnense sp. nov., PMU 26729, holotype, Kallholn. Scale bar = 10 mm for both figures.

Fig. 21. Reconstruction of the details of the septal necks and connecting ring of Nathorstoceras sp. nov., Kallholn, Boda Limestone. A. Nathorstoceras kallholnense sp. nov., PMU 26729, holotype. B. Nathorstoceras adnatum sp. nov., PMU 26733, holotype.