Nathorstoceras kallholnense, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 40-42

Nathorstoceras kallholnense sp. nov.

urn:lsid:zoobank.org:act:5393FB9D-6642-4D8D-9F4B-C2C741B7CC57

Figs 10D , 13E , 19 B-C, F, 20B, 21A

Diagnosis

Nathorstoceras gen. nov. with slightly curved annulated conch; angle of expansion of 10–13°; annulations more pronounced in late growth stages; shell sculpture of irregularly spaced, pronounced transverse growth lines, less than one millimeter apart; distance between annulations ca. 0.3 of corresponding conch height; annuli and growth lines with shallow sinus on prosiphuncular side; conch cross section compressed; width/height ratio ca. 0.87; septal distance ca. 0.3 of corresponding conch height; siphuncle eccentric (SPR ca. 0.3); septal perforation ca. 0.07 of corresponding conch height; siphuncular segments abruptly expanded at septal foramen with maximum diameter ca. 0.14 of conch diameter; epi- and hyposeptal deposits present; endosiphuncular deposits not known.

Etymology

Refers to the type locality.

Type material

Holotype

PMU 26729 .

Paratypes

Four paratypes: PMU d1376, 26730–26732, from Kallholn, Siljan District, Sweden; all Boda Limestone, Boda Core Member, late Katian.

Type locality and horizon

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

Description

The holotype is a piece of phragmocone ca. 60 mm long, a height of 27–33 mm and a width at the adoral end of 33 mm (width/height ratio 0.87, angle of expansion 10.4°) ( Figs 10 D , 19F ). The conch is weakly annulated with a distance between two successive annuli ca. 0.3 that of the corresponding conch cross section, and ornamented with pronounced, irregularly spaced growth lines or raised bands, each with a width of less than a millimeter. The conch is slightly crushed, and nearly straight, with an eccentrically positioned siphuncle. Because of the fragmented shell the siphuncle appears to be positioned on the convex side of the shell curvature. A reconstruction of the shell indicates that the siphuncle is on the convex side of a very slightly curved phragmocone. At the apical end of the specimen the distance of the siphuncle from the conch margin is 9 mm (SPR 0.33). The siphuncle is much wider than the septal perforation, and at the adapical end of the specimen the septal perforation is ca. 1.8 mm, and the maximum diameter of the siphuncle is 3.5 mm (0.07 and 0.13 of the corresponding conch cross section, respectively) ( Figs 20B , 21A ). The septa are 9 mm apart where the corresponding conch height is 30 mm. The septal necks are achoanitic and siphuncular segments (connecting rings) are abruptly expanded at the septal foramen. At the adapical end of the specimen epi- and hyposeptal deposits occur.

Specimen PMU 26730 is a fragment of a body chamber with a length of 72 mm, a height of 29–44 mm, and a width of 26–38 mm (width/height ratio 0.86–0.89, angle of expansion 11.8°) ( Fig. 19 B-C). The conch cross section is elliptical, with the antisiphuncular side slightly more sharply rounded. The conch is annulated with rounded, shallow annuli 12 mm apart at a corresponding conch height of 40 mm. In addition, pronounced, irregularly spaced growth lines run parallel to the annuli, both of which are slightly oblique and form a shallow sinus on the prosiphuncular side of the conch. The siphuncle is eccentric (SPR 0.28), lying on the convex side of the conch curvature. Specimen PMU 26731, which is a part of a phragmocone and body chamber, is the largest specimen of this species with a maximum conch height of ca. 60 mm.

Comparison

Nathorstoceras kallholnense sp. nov. differs from N. adnatum sp. nov. in having a more pronounced annulated conch and a less compressed conch cross section.

Stratigraphic and geographic range

Boda Limestone, Dalarna, Sweden, late Katian, Late Ordovician.

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.