Nathorstoceras kallholnense Kröger 2013, sp. nov.
Creators: name: Kröger, Björn
Publisher: name: Zenodo
Year: 2013
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Cephalopoda
Orthocerida
Eriditidae
Nathorstoceras
Nathorstoceras kallholnense

description: 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.
description: Published as part of Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, pp. 1-110 in European Journal of Taxonomy 41 on pages 40-42, DOI: 10.5852/ejt.2013.41, http://zenodo.org/record/3814221