Nilssonoceras latisiphonatum, Kröger, 2004

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 70

Nilssonoceras latisiphonatum sp. nov.

Fig. 9 .

Holotype: The specimen NRM−Mo 158037, which shows four chambers of a nearly mature specimen.

Type locality: Skarlöv of Öland ( Sweden).

Type horizon: Folkeslunda Limestone (Lasnamägian).

Derivation of the name: From Latin latius, wide: referring to the large size of the siphuncle.

Material.— Six specimens available in the NRM. The recrystallized shell is preserved in all specimens .

Diagnosis.— Nilssonoceras with a very wide siphuncle (nearly one fourth of the conch diameter) and well developed elongated endosiphuncular annuli.

Description.— Conch form: Cross section slightly compressed. Maximum diameter: 45 mm. Apical angle of the conch 2.9–5.7°, mean a: 4°.

Ornamentation: Shell transversally striated by sharp, acute striae as in Nilssonoceras nilssoni ( Fig. 6B ). Inner shell smooth.

Phragmocone: Chamber height 0.3–1.1 (mean 0.8) of the respective diameter. Sutures straight. Shape of the septa tube like, with a septal foramen resembling a mamma (herein called mammoidal). Orthochoanitic septal necks. Siphuncle eccentric to subcentral (sp = 0.33–0.48). Siphuncular diameter 0.25 of the respective diameter of the conch. Elongated endosiphuncular annuli (see Fig. 9 ). Endosiphuncular lining may occur. Thin hypo− and episeptal cameral deposits. Living chamber and juvenile chambers not known.

Comparison.— Nilssonoceras nilssoni differs from Nilssonoceras latisiphonatum only in the dimension of the siphuncle.

Distribution.— Nilssonoceras latisiphonatum occurs in the Seby and Folkeslunda Limestone (Lasnamägian) of Öland and the Holen Limestone (Kundan) of Dalarna.

Figures

Fig. 9. Longitudinal section of the siphuncle of Nilssonoceras latisiphonatum sp. nov., NRM−Mo 158037, holotype (diameter of the siphuncle 10 mm). The siphuncle shows different areas of sparite (dotted: fin grained calcite, medium hatched: medium grained sparite, coarsely hatched: coarse grained sparite) which are interpreted as residues of the spherulitic prismatic layer of the connecting ring (dotted), the endosiphonal annuli (medium hatched) and the endosiphuncular lining (coarsely hatched).

Fig. 6. Schematic sketches of the cross section of the shell. Shell ornamentation of Orthoceras (A), Nilssonoceras (B), and Kinnekulloceras (C) (aperture to the right).