Nilssonoceras, Kröger, 2004

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 68

Genus Nilssonoceras nov.

Type species: Orthoceras nilssoni Boll, 1857 .

Diagnosis.—Orthoceraconic to slightly cyrtoceraconic Geisonoceratidae with a slightly compressed cross section. The shell shows a sharp transversal striation with acute ribs at a distance of around 1 mm or lower. The tubular siphuncle is subcentral to midventral. The septal necks are suborthochoanitic–orthochoanitic. The endosiphonal deposits are highly variable but generally consist of annuli at the septal foramen which may fuse and form an irregular endosiphuncular lining. The cameral and endosiphuncular deposits are more strongly developed at the antisiphonal side of the conch.

Remarks.—The genus Nilssonoceras differs from all other Geisonoceratidae in its distinct ornamentation. It can be distinguished already by small fragments of the shell. The intraspecific variability of this common Middle Ordovician genus is very high. Especially the endosiphuncular and cameral deposits and the rate of expansion of the conch are strongly variable. So far the genus includes two species: Nilssonoceras nilssoni ( Boll, 1857) and Nilssonoceras latisiphonatum sp. nov.