Slemmestadoceras, Kröger, 2008

Kröger, Björn, 2008, Runter kommen sie immer: Kapi-Talfahrt, Acta Palaeontologica Polonica 53 (4), pp. 745-749 : 747

Genus Slemmestadoceras nov.

Etymology: From Slemmestad, Oslo Region Norway, the small harbour next to the type locality of the Bjørkåsholmen Formation ( Owen et al. 1990).

Type species: Orthoceras attavus Brøgger, 1882 , from the base of Bjørkåsholmen Formation , Vestfossen, Norway, by monotypy .

Diagnosis.—Orthoconic longicones similar to Kyminoceras and Protocycloceras with respect to the apical angle of approximately 6 °, and a number of four rounded annulations per distance similar to the conch diameter. In contrast to Kyminoceras septa are straight, sloping slightly adapically in direction of prosiphuncular conch side and instead of five, only four chambers occur at a distance similar to the conch cross section. Siphuncle marginal and nearly half as wide as in Kyminoceras with diameter 0.1 of conch cross section. Connecting ring tubular, septal necks orthochoanitic.

Remarks.—The new genus is based on the unique combination of straight sutures and a comparatively wide tubular siphuncle. Protocycloceras differs in having a siphuncle with concave segments and Semiannuloceras Evans, 2005 in having a wider apical angle and an annulation that fades out during growth.

Stratigraphic and geographic range.—Black nodule layer at the base of the Bjørkåsholmen Formation ( Bienvillia angelini bed), Paltodus deltifer Conodont Zone, Timeslice 1b of Webby et al. (2004), Tremadocian, Lower Ordovician; Oslo Region, Norway.