Phthanoncoceratidae Evans & King 1990

Order, gen. et sp. indet. B

Figs 11A, 45G, 49A

Specimen FMNH-P30424, from Profilstranda section, Ny Friesland, Spitsbergen, bed PO 4.8, 1.8 m above base of Olenidsletta Member, V 1 a trilobite zone, Blackhillsian, Floian.

The specimen is a relatively well-preserved fragment representing a weakly exogastrically curved slender phragmocone with a length of 72 mm and a conch height of 17–23 mm (Fig. 45G). Only the left part of the conch is preserved. Based on the preserved position of the siphuncle, the conch width can be reconstructed: the reconstructed width is ca 18–28 mm at either end of the preserved portion. The conch cross section, hence, is slightly compressed.

The surface is ornamented with distinct, straight, transverse, rounded lirae (ca six per mm at the adapical end of the specimen) and irregularly spaced growth lines. Additionally, the conch is very weakly and irregularly undulated with a distance between two subsequent undulations between 3 and 5 mm. The ornamentation is slightly oblique, shifted toward the aperture at the antisiphuncular side of the conch.

At the adoral end of the specimen the chamber distance is 7 mm. The septal perforation has a diameter of ca 7 mm at the adoral end of the specimen (rSD = 0.4) and is eccentrically positioned, with a distance of the siphuncle of ca 3 mm from the conch margin. The septal necks are long orthochoanitic to hemichoanitic and the shape of the connecting ring was presumably tubular or nearly so (Figs 11A, 49A).

Given the high number of longiconic orthocones with a relatively wide subcentral, tubular siphuncle and orthochoanitic septal necks within the Baltoceratidae, Troedssonellidae, and Proterocameroceratidae in Floian and Dapingian strata, a high-level classification of this specimen is not possible until more material is available. The relatively long septal necks of this specimen are similar to those of Hemichoanella, but the siphuncle is marginal in the latter genus. More material is needed, specifically to evaluate the presence or absence of endosiphunular deposits in apical portions of the shell.